monitoring database activity database activity monitoring A database administrator frequently wonders, What is the system doing right now? This chapter discusses how to find that out. Several tools are available for monitoring database activity and analyzing performance. Most of this chapter is devoted to describing PostgreSQL's statistics collector, but one should not neglect regular Unix monitoring programs such as ps, top, iostat, and vmstat. Also, once one has identified a poorly-performing query, further investigation might be needed using PostgreSQL's command. discusses EXPLAIN and other methods for understanding the behavior of an individual query. ps to monitor activity On most Unix platforms, PostgreSQL modifies its command title as reported by ps, so that individual server processes can readily be identified. A sample display is $ ps auxww | grep ^postgres postgres 15551 0.0 0.1 57536 7132 pts/0 S 18:02 0:00 postgres -i postgres 15554 0.0 0.0 57536 1184 ? Ss 18:02 0:00 postgres: background writer postgres 15555 0.0 0.0 57536 916 ? Ss 18:02 0:00 postgres: checkpointer postgres 15556 0.0 0.0 57536 916 ? Ss 18:02 0:00 postgres: walwriter postgres 15557 0.0 0.0 58504 2244 ? Ss 18:02 0:00 postgres: autovacuum launcher postgres 15558 0.0 0.0 17512 1068 ? Ss 18:02 0:00 postgres: stats collector postgres 15582 0.0 0.0 58772 3080 ? Ss 18:04 0:00 postgres: joe runbug 127.0.0.1 idle postgres 15606 0.0 0.0 58772 3052 ? Ss 18:07 0:00 postgres: tgl regression [local] SELECT waiting postgres 15610 0.0 0.0 58772 3056 ? Ss 18:07 0:00 postgres: tgl regression [local] idle in transaction (The appropriate invocation of ps varies across different platforms, as do the details of what is shown. This example is from a recent Linux system.) The first process listed here is the master server process. The command arguments shown for it are the same ones used when it was launched. The next five processes are background worker processes automatically launched by the master process. (The stats collector process will not be present if you have set the system not to start the statistics collector; likewise the autovacuum launcher process can be disabled.) Each of the remaining processes is a server process handling one client connection. Each such process sets its command line display in the form postgres: user database host activity The user, database, and (client) host items remain the same for the life of the client connection, but the activity indicator changes. The activity can be idle (i.e., waiting for a client command), idle in transaction (waiting for client inside a BEGIN block), or a command type name such as SELECT. Also, waiting is appended if the server process is presently waiting on a lock held by another session. In the above example we can infer that process 15606 is waiting for process 15610 to complete its transaction and thereby release some lock. (Process 15610 must be the blocker, because there is no other active session. In more complicated cases it would be necessary to look into the pg_locks system view to determine who is blocking whom.) If has been configured the cluster name will also be shown in ps output: $ psql -c 'SHOW cluster_name' cluster_name -------------- server1 (1 row) $ ps aux|grep server1 postgres 27093 0.0 0.0 30096 2752 ? Ss 11:34 0:00 postgres: server1: background writer ... If you have turned off then the activity indicator is not updated; the process title is set only once when a new process is launched. On some platforms this saves a measurable amount of per-command overhead; on others it's insignificant. Solaris requires special handling. You must use /usr/ucb/ps, rather than /bin/ps. You also must use two w flags, not just one. In addition, your original invocation of the postgres command must have a shorter ps status display than that provided by each server process. If you fail to do all three things, the ps output for each server process will be the original postgres command line. statistics PostgreSQL's statistics collector is a subsystem that supports collection and reporting of information about server activity. Presently, the collector can count accesses to tables and indexes in both disk-block and individual-row terms. It also tracks the total number of rows in each table, and information about vacuum and analyze actions for each table. It can also count calls to user-defined functions and the total time spent in each one. PostgreSQL also supports reporting dynamic information about exactly what is going on in the system right now, such as the exact command currently being executed by other server processes, and which other connections exist in the system. This facility is independent of the collector process. Since collection of statistics adds some overhead to query execution, the system can be configured to collect or not collect information. This is controlled by configuration parameters that are normally set in postgresql.conf. (See for details about setting configuration parameters.) The parameter enables monitoring of the current command being executed by any server process. The parameter controls whether statistics are collected about table and index accesses. The parameter enables tracking of usage of user-defined functions. The parameter enables monitoring of block read and write times. Normally these parameters are set in postgresql.conf so that they apply to all server processes, but it is possible to turn them on or off in individual sessions using the command. (To prevent ordinary users from hiding their activity from the administrator, only superusers are allowed to change these parameters with SET.) The statistics collector transmits the collected information to other PostgreSQL processes through temporary files. These files are stored in the directory named by the parameter, pg_stat_tmp by default. For better performance, stats_temp_directory can be pointed at a RAM-based file system, decreasing physical I/O requirements. When the server shuts down cleanly, a permanent copy of the statistics data is stored in the pg_stat subdirectory, so that statistics can be retained across server restarts. When recovery is performed at server start (e.g. after immediate shutdown, server crash, and point-in-time recovery), all statistics counters are reset. Several predefined views, listed in , are available to show the current state of the system. There are also several other views, listed in , available to show the results of statistics collection. Alternatively, one can build custom views using the underlying statistics functions, as discussed in . When using the statistics to monitor collected data, it is important to realize that the information does not update instantaneously. Each individual server process transmits new statistical counts to the collector just before going idle; so a query or transaction still in progress does not affect the displayed totals. Also, the collector itself emits a new report at most once per PGSTAT_STAT_INTERVAL milliseconds (500 ms unless altered while building the server). So the displayed information lags behind actual activity. However, current-query information collected by track_activities is always up-to-date. Another important point is that when a server process is asked to display any of these statistics, it first fetches the most recent report emitted by the collector process and then continues to use this snapshot for all statistical views and functions until the end of its current transaction. So the statistics will show static information as long as you continue the current transaction. Similarly, information about the current queries of all sessions is collected when any such information is first requested within a transaction, and the same information will be displayed throughout the transaction. This is a feature, not a bug, because it allows you to perform several queries on the statistics and correlate the results without worrying that the numbers are changing underneath you. But if you want to see new results with each query, be sure to do the queries outside any transaction block. Alternatively, you can invoke pg_stat_clear_snapshot(), which will discard the current transaction's statistics snapshot (if any). The next use of statistical information will cause a new snapshot to be fetched. A transaction can also see its own statistics (as yet untransmitted to the collector) in the views pg_stat_xact_all_tables, pg_stat_xact_sys_tables, pg_stat_xact_user_tables, and pg_stat_xact_user_functions. These numbers do not act as stated above; instead they update continuously throughout the transaction. Some of the information in the dynamic statistics views shown in is security restricted. Ordinary users can only see all the information about their own sessions (sessions belonging to a role that they are a member of). In rows about other sessions, many columns will be null. Note, however, that the existence of a session and its general properties such as its sessions user and database are visible to all users. Superusers and members of the built-in role pg_read_all_stats (see also ) can see all the information about all sessions. View Name Description pg_stat_activity pg_stat_activity One row per server process, showing information related to the current activity of that process, such as state and current query. See for details. pg_stat_replicationpg_stat_replication One row per WAL sender process, showing statistics about replication to that sender's connected standby server. See for details. pg_stat_wal_receiverpg_stat_wal_receiver Only one row, showing statistics about the WAL receiver from that receiver's connected server. See for details. pg_stat_subscriptionpg_stat_subscription At least one row per subscription, showing information about the subscription workers. See for details. pg_stat_sslpg_stat_ssl One row per connection (regular and replication), showing information about SSL used on this connection. See for details. pg_stat_gssapipg_stat_gssapi One row per connection (regular and replication), showing information about GSSAPI authentication and encryption used on this connection. See for details. pg_stat_progress_analyzepg_stat_progress_analyze One row for each backend (including autovacuum worker processes) running ANALYZE, showing current progress. See . pg_stat_progress_create_indexpg_stat_progress_create_index One row for each backend running CREATE INDEX or REINDEX, showing current progress. See . pg_stat_progress_vacuumpg_stat_progress_vacuum One row for each backend (including autovacuum worker processes) running VACUUM, showing current progress. See . pg_stat_progress_clusterpg_stat_progress_cluster One row for each backend running CLUSTER or VACUUM FULL, showing current progress. See . pg_stat_progress_basebackuppg_stat_progress_basebackup One row for each WAL sender process streaming a base backup, showing current progress. See . View Name Description pg_stat_archiverpg_stat_archiver One row only, showing statistics about the WAL archiver process's activity. See for details. pg_stat_bgwriterpg_stat_bgwriter One row only, showing statistics about the background writer process's activity. See for details. pg_stat_databasepg_stat_database One row per database, showing database-wide statistics. See for details. pg_stat_database_conflictspg_stat_database_conflicts One row per database, showing database-wide statistics about query cancels due to conflict with recovery on standby servers. See for details. pg_stat_all_tablespg_stat_all_tables One row for each table in the current database, showing statistics about accesses to that specific table. See for details. pg_stat_sys_tablespg_stat_sys_tables Same as pg_stat_all_tables, except that only system tables are shown. pg_stat_user_tablespg_stat_user_tables Same as pg_stat_all_tables, except that only user tables are shown. pg_stat_xact_all_tablespg_stat_xact_all_tables Similar to pg_stat_all_tables, but counts actions taken so far within the current transaction (which are not yet included in pg_stat_all_tables and related views). The columns for numbers of live and dead rows and vacuum and analyze actions are not present in this view. pg_stat_xact_sys_tablespg_stat_xact_sys_tables Same as pg_stat_xact_all_tables, except that only system tables are shown. pg_stat_xact_user_tablespg_stat_xact_user_tables Same as pg_stat_xact_all_tables, except that only user tables are shown. pg_stat_all_indexespg_stat_all_indexes One row for each index in the current database, showing statistics about accesses to that specific index. See for details. pg_stat_sys_indexespg_stat_sys_indexes Same as pg_stat_all_indexes, except that only indexes on system tables are shown. pg_stat_user_indexespg_stat_user_indexes Same as pg_stat_all_indexes, except that only indexes on user tables are shown. pg_statio_all_tablespg_statio_all_tables One row for each table in the current database, showing statistics about I/O on that specific table. See for details. pg_statio_sys_tablespg_statio_sys_tables Same as pg_statio_all_tables, except that only system tables are shown. pg_statio_user_tablespg_statio_user_tables Same as pg_statio_all_tables, except that only user tables are shown. pg_statio_all_indexespg_statio_all_indexes One row for each index in the current database, showing statistics about I/O on that specific index. See for details. pg_statio_sys_indexespg_statio_sys_indexes Same as pg_statio_all_indexes, except that only indexes on system tables are shown. pg_statio_user_indexespg_statio_user_indexes Same as pg_statio_all_indexes, except that only indexes on user tables are shown. pg_statio_all_sequencespg_statio_all_sequences One row for each sequence in the current database, showing statistics about I/O on that specific sequence. See for details. pg_statio_sys_sequencespg_statio_sys_sequences Same as pg_statio_all_sequences, except that only system sequences are shown. (Presently, no system sequences are defined, so this view is always empty.) pg_statio_user_sequencespg_statio_user_sequences Same as pg_statio_all_sequences, except that only user sequences are shown. pg_stat_user_functionspg_stat_user_functions One row for each tracked function, showing statistics about executions of that function. See for details. pg_stat_xact_user_functionspg_stat_xact_user_functions Similar to pg_stat_user_functions, but counts only calls during the current transaction (which are not yet included in pg_stat_user_functions). pg_stat_slrupg_stat_slru One row per SLRU, showing statistics of operations. See for details. The per-index statistics are particularly useful to determine which indexes are being used and how effective they are. The pg_statio_ views are primarily useful to determine the effectiveness of the buffer cache. When the number of actual disk reads is much smaller than the number of buffer hits, then the cache is satisfying most read requests without invoking a kernel call. However, these statistics do not give the entire story: due to the way in which PostgreSQL handles disk I/O, data that is not in the PostgreSQL buffer cache might still reside in the kernel's I/O cache, and might therefore still be fetched without requiring a physical read. Users interested in obtaining more detailed information on PostgreSQL I/O behavior are advised to use the PostgreSQL statistics collector in combination with operating system utilities that allow insight into the kernel's handling of I/O. Column Type Description datid oid OID of the database this backend is connected to datname name Name of the database this backend is connected to pid integer Process ID of this backend leader_pid integer Process ID of the parallel group leader if this process is or has been involved in parallel query, or null. This field is set when a process wants to cooperate with parallel workers, and remains set as long as the process exists. For a parallel group leader, this field is set to its own process ID. For a parallel worker, this field is set to the process ID of the parallel group leader. usesysid oid OID of the user logged into this backend usename name Name of the user logged into this backend application_name text Name of the application that is connected to this backend client_addr inet IP address of the client connected to this backend. If this field is null, it indicates either that the client is connected via a Unix socket on the server machine or that this is an internal process such as autovacuum. client_hostname text Host name of the connected client, as reported by a reverse DNS lookup of client_addr. This field will only be non-null for IP connections, and only when is enabled. client_port integer TCP port number that the client is using for communication with this backend, or -1 if a Unix socket is used. If this field is null, it indicates that this is an internal server process. backend_start timestamp with time zone Time when this process was started. For client backends, this is the time the client connected to the server. xact_start timestamp with time zone Time when this process' current transaction was started, or null if no transaction is active. If the current query is the first of its transaction, this column is equal to the query_start column. query_start timestamp with time zone Time when the currently active query was started, or if state is not active, when the last query was started state_change timestamp with time zone Time when the state was last changed wait_event_type text The type of event for which the backend is waiting, if any; otherwise NULL. See . wait_event text Wait event name if backend is currently waiting, otherwise NULL. See through . state text Current overall state of this backend. Possible values are: active: The backend is executing a query. idle: The backend is waiting for a new client command. idle in transaction: The backend is in a transaction, but is not currently executing a query. idle in transaction (aborted): This state is similar to idle in transaction, except one of the statements in the transaction caused an error. fastpath function call: The backend is executing a fast-path function. disabled: This state is reported if is disabled in this backend. backend_xid xid Top-level transaction identifier of this backend, if any. backend_xmin xid The current backend's xmin horizon. query text Text of this backend's most recent query. If state is active this field shows the currently executing query. In all other states, it shows the last query that was executed. By default the query text is truncated at 1024 characters; this value can be changed via the parameter . backend_type text Type of current backend. Possible types are autovacuum launcher, autovacuum worker, logical replication launcher, logical replication worker, parallel worker, background writer, client backend, checkpointer, startup, walreceiver, walsender and walwriter. In addition, background workers registered by extensions may have additional types. The pg_stat_activity view will have one row per server process, showing information related to the current activity of that process. The wait_event and state columns are independent. If a backend is in the active state, it may or may not be waiting on some event. If the state is active and wait_event is non-null, it means that a query is being executed, but is being blocked somewhere in the system. Wait Event Type Description Activity The server process is idle. This event type indicates a process waiting for activity in its main processing loop. wait_event will identify the specific wait point; see . BufferPin The server process is waiting for exclusive access to a data buffer. Buffer pin waits can be protracted if another process holds an open cursor that last read data from the buffer in question. See . Client The server process is waiting for activity on a socket connected to a user application. Thus, the server expects something to happen that is independent of its internal processes. wait_event will identify the specific wait point; see . Extension The server process is waiting for some condition defined by an extension module. See . IO The server process is waiting for an I/O operation to complete. wait_event will identify the specific wait point; see . IPC The server process is waiting for some interaction with another server process. wait_event will identify the specific wait point; see . Lock The server process is waiting for a heavyweight lock. Heavyweight locks, also known as lock manager locks or simply locks, primarily protect SQL-visible objects such as tables. However, they are also used to ensure mutual exclusion for certain internal operations such as relation extension. wait_event will identify the type of lock awaited; see . LWLock The server process is waiting for a lightweight lock. Most such locks protect a particular data structure in shared memory. wait_event will contain a name identifying the purpose of the lightweight lock. (Some locks have specific names; others are part of a group of locks each with a similar purpose.) See . Timeout The server process is waiting for a timeout to expire. wait_event will identify the specific wait point; see . Activity Wait Event Description ArchiverMain Waiting in main loop of archiver process. AutoVacuumMain Waiting in main loop of autovacuum launcher process. BgWriterHibernate Waiting in background writer process, hibernating. BgWriterMain Waiting in main loop of background writer process. CheckpointerMain Waiting in main loop of checkpointer process. LogicalApplyMain Waiting in main loop of logical replication apply process. LogicalLauncherMain Waiting in main loop of logical replication launcher process. PgStatMain Waiting in main loop of statistics collector process. RecoveryWalStream Waiting in main loop of startup process for WAL to arrive, during streaming recovery. SysLoggerMain Waiting in main loop of syslogger process. WalReceiverMain Waiting in main loop of WAL receiver process. WalSenderMain Waiting in main loop of WAL sender process. WalWriterMain Waiting in main loop of WAL writer process. BufferPin Wait Event Description BufferPin Waiting to acquire an exclusive pin on a buffer. Client Wait Event Description ClientRead Waiting to read data from the client. ClientWrite Waiting to write data to the client. GSSOpenServer Waiting to read data from the client while establishing a GSSAPI session. LibPQWalReceiverConnect Waiting in WAL receiver to establish connection to remote server. LibPQWalReceiverReceive Waiting in WAL receiver to receive data from remote server. SSLOpenServer Waiting for SSL while attempting connection. WalReceiverWaitStart Waiting for startup process to send initial data for streaming replication. WalSenderWaitForWAL Waiting for WAL to be flushed in WAL sender process. WalSenderWriteData Waiting for any activity when processing replies from WAL receiver in WAL sender process. Extension Wait Event Description Extension Waiting in an extension. IO Wait Event Description BufFileRead Waiting for a read from a buffered file. BufFileWrite Waiting for a write to a buffered file. ControlFileRead Waiting for a read from the control file. ControlFileSync Waiting for the control file to reach stable storage. ControlFileSyncUpdate Waiting for an update to the control file to reach stable storage. ControlFileWrite Waiting for a write to the control file. ControlFileWriteUpdate Waiting for a write to update the control file. CopyFileRead Waiting for a read during a file copy operation. CopyFileWrite Waiting for a write during a file copy operation. DSMFillZeroWrite Waiting to write zero bytes to a dynamic shared memory backing file. DataFileExtend Waiting for a relation data file to be extended. DataFileFlush Waiting for a relation data file to reach stable storage. DataFileImmediateSync Waiting for an immediate synchronization of a relation data file to stable storage. DataFilePrefetch Waiting for an asynchronous prefetch from a relation data file. DataFileRead Waiting for a read from a relation data file. DataFileSync Waiting for changes to a relation data file to reach stable storage. DataFileTruncate Waiting for a relation data file to be truncated. DataFileWrite Waiting for a write to a relation data file. LockFileAddToDataDirRead Waiting for a read while adding a line to the data directory lock file. LockFileAddToDataDirSync Waiting for data to reach stable storage while adding a line to the data directory lock file. LockFileAddToDataDirWrite Waiting for a write while adding a line to the data directory lock file. LockFileCreateRead Waiting to read while creating the data directory lock file. LockFileCreateSync Waiting for data to reach stable storage while creating the data directory lock file. LockFileCreateWrite Waiting for a write while creating the data directory lock file. LockFileReCheckDataDirRead Waiting for a read during recheck of the data directory lock file. LogicalRewriteCheckpointSync Waiting for logical rewrite mappings to reach stable storage during a checkpoint. LogicalRewriteMappingSync Waiting for mapping data to reach stable storage during a logical rewrite. LogicalRewriteMappingWrite Waiting for a write of mapping data during a logical rewrite. LogicalRewriteSync Waiting for logical rewrite mappings to reach stable storage. LogicalRewriteTruncate Waiting for truncate of mapping data during a logical rewrite. LogicalRewriteWrite Waiting for a write of logical rewrite mappings. ProcSignalBarrier Waiting for a barrier event to be processed by all backends. RelationMapRead Waiting for a read of the relation map file. RelationMapSync Waiting for the relation map file to reach stable storage. RelationMapWrite Waiting for a write to the relation map file. ReorderBufferRead Waiting for a read during reorder buffer management. ReorderBufferWrite Waiting for a write during reorder buffer management. ReorderLogicalMappingRead Waiting for a read of a logical mapping during reorder buffer management. ReplicationSlotRead Waiting for a read from a replication slot control file. ReplicationSlotRestoreSync Waiting for a replication slot control file to reach stable storage while restoring it to memory. ReplicationSlotSync Waiting for a replication slot control file to reach stable storage. ReplicationSlotWrite Waiting for a write to a replication slot control file. SLRUFlushSync Waiting for SLRU data to reach stable storage during a checkpoint or database shutdown. SLRURead Waiting for a read of an SLRU page. SLRUSync Waiting for SLRU data to reach stable storage following a page write. SLRUWrite Waiting for a write of an SLRU page. SnapbuildRead Waiting for a read of a serialized historical catalog snapshot. SnapbuildSync Waiting for a serialized historical catalog snapshot to reach stable storage. SnapbuildWrite Waiting for a write of a serialized historical catalog snapshot. TimelineHistoryFileSync Waiting for a timeline history file received via streaming replication to reach stable storage. TimelineHistoryFileWrite Waiting for a write of a timeline history file received via streaming replication. TimelineHistoryRead Waiting for a read of a timeline history file. TimelineHistorySync Waiting for a newly created timeline history file to reach stable storage. TimelineHistoryWrite Waiting for a write of a newly created timeline history file. TwophaseFileRead Waiting for a read of a two phase state file. TwophaseFileSync Waiting for a two phase state file to reach stable storage. TwophaseFileWrite Waiting for a write of a two phase state file. WALBootstrapSync Waiting for WAL to reach stable storage during bootstrapping. WALBootstrapWrite Waiting for a write of a WAL page during bootstrapping. WALCopyRead Waiting for a read when creating a new WAL segment by copying an existing one. WALCopySync Waiting for a new WAL segment created by copying an existing one to reach stable storage. WALCopyWrite Waiting for a write when creating a new WAL segment by copying an existing one. WALInitSync Waiting for a newly initialized WAL file to reach stable storage. WALInitWrite Waiting for a write while initializing a new WAL file. WALRead Waiting for a read from a WAL file. WALSenderTimelineHistoryRead Waiting for a read from a timeline history file during walsender timeline command. WALSync Waiting for a WAL file to reach stable storage. WALSyncMethodAssign Waiting for data to reach stable storage while assigning WAL sync method. WALWrite Waiting for a write to a WAL file. IPC Wait Event Description BackupWaitWalArchive Waiting for WAL files required for the backup to be successfully archived. BgWorkerShutdown Waiting for background worker to shut down. BgWorkerStartup Waiting for background worker to start up. BtreePage Waiting for the page number needed to continue a parallel B-tree scan to become available. CheckpointDone Waiting for a checkpoint to complete. CheckpointStart Waiting for a checkpoint to start. ClogGroupUpdate Waiting for group leader to update transaction status at transaction end. ExecuteGather Waiting for activity from child process while executing Gather node. Hash/Batch/Allocating Waiting for an elected Parallel Hash participant to allocate a hash table. Hash/Batch/Electing Electing a Parallel Hash participant to allocate a hash table. Hash/Batch/Loading Waiting for other Parallel Hash participants to finish loading a hash table. Hash/Build/Allocating Waiting for an elected Parallel Hash participant to allocate the initial hash table. Hash/Build/Electing Electing a Parallel Hash participant to allocate the initial hash table. Hash/Build/HashingInner Waiting for other Parallel Hash participants to finish hashing the inner relation. Hash/Build/HashingOuter Waiting for other Parallel Hash participants to finish partitioning the outer relation. Hash/GrowBatches/Allocating Waiting for an elected Parallel Hash participant to allocate more batches. Hash/GrowBatches/Deciding Electing a Parallel Hash participant to decide on future batch growth. Hash/GrowBatches/Electing Electing a Parallel Hash participant to allocate more batches. Hash/GrowBatches/Finishing Waiting for an elected Parallel Hash participant to decide on future batch growth. Hash/GrowBatches/Repartitioning Waiting for other Parallel Hash participants to finishing repartitioning. Hash/GrowBuckets/Allocating Waiting for an elected Parallel Hash participant to finish allocating more buckets. Hash/GrowBuckets/Electing Electing a Parallel Hash participant to allocate more buckets. Hash/GrowBuckets/Reinserting Waiting for other Parallel Hash participants to finish inserting tuples into new buckets. LogicalSyncData Waiting for logical replication remote server to send data for initial table synchronization. LogicalSyncStateChange Waiting for logical replication remote server to change state. MessageQueueInternal Waiting for other process to be attached in shared message queue. MessageQueuePutMessage Waiting to write a protocol message to a shared message queue. MessageQueueReceive Waiting to receive bytes from a shared message queue. MessageQueueSend Waiting to send bytes to a shared message queue. ParallelBitmapScan Waiting for parallel bitmap scan to become initialized. ParallelCreateIndexScan Waiting for parallel CREATE INDEX workers to finish heap scan. ParallelFinish Waiting for parallel workers to finish computing. ProcArrayGroupUpdate Waiting for group leader to clear transaction id at transaction end. Promote Waiting for standby promotion. RecoveryConflictSnapshot Waiting for recovery conflict resolution on a vacuum cleanup. RecoveryConflictTablespace Waiting for recovery conflict resolution on dropping tablespace. RecoveryPause Waiting for recovery to be resumed. ReplicationOriginDrop Waiting for a replication origin to become inactive to be dropped. ReplicationSlotDrop Waiting for a replication slot to become inactive to be dropped. SafeSnapshot Waiting for a snapshot for a READ ONLY DEFERRABLE transaction. SyncRep Waiting for confirmation from remote server during synchronous replication. Lock Wait Event Description advisory Waiting to acquire an advisory user lock. extend Waiting to extend a relation. object Waiting to acquire a lock on a non-relation database object. page Waiting to acquire a lock on a page of a relation. relation Waiting to acquire a lock on a relation. spectoken Waiting to acquire a speculative insertion lock. transactionid Waiting for a transaction to finish. tuple Waiting to acquire a lock on a tuple. userlock Waiting to acquire a user lock. virtualxid Waiting to acquire a virtual transaction ID lock. LWLock Wait Event Description AddinShmemInit Waiting to manage an extension's space allocation in shared memory. AutoFile Waiting to update the postgresql.auto.conf file. Autovacuum Waiting to read or update the current state of autovacuum workers. AutovacuumSchedule Waiting to ensure that a table selected for autovacuum still needs vacuuming. BackgroundWorker Waiting to read or update background worker state. BtreeVacuum Waiting to read or update vacuum-related information for a B-tree index. BufferContent Waiting to access a data page in memory. BufferIO Waiting for I/O on a data page. BufferMapping Waiting to associate a data block with a buffer in the buffer pool. Checkpoint Waiting to begin a checkpoint. CheckpointerComm Waiting to manage fsync requests. CommitTs Waiting to read or update the last value set for a transaction commit timestamp. CommitTsBuffer Waiting for I/O on a commit timestamp SLRU buffer. CommitTsSLRU Waiting to access the commit timestamp SLRU cache. ControlFile Waiting to read or update the pg_control file or create a new WAL file. DynamicSharedMemoryControl Waiting to read or update dynamic shared memory allocation information. LockFastPath Waiting to read or update a process' fast-path lock information. LockManager Waiting to read or update information about heavyweight locks. LogicalRepWorker Waiting to read or update the state of logical replication workers. MultiXactGen Waiting to read or update shared multixact state. MultiXactMemberBuffer Waiting for I/O on a multixact member SLRU buffer. MultiXactMemberSLRU Waiting to access the multixact member SLRU cache. MultiXactOffsetBuffer Waiting for I/O on a multixact offset SLRU buffer. MultiXactOffsetSLRU Waiting to access the multixact offset SLRU cache. MultiXactTruncation Waiting to read or truncate multixact information. NotifyBuffer Waiting for I/O on a NOTIFY message SLRU buffer. NotifyQueue Waiting to read or update NOTIFY messages. NotifySLRU Waiting to access the NOTIFY message SLRU cache. OidGen Waiting to allocate a new OID. OldSnapshotTimeMap Waiting to read or update old snapshot control information. ParallelAppend Waiting to choose the next subplan during Parallel Append plan execution. ParallelHashJoin Waiting to synchronize workers during Parallel Hash Join plan execution. ParallelQueryDSA Waiting for parallel query dynamic shared memory allocation. PerSessionDSA Waiting for parallel query dynamic shared memory allocation. PerSessionRecordType Waiting to access a parallel query's information about composite types. PerSessionRecordTypmod Waiting to access a parallel query's information about type modifiers that identify anonymous record types. PerXactPredicateList Waiting to access the list of predicate locks held by the current serializable transaction during a parallel query. PredicateLockManager Waiting to access predicate lock information used by serializable transactions. ProcArray Waiting to access the shared per-process data structures (typically, to get a snapshot or report a session's transaction ID). RelationMapping Waiting to read or update a pg_filenode.map file (used to track the filenode assignments of certain system catalogs). RelCacheInit Waiting to read or update a pg_internal.init relation cache initialization file. ReplicationOrigin Waiting to create, drop or use a replication origin. ReplicationOriginState Waiting to read or update the progress of one replication origin. ReplicationSlotAllocation Waiting to allocate or free a replication slot. ReplicationSlotControl Waiting to read or update replication slot state. ReplicationSlotIO Waiting for I/O on a replication slot. SerialBuffer Waiting for I/O on a serializable transaction conflict SLRU buffer. SerializableFinishedList Waiting to access the list of finished serializable transactions. SerializablePredicateList Waiting to access the list of predicate locks held by serializable transactions. SerializableXactHash Waiting to read or update information about serializable transactions. SerialSLRU Waiting to access the serializable transaction conflict SLRU cache. SharedTidBitmap Waiting to access a shared TID bitmap during a parallel bitmap index scan. SharedTupleStore Waiting to access a shared tuple store during parallel query. ShmemIndex Waiting to find or allocate space in shared memory. SInvalRead Waiting to retrieve messages from the shared catalog invalidation queue. SInvalWrite Waiting to add a message to the shared catalog invalidation queue. SubtransBuffer Waiting for I/O on a sub-transaction SLRU buffer. SubtransSLRU Waiting to access the sub-transaction SLRU cache. SyncRep Waiting to read or update information about the state of synchronous replication. SyncScan Waiting to select the starting location of a synchronized table scan. TablespaceCreate Waiting to create or drop a tablespace. TwoPhaseState Waiting to read or update the state of prepared transactions. WALBufMapping Waiting to replace a page in WAL buffers. WALInsert Waiting to insert WAL data into a memory buffer. WALWrite Waiting for WAL buffers to be written to disk. XactBuffer Waiting for I/O on a transaction status SLRU buffer. XactSLRU Waiting to access the transaction status SLRU cache. XactTruncation Waiting to execute pg_xact_status or update the oldest transaction ID available to it. XidGen Waiting to allocate a new transaction ID. Extensions can add LWLock types to the list shown in . In some cases, the name assigned by an extension will not be available in all server processes; so an LWLock wait event might be reported as just extension rather than the extension-assigned name. Timeout Wait Event Description BaseBackupThrottle Waiting during base backup when throttling activity. PgSleep Waiting due to a call to pg_sleep or a sibling function. RecoveryApplyDelay Waiting to apply WAL during recovery because of a delay setting. RecoveryRetrieveRetryInterval Waiting during recovery when WAL data is not available from any source (pg_wal, archive or stream). VacuumDelay Waiting in a cost-based vacuum delay point. Here is an example of how wait events can be viewed: SELECT pid, wait_event_type, wait_event FROM pg_stat_activity WHERE wait_event is NOT NULL; pid | wait_event_type | wait_event ------+-----------------+--------------- 2540 | Lock | relation 6644 | LWLock | ProcArrayLock (2 rows) Column Type Description pid integer Process ID of a WAL sender process usesysid oid OID of the user logged into this WAL sender process usename name Name of the user logged into this WAL sender process application_name text Name of the application that is connected to this WAL sender client_addr inet IP address of the client connected to this WAL sender. If this field is null, it indicates that the client is connected via a Unix socket on the server machine. client_hostname text Host name of the connected client, as reported by a reverse DNS lookup of client_addr. This field will only be non-null for IP connections, and only when is enabled. client_port integer TCP port number that the client is using for communication with this WAL sender, or -1 if a Unix socket is used backend_start timestamp with time zone Time when this process was started, i.e., when the client connected to this WAL sender backend_xmin xid This standby's xmin horizon reported by . state text Current WAL sender state. Possible values are: startup: This WAL sender is starting up. catchup: This WAL sender's connected standby is catching up with the primary. streaming: This WAL sender is streaming changes after its connected standby server has caught up with the primary. backup: This WAL sender is sending a backup. stopping: This WAL sender is stopping. sent_lsn pg_lsn Last write-ahead log location sent on this connection write_lsn pg_lsn Last write-ahead log location written to disk by this standby server flush_lsn pg_lsn Last write-ahead log location flushed to disk by this standby server replay_lsn pg_lsn Last write-ahead log location replayed into the database on this standby server write_lag interval Time elapsed between flushing recent WAL locally and receiving notification that this standby server has written it (but not yet flushed it or applied it). This can be used to gauge the delay that synchronous_commit level remote_write incurred while committing if this server was configured as a synchronous standby. flush_lag interval Time elapsed between flushing recent WAL locally and receiving notification that this standby server has written and flushed it (but not yet applied it). This can be used to gauge the delay that synchronous_commit level on incurred while committing if this server was configured as a synchronous standby. replay_lag interval Time elapsed between flushing recent WAL locally and receiving notification that this standby server has written, flushed and applied it. This can be used to gauge the delay that synchronous_commit level remote_apply incurred while committing if this server was configured as a synchronous standby. sync_priority integer Priority of this standby server for being chosen as the synchronous standby in a priority-based synchronous replication. This has no effect in a quorum-based synchronous replication. sync_state text Synchronous state of this standby server. Possible values are: async: This standby server is asynchronous. potential: This standby server is now asynchronous, but can potentially become synchronous if one of current synchronous ones fails. sync: This standby server is synchronous. quorum: This standby server is considered as a candidate for quorum standbys. reply_time timestamp with time zone Send time of last reply message received from standby server spill_txns bigint Number of transactions spilled to disk after the memory used by logical decoding exceeds logical_decoding_work_mem. The counter gets incremented both for toplevel transactions and subtransactions. spill_count bigint Number of times transactions were spilled to disk. Transactions may get spilled repeatedly, and this counter gets incremented on every such invocation. spill_bytes bigint Amount of decoded transaction data spilled to disk. The pg_stat_replication view will contain one row per WAL sender process, showing statistics about replication to that sender's connected standby server. Only directly connected standbys are listed; no information is available about downstream standby servers. The lag times reported in the pg_stat_replication view are measurements of the time taken for recent WAL to be written, flushed and replayed and for the sender to know about it. These times represent the commit delay that was (or would have been) introduced by each synchronous commit level, if the remote server was configured as a synchronous standby. For an asynchronous standby, the replay_lag column approximates the delay before recent transactions became visible to queries. If the standby server has entirely caught up with the sending server and there is no more WAL activity, the most recently measured lag times will continue to be displayed for a short time and then show NULL. Lag times work automatically for physical replication. Logical decoding plugins may optionally emit tracking messages; if they do not, the tracking mechanism will simply display NULL lag. The reported lag times are not predictions of how long it will take for the standby to catch up with the sending server assuming the current rate of replay. Such a system would show similar times while new WAL is being generated, but would differ when the sender becomes idle. In particular, when the standby has caught up completely, pg_stat_replication shows the time taken to write, flush and replay the most recent reported WAL location rather than zero as some users might expect. This is consistent with the goal of measuring synchronous commit and transaction visibility delays for recent write transactions. To reduce confusion for users expecting a different model of lag, the lag columns revert to NULL after a short time on a fully replayed idle system. Monitoring systems should choose whether to represent this as missing data, zero or continue to display the last known value. Column Type Description pid integer Process ID of the WAL receiver process status text Activity status of the WAL receiver process receive_start_lsn pg_lsn First write-ahead log location used when WAL receiver is started receive_start_tli integer First timeline number used when WAL receiver is started written_lsn pg_lsn Last write-ahead log location already received and written to disk, but not flushed. This should not be used for data integrity checks. flushed_lsn pg_lsn Last write-ahead log location already received and flushed to disk, the initial value of this field being the first log location used when WAL receiver is started received_tli integer Timeline number of last write-ahead log location received and flushed to disk, the initial value of this field being the timeline number of the first log location used when WAL receiver is started last_msg_send_time timestamp with time zone Send time of last message received from origin WAL sender last_msg_receipt_time timestamp with time zone Receipt time of last message received from origin WAL sender latest_end_lsn pg_lsn Last write-ahead log location reported to origin WAL sender latest_end_time timestamp with time zone Time of last write-ahead log location reported to origin WAL sender slot_name text Replication slot name used by this WAL receiver sender_host text Host of the PostgreSQL instance this WAL receiver is connected to. This can be a host name, an IP address, or a directory path if the connection is via Unix socket. (The path case can be distinguished because it will always be an absolute path, beginning with /.) sender_port integer Port number of the PostgreSQL instance this WAL receiver is connected to. conninfo text Connection string used by this WAL receiver, with security-sensitive fields obfuscated. The pg_stat_wal_receiver view will contain only one row, showing statistics about the WAL receiver from that receiver's connected server. Column Type Description subid oid OID of the subscription subname name Name of the subscription pid integer Process ID of the subscription worker process relid oid OID of the relation that the worker is synchronizing; null for the main apply worker received_lsn pg_lsn Last write-ahead log location received, the initial value of this field being 0 last_msg_send_time timestamp with time zone Send time of last message received from origin WAL sender last_msg_receipt_time timestamp with time zone Receipt time of last message received from origin WAL sender latest_end_lsn pg_lsn Last write-ahead log location reported to origin WAL sender latest_end_time timestamp with time zone Time of last write-ahead log location reported to origin WAL sender The pg_stat_subscription view will contain one row per subscription for main worker (with null PID if the worker is not running), and additional rows for workers handling the initial data copy of the subscribed tables. Column Type Description pid integer Process ID of a backend or WAL sender process ssl boolean True if SSL is used on this connection version text Version of SSL in use, or NULL if SSL is not in use on this connection cipher text Name of SSL cipher in use, or NULL if SSL is not in use on this connection bits integer Number of bits in the encryption algorithm used, or NULL if SSL is not used on this connection compression boolean True if SSL compression is in use, false if not, or NULL if SSL is not in use on this connection client_dn text Distinguished Name (DN) field from the client certificate used, or NULL if no client certificate was supplied or if SSL is not in use on this connection. This field is truncated if the DN field is longer than NAMEDATALEN (64 characters in a standard build). client_serial numeric Serial number of the client certificate, or NULL if no client certificate was supplied or if SSL is not in use on this connection. The combination of certificate serial number and certificate issuer uniquely identifies a certificate (unless the issuer erroneously reuses serial numbers). issuer_dn text DN of the issuer of the client certificate, or NULL if no client certificate was supplied or if SSL is not in use on this connection. This field is truncated like client_dn. The pg_stat_ssl view will contain one row per backend or WAL sender process, showing statistics about SSL usage on this connection. It can be joined to pg_stat_activity or pg_stat_replication on the pid column to get more details about the connection. Column Type Description pid integer Process ID of a backend gss_authenticated boolean True if GSSAPI authentication was used for this connection principal text Principal used to authenticate this connection, or NULL if GSSAPI was not used to authenticate this connection. This field is truncated if the principal is longer than NAMEDATALEN (64 characters in a standard build). encrypted boolean True if GSSAPI encryption is in use on this connection The pg_stat_gssapi view will contain one row per backend, showing information about GSSAPI usage on this connection. It can be joined to pg_stat_activity or pg_stat_replication on the pid column to get more details about the connection. Column Type Description archived_count bigint Number of WAL files that have been successfully archived last_archived_wal text Name of the last WAL file successfully archived last_archived_time timestamp with time zone Time of the last successful archive operation failed_count bigint Number of failed attempts for archiving WAL files last_failed_wal text Name of the WAL file of the last failed archival operation last_failed_time timestamp with time zone Time of the last failed archival operation stats_reset timestamp with time zone Time at which these statistics were last reset The pg_stat_archiver view will always have a single row, containing data about the archiver process of the cluster. Column Type Description checkpoints_timed bigint Number of scheduled checkpoints that have been performed checkpoints_req bigint Number of requested checkpoints that have been performed checkpoint_write_time double precision Total amount of time that has been spent in the portion of checkpoint processing where files are written to disk, in milliseconds checkpoint_sync_time double precision Total amount of time that has been spent in the portion of checkpoint processing where files are synchronized to disk, in milliseconds buffers_checkpoint bigint Number of buffers written during checkpoints buffers_clean bigint Number of buffers written by the background writer maxwritten_clean bigint Number of times the background writer stopped a cleaning scan because it had written too many buffers buffers_backend bigint Number of buffers written directly by a backend buffers_backend_fsync bigint Number of times a backend had to execute its own fsync call (normally the background writer handles those even when the backend does its own write) buffers_alloc bigint Number of buffers allocated stats_reset timestamp with time zone Time at which these statistics were last reset The pg_stat_bgwriter view will always have a single row, containing global data for the cluster. Column Type Description datid oid OID of this database, or 0 for objects belonging to a shared relation datname name Name of this database, or NULL for shared objects. numbackends integer Number of backends currently connected to this database, or NULL for shared objects. This is the only column in this view that returns a value reflecting current state; all other columns return the accumulated values since the last reset. xact_commit bigint Number of transactions in this database that have been committed xact_rollback bigint Number of transactions in this database that have been rolled back blks_read bigint Number of disk blocks read in this database blks_hit bigint Number of times disk blocks were found already in the buffer cache, so that a read was not necessary (this only includes hits in the PostgreSQL buffer cache, not the operating system's file system cache) tup_returned bigint Number of rows returned by queries in this database tup_fetched bigint Number of rows fetched by queries in this database tup_inserted bigint Number of rows inserted by queries in this database tup_updated bigint Number of rows updated by queries in this database tup_deleted bigint Number of rows deleted by queries in this database conflicts bigint Number of queries canceled due to conflicts with recovery in this database. (Conflicts occur only on standby servers; see for details.) temp_files bigint Number of temporary files created by queries in this database. All temporary files are counted, regardless of why the temporary file was created (e.g., sorting or hashing), and regardless of the setting. temp_bytes bigint Total amount of data written to temporary files by queries in this database. All temporary files are counted, regardless of why the temporary file was created, and regardless of the setting. deadlocks bigint Number of deadlocks detected in this database checksum_failures bigint Number of data page checksum failures detected in this database (or on a shared object), or NULL if data checksums are not enabled. checksum_last_failure timestamp with time zone Time at which the last data page checksum failure was detected in this database (or on a shared object), or NULL if data checksums are not enabled. blk_read_time double precision Time spent reading data file blocks by backends in this database, in milliseconds blk_write_time double precision Time spent writing data file blocks by backends in this database, in milliseconds stats_reset timestamp with time zone Time at which these statistics were last reset The pg_stat_database view will contain one row for each database in the cluster, plus one for shared objects, showing database-wide statistics. Column Type Description datid oid OID of a database datname name Name of this database confl_tablespace bigint Number of queries in this database that have been canceled due to dropped tablespaces confl_lock bigint Number of queries in this database that have been canceled due to lock timeouts confl_snapshot bigint Number of queries in this database that have been canceled due to old snapshots confl_bufferpin bigint Number of queries in this database that have been canceled due to pinned buffers confl_deadlock bigint Number of queries in this database that have been canceled due to deadlocks The pg_stat_database_conflicts view will contain one row per database, showing database-wide statistics about query cancels occurring due to conflicts with recovery on standby servers. This view will only contain information on standby servers, since conflicts do not occur on master servers. Column Type Description relid oid OID of a table schemaname name Name of the schema that this table is in relname name Name of this table seq_scan bigint Number of sequential scans initiated on this table seq_tup_read bigint Number of live rows fetched by sequential scans idx_scan bigint Number of index scans initiated on this table idx_tup_fetch bigint Number of live rows fetched by index scans n_tup_ins bigint Number of rows inserted n_tup_upd bigint Number of rows updated (includes HOT updated rows) n_tup_del bigint Number of rows deleted n_tup_hot_upd bigint Number of rows HOT updated (i.e., with no separate index update required) n_live_tup bigint Estimated number of live rows n_dead_tup bigint Estimated number of dead rows n_mod_since_analyze bigint Estimated number of rows modified since this table was last analyzed n_ins_since_vacuum bigint Estimated number of rows inserted since this table was last vacuumed last_vacuum timestamp with time zone Last time at which this table was manually vacuumed (not counting VACUUM FULL) last_autovacuum timestamp with time zone Last time at which this table was vacuumed by the autovacuum daemon last_analyze timestamp with time zone Last time at which this table was manually analyzed last_autoanalyze timestamp with time zone Last time at which this table was analyzed by the autovacuum daemon vacuum_count bigint Number of times this table has been manually vacuumed (not counting VACUUM FULL) autovacuum_count bigint Number of times this table has been vacuumed by the autovacuum daemon analyze_count bigint Number of times this table has been manually analyzed autoanalyze_count bigint Number of times this table has been analyzed by the autovacuum daemon The pg_stat_all_tables view will contain one row for each table in the current database (including TOAST tables), showing statistics about accesses to that specific table. The pg_stat_user_tables and pg_stat_sys_tables views contain the same information, but filtered to only show user and system tables respectively. Column Type Description relid oid OID of the table for this index indexrelid oid OID of this index schemaname name Name of the schema this index is in relname name Name of the table for this index indexrelname name Name of this index idx_scan bigint Number of index scans initiated on this index idx_tup_read bigint Number of index entries returned by scans on this index idx_tup_fetch bigint Number of live table rows fetched by simple index scans using this index The pg_stat_all_indexes view will contain one row for each index in the current database, showing statistics about accesses to that specific index. The pg_stat_user_indexes and pg_stat_sys_indexes views contain the same information, but filtered to only show user and system indexes respectively. Indexes can be used by simple index scans, bitmap index scans, and the optimizer. In a bitmap scan the output of several indexes can be combined via AND or OR rules, so it is difficult to associate individual heap row fetches with specific indexes when a bitmap scan is used. Therefore, a bitmap scan increments the pg_stat_all_indexes.idx_tup_read count(s) for the index(es) it uses, and it increments the pg_stat_all_tables.idx_tup_fetch count for the table, but it does not affect pg_stat_all_indexes.idx_tup_fetch. The optimizer also accesses indexes to check for supplied constants whose values are outside the recorded range of the optimizer statistics because the optimizer statistics might be stale. The idx_tup_read and idx_tup_fetch counts can be different even without any use of bitmap scans, because idx_tup_read counts index entries retrieved from the index while idx_tup_fetch counts live rows fetched from the table. The latter will be less if any dead or not-yet-committed rows are fetched using the index, or if any heap fetches are avoided by means of an index-only scan. Column Type Description relid oid OID of a table schemaname name Name of the schema that this table is in relname name Name of this table heap_blks_read bigint Number of disk blocks read from this table heap_blks_hit bigint Number of buffer hits in this table idx_blks_read bigint Number of disk blocks read from all indexes on this table idx_blks_hit bigint Number of buffer hits in all indexes on this table toast_blks_read bigint Number of disk blocks read from this table's TOAST table (if any) toast_blks_hit bigint Number of buffer hits in this table's TOAST table (if any) tidx_blks_read bigint Number of disk blocks read from this table's TOAST table indexes (if any) tidx_blks_hit bigint Number of buffer hits in this table's TOAST table indexes (if any) The pg_statio_all_tables view will contain one row for each table in the current database (including TOAST tables), showing statistics about I/O on that specific table. The pg_statio_user_tables and pg_statio_sys_tables views contain the same information, but filtered to only show user and system tables respectively. Column Type Description relid oid OID of the table for this index indexrelid oid OID of this index schemaname name Name of the schema this index is in relname name Name of the table for this index indexrelname name Name of this index idx_blks_read bigint Number of disk blocks read from this index idx_blks_hit bigint Number of buffer hits in this index The pg_statio_all_indexes view will contain one row for each index in the current database, showing statistics about I/O on that specific index. The pg_statio_user_indexes and pg_statio_sys_indexes views contain the same information, but filtered to only show user and system indexes respectively. Column Type Description relid oid OID of a sequence schemaname name Name of the schema this sequence is in relname name Name of this sequence blks_read bigint Number of disk blocks read from this sequence blks_hit bigint Number of buffer hits in this sequence The pg_statio_all_sequences view will contain one row for each sequence in the current database, showing statistics about I/O on that specific sequence. Column Type Description funcid oid OID of a function schemaname name Name of the schema this function is in funcname name Name of this function calls bigint Number of times this function has been called total_time double precision Total time spent in this function and all other functions called by it, in milliseconds self_time double precision Total time spent in this function itself, not including other functions called by it, in milliseconds SLRU PostgreSQL accesses certain on-disk information via SLRU (simple least-recently-used) caches. The pg_stat_slru view will contain one row for each tracked SLRU cache, showing statistics about access to cached pages. Column Type Description name text name of the SLRU blks_zeroed bigint Number of blocks zeroed during initializations blks_hit bigint Number of times disk blocks were found already in the SLRU, so that a read was not necessary (this only includes hits in the SLRU, not the operating system's file system cache) blks_read bigint Number of disk blocks read for this SLRU blks_written bigint Number of disk blocks written for this SLRU blks_exists bigint Number of blocks checked for existence for this SLRU flushes bigint Number of flushes of dirty data for this SLRU truncates bigint Number of truncates for this SLRU stats_reset timestamp with time zone Time at which these statistics were last reset The pg_stat_user_functions view will contain one row for each tracked function, showing statistics about executions of that function. The parameter controls exactly which functions are tracked. Other ways of looking at the statistics can be set up by writing queries that use the same underlying statistics access functions used by the standard views shown above. For details such as the functions' names, consult the definitions of the standard views. (For example, in psql you could issue \d+ pg_stat_activity.) The access functions for per-database statistics take a database OID as an argument to identify which database to report on. The per-table and per-index functions take a table or index OID. The functions for per-function statistics take a function OID. Note that only tables, indexes, and functions in the current database can be seen with these functions. Additional functions related to statistics collection are listed in . Function Description pg_backend_pid () integer Returns the process ID of the server process attached to the current session. pg_stat_get_activity pg_stat_get_activity ( integer ) setof record Returns a record of information about the backend with the specified process ID, or one record for each active backend in the system if NULL is specified. The fields returned are a subset of those in the pg_stat_activity view. pg_stat_get_snapshot_timestamp pg_stat_get_snapshot_timestamp () timestamp with time zone Returns the timestamp of the current statistics snapshot. pg_stat_clear_snapshot pg_stat_clear_snapshot () void Discards the current statistics snapshot. pg_stat_reset pg_stat_reset () void Resets all statistics counters for the current database to zero. This function is restricted to superusers by default, but other users can be granted EXECUTE to run the function. pg_stat_reset_shared pg_stat_reset_shared ( text ) void Resets some cluster-wide statistics counters to zero, depending on the argument. The argument can be bgwriter to reset all the counters shown in the pg_stat_bgwriter view,or archiver to reset all the counters shown in the pg_stat_archiver view. This function is restricted to superusers by default, but other users can be granted EXECUTE to run the function. pg_stat_reset_single_table_counters pg_stat_reset_single_table_counters ( oid ) void Resets statistics for a single table or index in the current database to zero. This function is restricted to superusers by default, but other users can be granted EXECUTE to run the function. pg_stat_reset_single_function_counters pg_stat_reset_single_function_counters ( oid ) void Resets statistics for a single function in the current database to zero. This function is restricted to superusers by default, but other users can be granted EXECUTE to run the function. pg_stat_reset_slru pg_stat_reset_slru ( text ) void Resets statistics to zero for a single SLRU cache, or for all SLRUs in the cluster. If the argument is NULL, all counters shown in the pg_stat_slru view for all SLRU caches are reset. The argument can be one of CommitTs, MultiXactMember, MultiXactOffset, Notify, Serial, Subtrans, or Xact to reset the counters for only that entry. If the argument is other (or indeed, any unrecognized name), then the counters for all other SLRU caches, such as extension-defined caches, are reset. This function is restricted to superusers by default, but other users can be granted EXECUTE to run the function. pg_stat_get_activity, the underlying function of the pg_stat_activity view, returns a set of records containing all the available information about each backend process. Sometimes it may be more convenient to obtain just a subset of this information. In such cases, an older set of per-backend statistics access functions can be used; these are shown in . These access functions use a backend ID number, which ranges from one to the number of currently active backends. The function pg_stat_get_backend_idset provides a convenient way to generate one row for each active backend for invoking these functions. For example, to show the PIDs and current queries of all backends: SELECT pg_stat_get_backend_pid(s.backendid) AS pid, pg_stat_get_backend_activity(s.backendid) AS query FROM (SELECT pg_stat_get_backend_idset() AS backendid) AS s; Function Description pg_stat_get_backend_idset pg_stat_get_backend_idset () setof integer Returns the set of currently active backend ID numbers (from 1 to the number of active backends). pg_stat_get_backend_activity pg_stat_get_backend_activity ( integer ) text Returns the text of this backend's most recent query. pg_stat_get_backend_activity_start pg_stat_get_backend_activity_start ( integer ) timestamp with time zone Returns the time when the backend's most recent query was started. pg_stat_get_backend_client_addr pg_stat_get_backend_client_addr ( integer ) inet Returns the IP address of the client connected to this backend. pg_stat_get_backend_client_port pg_stat_get_backend_client_port ( integer ) integer Returns the TCP port number that the client is using for communication. pg_stat_get_backend_dbid pg_stat_get_backend_dbid ( integer ) oid Returns the OID of the database this backend is connected to. pg_stat_get_backend_pid pg_stat_get_backend_pid ( integer ) integer Returns the process ID of this backend. pg_stat_get_backend_start pg_stat_get_backend_start ( integer ) timestamp with time zone Returns the time when this process was started. pg_stat_get_backend_userid pg_stat_get_backend_userid ( integer ) oid Returns the OID of the user logged into this backend. pg_stat_get_backend_wait_event_type pg_stat_get_backend_wait_event_type ( integer ) text Returns the wait event type name if this backend is currently waiting, otherwise NULL. See for details. pg_stat_get_backend_wait_event pg_stat_get_backend_wait_event ( integer ) text Returns the wait event name if this backend is currently waiting, otherwise NULL. See through . pg_stat_get_backend_xact_start pg_stat_get_backend_xact_start ( integer ) timestamp with time zone Returns the time when the backend's current transaction was started. lock monitoring Another useful tool for monitoring database activity is the pg_locks system table. It allows the database administrator to view information about the outstanding locks in the lock manager. For example, this capability can be used to: View all the locks currently outstanding, all the locks on relations in a particular database, all the locks on a particular relation, or all the locks held by a particular PostgreSQL session. Determine the relation in the current database with the most ungranted locks (which might be a source of contention among database clients). Determine the effect of lock contention on overall database performance, as well as the extent to which contention varies with overall database traffic. Details of the pg_locks view appear in . For more information on locking and managing concurrency with PostgreSQL, refer to . PostgreSQL has the ability to report the progress of certain commands during command execution. Currently, the only commands which support progress reporting are ANALYZE, CLUSTER, CREATE INDEX, VACUUM, and (i.e., replication command that issues to take a base backup). This may be expanded in the future. Whenever ANALYZE is running, the pg_stat_progress_analyze view will contain a row for each backend that is currently running that command. The tables below describe the information that will be reported and provide information about how to interpret it. Column Type Description pid integer Process ID of backend. datid oid OID of the database to which this backend is connected. datname name Name of the database to which this backend is connected. relid oid OID of the table being analyzed. phase text Current processing phase. See . sample_blks_total bigint Total number of heap blocks that will be sampled. sample_blks_scanned bigint Number of heap blocks scanned. ext_stats_total bigint Number of extended statistics. ext_stats_computed bigint Number of extended statistics computed. This counter only advances when the phase is computing extended statistics. child_tables_total bigint Number of child tables. child_tables_done bigint Number of child tables scanned. This counter only advances when the phase is acquiring inherited sample rows. current_child_table_relid oid OID of the child table currently being scanned. This field is only valid when the phase is acquiring inherited sample rows. Phase Description initializing The command is preparing to begin scanning the heap. This phase is expected to be very brief. acquiring sample rows The command is currently scanning the table given by relid to obtain sample rows. acquiring inherited sample rows The command is currently scanning child tables to obtain sample rows. Columns child_tables_total, child_tables_done, and current_child_table_relid contain the progress information for this phase. computing statistics The command is computing statistics from the sample rows obtained during the table scan. computing extended statistics The command is computing extended statistics from the sample rows obtained during the table scan. finalizing analyze The command is updating pg_class. When this phase is completed, ANALYZE will end. Note that when ANALYZE is run on a partitioned table, all of its partitions are also recursively analyzed as also mentioned in . In that case, ANALYZE progress is reported first for the parent table, whereby its inheritance statistics are collected, followed by that for each partition. Whenever CREATE INDEX or REINDEX is running, the pg_stat_progress_create_index view will contain one row for each backend that is currently creating indexes. The tables below describe the information that will be reported and provide information about how to interpret it. Column Type Description pid integer Process ID of backend. datid oid OID of the database to which this backend is connected. datname name Name of the database to which this backend is connected. relid oid OID of the table on which the index is being created. index_relid oid OID of the index being created or reindexed. During a non-concurrent CREATE INDEX, this is 0. command text The command that is running: CREATE INDEX, CREATE INDEX CONCURRENTLY, REINDEX, or REINDEX CONCURRENTLY. phase text Current processing phase of index creation. See . lockers_total bigint Total number of lockers to wait for, when applicable. lockers_done bigint Number of lockers already waited for. current_locker_pid bigint Process ID of the locker currently being waited for. blocks_total bigint Total number of blocks to be processed in the current phase. blocks_done bigint Number of blocks already processed in the current phase. tuples_total bigint Total number of tuples to be processed in the current phase. tuples_done bigint Number of tuples already processed in the current phase. partitions_total bigint When creating an index on a partitioned table, this column is set to the total number of partitions on which the index is to be created. partitions_done bigint When creating an index on a partitioned table, this column is set to the number of partitions on which the index has been completed. Phase Description initializing CREATE INDEX or REINDEX is preparing to create the index. This phase is expected to be very brief. waiting for writers before build CREATE INDEX CONCURRENTLY or REINDEX CONCURRENTLY is waiting for transactions with write locks that can potentially see the table to finish. This phase is skipped when not in concurrent mode. Columns lockers_total, lockers_done and current_locker_pid contain the progress information for this phase. building index The index is being built by the access method-specific code. In this phase, access methods that support progress reporting fill in their own progress data, and the subphase is indicated in this column. Typically, blocks_total and blocks_done will contain progress data, as well as potentially tuples_total and tuples_done. waiting for writers before validation CREATE INDEX CONCURRENTLY or REINDEX CONCURRENTLY is waiting for transactions with write locks that can potentially write into the table to finish. This phase is skipped when not in concurrent mode. Columns lockers_total, lockers_done and current_locker_pid contain the progress information for this phase. index validation: scanning index CREATE INDEX CONCURRENTLY is scanning the index searching for tuples that need to be validated. This phase is skipped when not in concurrent mode. Columns blocks_total (set to the total size of the index) and blocks_done contain the progress information for this phase. index validation: sorting tuples CREATE INDEX CONCURRENTLY is sorting the output of the index scanning phase. index validation: scanning table CREATE INDEX CONCURRENTLY is scanning the table to validate the index tuples collected in the previous two phases. This phase is skipped when not in concurrent mode. Columns blocks_total (set to the total size of the table) and blocks_done contain the progress information for this phase. waiting for old snapshots CREATE INDEX CONCURRENTLY or REINDEX CONCURRENTLY is waiting for transactions that can potentially see the table to release their snapshots. This phase is skipped when not in concurrent mode. Columns lockers_total, lockers_done and current_locker_pid contain the progress information for this phase. waiting for readers before marking dead REINDEX CONCURRENTLY is waiting for transactions with read locks on the table to finish, before marking the old index dead. This phase is skipped when not in concurrent mode. Columns lockers_total, lockers_done and current_locker_pid contain the progress information for this phase. waiting for readers before dropping REINDEX CONCURRENTLY is waiting for transactions with read locks on the table to finish, before dropping the old index. This phase is skipped when not in concurrent mode. Columns lockers_total, lockers_done and current_locker_pid contain the progress information for this phase. Whenever VACUUM is running, the pg_stat_progress_vacuum view will contain one row for each backend (including autovacuum worker processes) that is currently vacuuming. The tables below describe the information that will be reported and provide information about how to interpret it. Progress for VACUUM FULL commands is reported via pg_stat_progress_cluster because both VACUUM FULL and CLUSTER rewrite the table, while regular VACUUM only modifies it in place. See . Column Type Description pid integer Process ID of backend. datid oid OID of the database to which this backend is connected. datname name Name of the database to which this backend is connected. relid oid OID of the table being vacuumed. phase text Current processing phase of vacuum. See . heap_blks_total bigint Total number of heap blocks in the table. This number is reported as of the beginning of the scan; blocks added later will not be (and need not be) visited by this VACUUM. heap_blks_scanned bigint Number of heap blocks scanned. Because the visibility map is used to optimize scans, some blocks will be skipped without inspection; skipped blocks are included in this total, so that this number will eventually become equal to heap_blks_total when the vacuum is complete. This counter only advances when the phase is scanning heap. heap_blks_vacuumed bigint Number of heap blocks vacuumed. Unless the table has no indexes, this counter only advances when the phase is vacuuming heap. Blocks that contain no dead tuples are skipped, so the counter may sometimes skip forward in large increments. index_vacuum_count bigint Number of completed index vacuum cycles. max_dead_tuples bigint Number of dead tuples that we can store before needing to perform an index vacuum cycle, based on . num_dead_tuples bigint Number of dead tuples collected since the last index vacuum cycle. Phase Description initializing VACUUM is preparing to begin scanning the heap. This phase is expected to be very brief. scanning heap VACUUM is currently scanning the heap. It will prune and defragment each page if required, and possibly perform freezing activity. The heap_blks_scanned column can be used to monitor the progress of the scan. vacuuming indexes VACUUM is currently vacuuming the indexes. If a table has any indexes, this will happen at least once per vacuum, after the heap has been completely scanned. It may happen multiple times per vacuum if is insufficient to store the number of dead tuples found. vacuuming heap VACUUM is currently vacuuming the heap. Vacuuming the heap is distinct from scanning the heap, and occurs after each instance of vacuuming indexes. If heap_blks_scanned is less than heap_blks_total, the system will return to scanning the heap after this phase is completed; otherwise, it will begin cleaning up indexes after this phase is completed. cleaning up indexes VACUUM is currently cleaning up indexes. This occurs after the heap has been completely scanned and all vacuuming of the indexes and the heap has been completed. truncating heap VACUUM is currently truncating the heap so as to return empty pages at the end of the relation to the operating system. This occurs after cleaning up indexes. performing final cleanup VACUUM is performing final cleanup. During this phase, VACUUM will vacuum the free space map, update statistics in pg_class, and report statistics to the statistics collector. When this phase is completed, VACUUM will end. Whenever CLUSTER or VACUUM FULL is running, the pg_stat_progress_cluster view will contain a row for each backend that is currently running either command. The tables below describe the information that will be reported and provide information about how to interpret it. Column Type Description pid integer Process ID of backend. datid oid OID of the database to which this backend is connected. datname name Name of the database to which this backend is connected. relid oid OID of the table being clustered. command text The command that is running. Either CLUSTER or VACUUM FULL. phase text Current processing phase. See . cluster_index_relid oid If the table is being scanned using an index, this is the OID of the index being used; otherwise, it is zero. heap_tuples_scanned bigint Number of heap tuples scanned. This counter only advances when the phase is seq scanning heap, index scanning heap or writing new heap. heap_tuples_written bigint Number of heap tuples written. This counter only advances when the phase is seq scanning heap, index scanning heap or writing new heap. heap_blks_total bigint Total number of heap blocks in the table. This number is reported as of the beginning of seq scanning heap. heap_blks_scanned bigint Number of heap blocks scanned. This counter only advances when the phase is seq scanning heap. index_rebuild_count bigint Number of indexes rebuilt. This counter only advances when the phase is rebuilding index. Phase Description initializing The command is preparing to begin scanning the heap. This phase is expected to be very brief. seq scanning heap The command is currently scanning the table using a sequential scan. index scanning heap CLUSTER is currently scanning the table using an index scan. sorting tuples CLUSTER is currently sorting tuples. writing new heap CLUSTER is currently writing the new heap. swapping relation files The command is currently swapping newly-built files into place. rebuilding index The command is currently rebuilding an index. performing final cleanup The command is performing final cleanup. When this phase is completed, CLUSTER or VACUUM FULL will end. Whenever an application like pg_basebackup is taking a base backup, the pg_stat_progress_basebackup view will contain a row for each WAL sender process that is currently running the BASE_BACKUP replication command and streaming the backup. The tables below describe the information that will be reported and provide information about how to interpret it. Column Type Description pid integer Process ID of a WAL sender process. phase text Current processing phase. See . backup_total bigint Total amount of data that will be streamed. This is estimated and reported as of the beginning of streaming database files phase. Note that this is only an approximation since the database may change during streaming database files phase and WAL log may be included in the backup later. This is always the same value as backup_streamed once the amount of data streamed exceeds the estimated total size. If the estimation is disabled in pg_basebackup (i.e., --no-estimate-size option is specified), this is NULL. backup_streamed bigint Amount of data streamed. This counter only advances when the phase is streaming database files or transferring wal files. tablespaces_total bigint Total number of tablespaces that will be streamed. tablespaces_streamed bigint Number of tablespaces streamed. This counter only advances when the phase is streaming database files. Phase Description initializing The WAL sender process is preparing to begin the backup. This phase is expected to be very brief. waiting for checkpoint to finish The WAL sender process is currently performing pg_start_backup to set up for taking a base backup, and waiting for backup start checkpoint to finish. estimating backup size The WAL sender process is currently estimating the total amount of database files that will be streamed as a base backup. streaming database files The WAL sender process is currently streaming database files as a base backup. waiting for wal archiving to finish The WAL sender process is currently performing pg_stop_backup to finish the backup, and waiting for all the WAL files required for the base backup to be successfully archived. If either --wal-method=none or --wal-method=stream is specified in pg_basebackup, the backup will end when this phase is completed. transferring wal files The WAL sender process is currently transferring all WAL logs generated during the backup. This phase occurs after waiting for wal archiving to finish phase if --wal-method=fetch is specified in pg_basebackup. The backup will end when this phase is completed. DTrace PostgreSQL provides facilities to support dynamic tracing of the database server. This allows an external utility to be called at specific points in the code and thereby trace execution. A number of probes or trace points are already inserted into the source code. These probes are intended to be used by database developers and administrators. By default the probes are not compiled into PostgreSQL; the user needs to explicitly tell the configure script to make the probes available. Currently, the DTrace utility is supported, which, at the time of this writing, is available on Solaris, macOS, FreeBSD, NetBSD, and Oracle Linux. The SystemTap project for Linux provides a DTrace equivalent and can also be used. Supporting other dynamic tracing utilities is theoretically possible by changing the definitions for the macros in src/include/utils/probes.h. By default, probes are not available, so you will need to explicitly tell the configure script to make the probes available in PostgreSQL. To include DTrace support specify --enable-dtrace to configure. See for further information. A number of standard probes are provided in the source code, as shown in ; shows the types used in the probes. More probes can certainly be added to enhance PostgreSQL's observability. Name Parameters Description transaction-start (LocalTransactionId) Probe that fires at the start of a new transaction. arg0 is the transaction ID. transaction-commit (LocalTransactionId) Probe that fires when a transaction completes successfully. arg0 is the transaction ID. transaction-abort (LocalTransactionId) Probe that fires when a transaction completes unsuccessfully. arg0 is the transaction ID. query-start (const char *) Probe that fires when the processing of a query is started. arg0 is the query string. query-done (const char *) Probe that fires when the processing of a query is complete. arg0 is the query string. query-parse-start (const char *) Probe that fires when the parsing of a query is started. arg0 is the query string. query-parse-done (const char *) Probe that fires when the parsing of a query is complete. arg0 is the query string. query-rewrite-start (const char *) Probe that fires when the rewriting of a query is started. arg0 is the query string. query-rewrite-done (const char *) Probe that fires when the rewriting of a query is complete. arg0 is the query string. query-plan-start () Probe that fires when the planning of a query is started. query-plan-done () Probe that fires when the planning of a query is complete. query-execute-start () Probe that fires when the execution of a query is started. query-execute-done () Probe that fires when the execution of a query is complete. statement-status (const char *) Probe that fires anytime the server process updates its pg_stat_activity.status. arg0 is the new status string. checkpoint-start (int) Probe that fires when a checkpoint is started. arg0 holds the bitwise flags used to distinguish different checkpoint types, such as shutdown, immediate or force. checkpoint-done (int, int, int, int, int) Probe that fires when a checkpoint is complete. (The probes listed next fire in sequence during checkpoint processing.) arg0 is the number of buffers written. arg1 is the total number of buffers. arg2, arg3 and arg4 contain the number of WAL files added, removed and recycled respectively. clog-checkpoint-start (bool) Probe that fires when the CLOG portion of a checkpoint is started. arg0 is true for normal checkpoint, false for shutdown checkpoint. clog-checkpoint-done (bool) Probe that fires when the CLOG portion of a checkpoint is complete. arg0 has the same meaning as for clog-checkpoint-start. subtrans-checkpoint-start (bool) Probe that fires when the SUBTRANS portion of a checkpoint is started. arg0 is true for normal checkpoint, false for shutdown checkpoint. subtrans-checkpoint-done (bool) Probe that fires when the SUBTRANS portion of a checkpoint is complete. arg0 has the same meaning as for subtrans-checkpoint-start. multixact-checkpoint-start (bool) Probe that fires when the MultiXact portion of a checkpoint is started. arg0 is true for normal checkpoint, false for shutdown checkpoint. multixact-checkpoint-done (bool) Probe that fires when the MultiXact portion of a checkpoint is complete. arg0 has the same meaning as for multixact-checkpoint-start. buffer-checkpoint-start (int) Probe that fires when the buffer-writing portion of a checkpoint is started. arg0 holds the bitwise flags used to distinguish different checkpoint types, such as shutdown, immediate or force. buffer-sync-start (int, int) Probe that fires when we begin to write dirty buffers during checkpoint (after identifying which buffers must be written). arg0 is the total number of buffers. arg1 is the number that are currently dirty and need to be written. buffer-sync-written (int) Probe that fires after each buffer is written during checkpoint. arg0 is the ID number of the buffer. buffer-sync-done (int, int, int) Probe that fires when all dirty buffers have been written. arg0 is the total number of buffers. arg1 is the number of buffers actually written by the checkpoint process. arg2 is the number that were expected to be written (arg1 of buffer-sync-start); any difference reflects other processes flushing buffers during the checkpoint. buffer-checkpoint-sync-start () Probe that fires after dirty buffers have been written to the kernel, and before starting to issue fsync requests. buffer-checkpoint-done () Probe that fires when syncing of buffers to disk is complete. twophase-checkpoint-start () Probe that fires when the two-phase portion of a checkpoint is started. twophase-checkpoint-done () Probe that fires when the two-phase portion of a checkpoint is complete. buffer-read-start (ForkNumber, BlockNumber, Oid, Oid, Oid, int, bool) Probe that fires when a buffer read is started. arg0 and arg1 contain the fork and block numbers of the page (but arg1 will be -1 if this is a relation extension request). arg2, arg3, and arg4 contain the tablespace, database, and relation OIDs identifying the relation. arg5 is the ID of the backend which created the temporary relation for a local buffer, or InvalidBackendId (-1) for a shared buffer. arg6 is true for a relation extension request, false for normal read. buffer-read-done (ForkNumber, BlockNumber, Oid, Oid, Oid, int, bool, bool) Probe that fires when a buffer read is complete. arg0 and arg1 contain the fork and block numbers of the page (if this is a relation extension request, arg1 now contains the block number of the newly added block). arg2, arg3, and arg4 contain the tablespace, database, and relation OIDs identifying the relation. arg5 is the ID of the backend which created the temporary relation for a local buffer, or InvalidBackendId (-1) for a shared buffer. arg6 is true for a relation extension request, false for normal read. arg7 is true if the buffer was found in the pool, false if not. buffer-flush-start (ForkNumber, BlockNumber, Oid, Oid, Oid) Probe that fires before issuing any write request for a shared buffer. arg0 and arg1 contain the fork and block numbers of the page. arg2, arg3, and arg4 contain the tablespace, database, and relation OIDs identifying the relation. buffer-flush-done (ForkNumber, BlockNumber, Oid, Oid, Oid) Probe that fires when a write request is complete. (Note that this just reflects the time to pass the data to the kernel; it's typically not actually been written to disk yet.) The arguments are the same as for buffer-flush-start. buffer-write-dirty-start (ForkNumber, BlockNumber, Oid, Oid, Oid) Probe that fires when a server process begins to write a dirty buffer. (If this happens often, it implies that is too small or the background writer control parameters need adjustment.) arg0 and arg1 contain the fork and block numbers of the page. arg2, arg3, and arg4 contain the tablespace, database, and relation OIDs identifying the relation. buffer-write-dirty-done (ForkNumber, BlockNumber, Oid, Oid, Oid) Probe that fires when a dirty-buffer write is complete. The arguments are the same as for buffer-write-dirty-start. wal-buffer-write-dirty-start () Probe that fires when a server process begins to write a dirty WAL buffer because no more WAL buffer space is available. (If this happens often, it implies that is too small.) wal-buffer-write-dirty-done () Probe that fires when a dirty WAL buffer write is complete. wal-insert (unsigned char, unsigned char) Probe that fires when a WAL record is inserted. arg0 is the resource manager (rmid) for the record. arg1 contains the info flags. wal-switch () Probe that fires when a WAL segment switch is requested. smgr-md-read-start (ForkNumber, BlockNumber, Oid, Oid, Oid, int) Probe that fires when beginning to read a block from a relation. arg0 and arg1 contain the fork and block numbers of the page. arg2, arg3, and arg4 contain the tablespace, database, and relation OIDs identifying the relation. arg5 is the ID of the backend which created the temporary relation for a local buffer, or InvalidBackendId (-1) for a shared buffer. smgr-md-read-done (ForkNumber, BlockNumber, Oid, Oid, Oid, int, int, int) Probe that fires when a block read is complete. arg0 and arg1 contain the fork and block numbers of the page. arg2, arg3, and arg4 contain the tablespace, database, and relation OIDs identifying the relation. arg5 is the ID of the backend which created the temporary relation for a local buffer, or InvalidBackendId (-1) for a shared buffer. arg6 is the number of bytes actually read, while arg7 is the number requested (if these are different it indicates trouble). smgr-md-write-start (ForkNumber, BlockNumber, Oid, Oid, Oid, int) Probe that fires when beginning to write a block to a relation. arg0 and arg1 contain the fork and block numbers of the page. arg2, arg3, and arg4 contain the tablespace, database, and relation OIDs identifying the relation. arg5 is the ID of the backend which created the temporary relation for a local buffer, or InvalidBackendId (-1) for a shared buffer. smgr-md-write-done (ForkNumber, BlockNumber, Oid, Oid, Oid, int, int, int) Probe that fires when a block write is complete. arg0 and arg1 contain the fork and block numbers of the page. arg2, arg3, and arg4 contain the tablespace, database, and relation OIDs identifying the relation. arg5 is the ID of the backend which created the temporary relation for a local buffer, or InvalidBackendId (-1) for a shared buffer. arg6 is the number of bytes actually written, while arg7 is the number requested (if these are different it indicates trouble). sort-start (int, bool, int, int, bool, int) Probe that fires when a sort operation is started. arg0 indicates heap, index or datum sort. arg1 is true for unique-value enforcement. arg2 is the number of key columns. arg3 is the number of kilobytes of work memory allowed. arg4 is true if random access to the sort result is required. arg5 indicates serial when 0, parallel worker when 1, or parallel leader when 2. sort-done (bool, long) Probe that fires when a sort is complete. arg0 is true for external sort, false for internal sort. arg1 is the number of disk blocks used for an external sort, or kilobytes of memory used for an internal sort. lwlock-acquire (char *, LWLockMode) Probe that fires when an LWLock has been acquired. arg0 is the LWLock's tranche. arg1 is the requested lock mode, either exclusive or shared. lwlock-release (char *) Probe that fires when an LWLock has been released (but note that any released waiters have not yet been awakened). arg0 is the LWLock's tranche. lwlock-wait-start (char *, LWLockMode) Probe that fires when an LWLock was not immediately available and a server process has begun to wait for the lock to become available. arg0 is the LWLock's tranche. arg1 is the requested lock mode, either exclusive or shared. lwlock-wait-done (char *, LWLockMode) Probe that fires when a server process has been released from its wait for an LWLock (it does not actually have the lock yet). arg0 is the LWLock's tranche. arg1 is the requested lock mode, either exclusive or shared. lwlock-condacquire (char *, LWLockMode) Probe that fires when an LWLock was successfully acquired when the caller specified no waiting. arg0 is the LWLock's tranche. arg1 is the requested lock mode, either exclusive or shared. lwlock-condacquire-fail (char *, LWLockMode) Probe that fires when an LWLock was not successfully acquired when the caller specified no waiting. arg0 is the LWLock's tranche. arg1 is the requested lock mode, either exclusive or shared. lock-wait-start (unsigned int, unsigned int, unsigned int, unsigned int, unsigned int, LOCKMODE) Probe that fires when a request for a heavyweight lock (lmgr lock) has begun to wait because the lock is not available. arg0 through arg3 are the tag fields identifying the object being locked. arg4 indicates the type of object being locked. arg5 indicates the lock type being requested. lock-wait-done (unsigned int, unsigned int, unsigned int, unsigned int, unsigned int, LOCKMODE) Probe that fires when a request for a heavyweight lock (lmgr lock) has finished waiting (i.e., has acquired the lock). The arguments are the same as for lock-wait-start. deadlock-found () Probe that fires when a deadlock is found by the deadlock detector. Type Definition LocalTransactionId unsigned int LWLockMode int LOCKMODE int BlockNumber unsigned int Oid unsigned int ForkNumber int bool unsigned char The example below shows a DTrace script for analyzing transaction counts in the system, as an alternative to snapshotting pg_stat_database before and after a performance test: #!/usr/sbin/dtrace -qs postgresql$1:::transaction-start { @start["Start"] = count(); self->ts = timestamp; } postgresql$1:::transaction-abort { @abort["Abort"] = count(); } postgresql$1:::transaction-commit /self->ts/ { @commit["Commit"] = count(); @time["Total time (ns)"] = sum(timestamp - self->ts); self->ts=0; } When executed, the example D script gives output such as: # ./txn_count.d `pgrep -n postgres` or ./txn_count.d <PID> ^C Start 71 Commit 70 Total time (ns) 2312105013 SystemTap uses a different notation for trace scripts than DTrace does, even though the underlying trace points are compatible. One point worth noting is that at this writing, SystemTap scripts must reference probe names using double underscores in place of hyphens. This is expected to be fixed in future SystemTap releases. You should remember that DTrace scripts need to be carefully written and debugged, otherwise the trace information collected might be meaningless. In most cases where problems are found it is the instrumentation that is at fault, not the underlying system. When discussing information found using dynamic tracing, be sure to enclose the script used to allow that too to be checked and discussed. New probes can be defined within the code wherever the developer desires, though this will require a recompilation. Below are the steps for inserting new probes: Decide on probe names and data to be made available through the probes Add the probe definitions to src/backend/utils/probes.d Include pg_trace.h if it is not already present in the module(s) containing the probe points, and insert TRACE_POSTGRESQL probe macros at the desired locations in the source code Recompile and verify that the new probes are available Here is an example of how you would add a probe to trace all new transactions by transaction ID. Decide that the probe will be named transaction-start and requires a parameter of type LocalTransactionId Add the probe definition to src/backend/utils/probes.d: probe transaction__start(LocalTransactionId); Note the use of the double underline in the probe name. In a DTrace script using the probe, the double underline needs to be replaced with a hyphen, so transaction-start is the name to document for users. At compile time, transaction__start is converted to a macro called TRACE_POSTGRESQL_TRANSACTION_START (notice the underscores are single here), which is available by including pg_trace.h. Add the macro call to the appropriate location in the source code. In this case, it looks like the following: TRACE_POSTGRESQL_TRANSACTION_START(vxid.localTransactionId); After recompiling and running the new binary, check that your newly added probe is available by executing the following DTrace command. You should see similar output: # dtrace -ln transaction-start ID PROVIDER MODULE FUNCTION NAME 18705 postgresql49878 postgres StartTransactionCommand transaction-start 18755 postgresql49877 postgres StartTransactionCommand transaction-start 18805 postgresql49876 postgres StartTransactionCommand transaction-start 18855 postgresql49875 postgres StartTransactionCommand transaction-start 18986 postgresql49873 postgres StartTransactionCommand transaction-start There are a few things to be careful about when adding trace macros to the C code: You should take care that the data types specified for a probe's parameters match the data types of the variables used in the macro. Otherwise, you will get compilation errors. On most platforms, if PostgreSQL is built with --enable-dtrace, the arguments to a trace macro will be evaluated whenever control passes through the macro, even if no tracing is being done. This is usually not worth worrying about if you are just reporting the values of a few local variables. But beware of putting expensive function calls into the arguments. If you need to do that, consider protecting the macro with a check to see if the trace is actually enabled: if (TRACE_POSTGRESQL_TRANSACTION_START_ENABLED()) TRACE_POSTGRESQL_TRANSACTION_START(some_function(...)); Each trace macro has a corresponding ENABLED macro.