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 and top. Also, once one has identified a poorly-performing query, further investigation may be needed using PostgreSQL's EXPLAIN command. The User's Guide discusses EXPLAIN and other methods for understanding the behavior of an individual query. ps to monitor activity On most 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 960 0.0 1.1 6104 1480 pts/1 SN 13:17 0:00 postmaster -i postgres 963 0.0 1.1 7084 1472 pts/1 SN 13:17 0:00 postgres: stats buffer process postgres 965 0.0 1.1 6152 1512 pts/1 SN 13:17 0:00 postgres: stats collector process postgres 998 0.0 2.3 6532 2992 pts/1 SN 13:18 0:00 postgres: tgl runbug 127.0.0.1 idle postgres 1003 0.0 2.4 6532 3128 pts/1 SN 13:19 0:00 postgres: tgl regression [local] SELECT waiting postgres 1016 0.1 2.4 6532 3080 pts/1 SN 13:19 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 postmaster, the master server process. The command arguments shown for it are the same ones given when it was launched. The next two processes implement the statistics collector, which will be described in detail in the next section. (These will not be present if you have set the system not to start the statistics collector.) 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 connection source host items remain the same for the life of the client connection, but the activity indicator changes. The activity may be idle (ie, 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 attached if the server is presently waiting on a lock held by another server process. In the above example we can infer that process 1003 is waiting for process 1016 to complete its transaction and thereby release some lock or other. 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 postmaster must have a shorter ps status display than that provided by each backend. If you fail to do all three things, the ps output for each backend will be the original postmaster 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 supports determining the exact query currently being executed by other server processes. 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 variables that are normally set in postgresql.conf (see for details about setting configuration variables). The variable STATS_START_COLLECTOR must be set to true for the statistics collector to be launched at all. This is the default and recommended setting, but it may be turned off if you have no interest in statistics and want to squeeze out every last drop of overhead. (The savings is likely to be small, however.) Note that this option cannot be changed while the server is running. The variables STATS_COMMAND_STRING, STATS_BLOCK_LEVEL, and STATS_ROW_LEVEL control how much information is actually sent to the collector, and thus determine how much run-time overhead occurs. These respectively determine whether a server process sends its current command string, disk-block-level access statistics, and row-level access statistics to the collector. Normally these variables 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 server processes using the SET command. (To prevent ordinary users from hiding their activity from the administrator, only superusers are allowed to change these variables with SET.) Since the variables STATS_COMMAND_STRING, STATS_BLOCK_LEVEL, and STATS_ROW_LEVEL default to false, no statistics are actually collected in the default configuration! You must turn one or more of them on before you will get useful results from the statistical display functions. Several predefined views are available to show the results of statistics collection. Alternatively, one can build custom views using the underlying statistics functions. When using the statistics to monitor current activity, it is important to realize that the information does not update instantaneously. Each individual server process transmits new access counts to the collector just before waiting for another client command; so a query still in progress does not affect the displayed totals. Also, the collector itself emits new totals at most once per pgstat_stat_interval (500 milliseconds by default). So the displayed totals lag behind actual activity. Another important point is that when a server process is asked to display any of these statistics, it first fetches the most recent totals emitted by the collector process. It then continues to use this snapshot for all statistical views and functions until the end of its current transaction. So the statistics will appear not to change as long as you continue the current 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. View Name Description pg_stat_activity One row per server process, showing process PID, database, user, and current query. The current query column is only available to superusers; for others it reads as NULL. (Note that because of the collector's reporting delay, current query will only be up-to-date for long-running queries.) pg_stat_database One row per database, showing number of active backends, total transactions committed and total rolled back in that database, total disk blocks read, and total number of buffer hits (ie, block read requests avoided by finding the block already in buffer cache). pg_stat_all_tables For each table in the current database, total numbers of sequential and index scans, total numbers of tuples returned by each type of scan, and totals of tuple insertions, updates, and deletes. pg_stat_sys_tables Same as pg_stat_all_tables, except that only system tables are shown. pg_stat_user_tables Same as pg_stat_all_tables, except that only user tables are shown. pg_stat_all_indexes For each index in the current database, the total number of index scans that have used that index, the number of index tuples read, and the number of successfully fetched heap tuples (this may be less when there are index entries pointing to expired heap tuples). pg_stat_sys_indexes Same as pg_stat_all_indexes, except that only indexes on system tables are shown. pg_stat_user_indexes Same as pg_stat_all_indexes, except that only indexes on user tables are shown. pg_statio_all_tables For each table in the current database, the total number of disk blocks read from that table, the number of buffer hits, the numbers of disk blocks read and buffer hits in all the indexes of that table, the numbers of disk blocks read and buffer hits from the table's auxiliary TOAST table (if any), and the numbers of disk blocks read and buffer hits for the TOAST table's index. pg_statio_sys_tables Same as pg_statio_all_tables, except that only system tables are shown. pg_statio_user_tables Same as pg_statio_all_tables, except that only user tables are shown. pg_statio_all_indexes For each index in the current database, the numbers of disk blocks read and buffer hits in that index. pg_statio_sys_indexes Same as pg_statio_all_indexes, except that only indexes on system tables are shown. pg_statio_user_indexes Same as pg_statio_all_indexes, except that only indexes on user tables are shown. pg_statio_all_sequences For each sequence object in the current database, the numbers of disk blocks read and buffer hits in that sequence. pg_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_sequences Same as pg_statio_all_sequences, except that only user sequences are shown. 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. Other ways of looking at the statistics can be set up by writing queries that use the same underlying statistics access functions as these standard views do. The per-database access functions accept a database OID to identify which database to report on. The per-table and per-index functions accept a table or index OID (note that only tables and indexes in the current database can be seen with these functions). The per-backend access functions accept a backend ID number, which ranges from one to the number of currently active backends. Function Return Type Description pg_stat_get_db_numbackends(oid) integer Number of active backends in database pg_stat_get_db_xact_commit(oid) bigint Transactions committed in database pg_stat_get_db_xact_rollback(oid) bigint Transactions rolled back in database pg_stat_get_db_blocks_fetched(oid) bigint Number of disk block fetch requests for database pg_stat_get_db_blocks_hit(oid) bigint Number of disk block requests found in cache for database pg_stat_get_numscans(oid) bigint Number of sequential scans done when argument is a table, or number of index scans done when argument is an index pg_stat_get_tuples_returned(oid) bigint Number of tuples read by sequential scans when argument is a table, or number of index tuples read when argument is an index pg_stat_get_tuples_fetched(oid) bigint Number of valid (unexpired) table tuples fetched by sequential scans when argument is a table, or fetched by index scans using this index when argument is an index pg_stat_get_tuples_inserted(oid) bigint Number of tuples inserted into table pg_stat_get_tuples_updated(oid) bigint Number of tuples updated in table pg_stat_get_tuples_deleted(oid) bigint Number of tuples deleted from table pg_stat_get_blocks_fetched(oid) bigint Number of disk block fetch requests for table or index pg_stat_get_blocks_hit(oid) bigint Number of disk block requests found in cache for table or index pg_stat_get_backend_idset() set of integer Set of currently active backend IDs (from 1 to N where N is the number of active backends). See usage example below. pg_backend_pid() integer Process ID of the attached backend pg_stat_get_backend_pid(integer) integer Process ID of all backend processes pg_stat_get_backend_dbid(integer) oid Database ID of backend process pg_stat_get_backend_userid(integer) oid User ID of backend process pg_stat_get_backend_activity(integer) text Current query of backend process (NULL if caller is not superuser) pg_stat_reset() boolean Reset all currently collected statistics. Note: blocks_fetched minus blocks_hit gives the number of kernel read() calls issued for the table, index, or database; but the actual number of physical reads is usually lower due to kernel-level buffering. The function pg_stat_get_backend_idset provides a convenient way to generate one row for each active backend. For example, to show the PIDs and current queries of all backends: SELECT pg_stat_get_backend_pid(S.backendid) AS procpid, pg_stat_get_backend_activity(S.backendid) AS current_query FROM (SELECT pg_stat_get_backend_idset() AS backendid) AS S; Another useful tool for monitoring database activity is the pg_locks system catalog. This 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 backend. View the relation in the current database with the most un-granted 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. For more information on locking and managing concurrency with PostgreSQL, refer to the Administrator's Guide. When the pg_locks view is accessed, the internal lock manager data structures are momentarily locked, and a copy is made for the view to display. This ensures that the view produces a consistent set of results, while not blocking normal lock manager operations longer than necessary. Nonetheless there could be some impact on database performance if this view is examined often. The pg_locks view contains one row per lockable object and requested lock mode. Thus, the same lockable object may appear many times, if multiple transactions are holding or waiting for locks on it. A lockable object is either a relation or a transaction ID. (Note that this view includes only table-level locks, not row-level ones. If a transaction is waiting for a row-level lock, it will appear in the view as waiting for the transaction ID of the current holder of that row lock.) Column Name Type Description relation oid The OID of the locked relation, or NULL if the lockable object is a transaction ID. This column can be joined with the pg_class system catalog to get more information on the locked relation. Note however that this will only work for relations in the current database (those for which the database column is either the current database's OID or zero). database oid The OID of the database in which the locked relation exists, or NULL if the lockable object is a transaction ID. If the lock is on a globally-shared table, this field will be zero. This column can be joined with the pg_database system catalog to get more information on the locked object's database. transaction xid The ID of a transaction, or NULL if the lockable object is a relation. Every transaction holds an exclusive lock on its transaction ID for its entire duration. If one transaction finds it necessary to wait specifically for another transaction, it does so by attempting to acquire share lock on the other transaction ID. That will succeed only when the other transaction terminates and releases its locks. pid int4 The process ID of the PostgreSQL backend that has acquired or is attempting to acquire the lock. If you have enabled the statistics collector, this column can be joined with the pg_stat_activity view to get more information on the backend holding or waiting to hold the lock. mode text The mode of the requested or held lock on the lockable object. For more information on the different lock modes available in PostgreSQL, refer to the User's Guide. isgranted bool True if this lock has been granted (is held by this backend). False indicates that this backend is currently waiting to acquire this lock, which implies that some other backend is holding a conflicting lock mode on the same lockable object. This backend will sleep until the other lock is released (or a deadlock situation is detected). A single backend can be waiting to acquire at most one lock at a time.