Monitoring Database Activitymonitoringdatabase activitydatabase activitymonitoring
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.
Standard Unix Toolspsto 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 960 0.0 1.1 6104 1480 pts/1 SN 13:17 0:00 postgres -i
postgres 963 0.0 1.1 7084 1472 pts/1 SN 13:17 0:00 postgres: writer 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
master server process. The command arguments
shown for it are the same ones used when it was launched. The next two
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.) 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 1003 is waiting for process 1016 to complete its transaction and
thereby release some lock.
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
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.
The Statistics CollectorstatisticsPostgreSQL'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 of the exact
command currently being executed by other server processes. This
facility is independent of the collector process.
Statistics Collection Configuration
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 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 the current command being executed by any server process.
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 backends (including autovacuum) through temporary files.
These files are stored in the pg_stat_tmp subdirectory.
When the postmaster shuts down, a permanent copy of the statistics
data is stored in the global subdirectory. For increased
performance, the parameter can
be pointed at a RAM-based file system, decreasing physical I/O requirements.
Viewing Collected Statistics
Several predefined views, listed in , 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 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 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>, or via these views' underlying
functions (named the same as the standard statistics functions but with the
prefix pg_stat_get_xact_). These numbers do not act
as stated above; instead they update continuously throughout the transaction.
Standard Statistics ViewsView NameDescriptionpg_stat_activitypg_stat_activityOne row per server process, showing information related to
each connection to the server. Unless the
parameter has been turned
off, it is possible to monitor state and query information of
all running processes.
See for more details.
pg_stat_bgwriter>pg_stat_bgwriterOne row only, showing cluster-wide statistics. See
for more details.
pg_stat_database>pg_stat_databaseOne row per database, showing database-wide statistics. See
for more details.
pg_stat_database_conflicts>pg_stat_database_conflicts
One row per database showing database-wide statistics about
query cancels due to conflict with recovery on standby servers.
Will only contain information on standby servers, since
conflicts do not occur on master servers.
See for more details.
pg_stat_replication>pg_stat_replicationOne row per WAL sender process, showing statistics about the
replication to this slave. See
for more details. Only directly connected standbys are listed; no
information about downstream standby servers is recorded.
pg_stat_all_tables>pg_stat_all_tables
One row for each table in the current database (including TOAST
tables) with information about accesses to this specific table.
See for more details.
pg_stat_sys_tables>pg_stat_sys_tablesSame as pg_stat_all_tables>, except that only
system tables are shown.pg_stat_user_tables>pg_stat_user_tablesSame as pg_stat_all_tables>, except that only user
tables are shown.pg_stat_xact_all_tables>pg_stat_xact_all_tablesSimilar 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_tables>pg_stat_xact_sys_tablesSame as pg_stat_xact_all_tables>, except that only
system tables are shown.pg_stat_xact_user_tables>pg_stat_xact_user_tablesSame as pg_stat_xact_all_tables>, except that only
user tables are shown.pg_stat_all_indexes>pg_stat_all_indexes
One row for each index in the current database with information
about accesses to this specific index.
See for more details.
pg_stat_sys_indexes>pg_stat_sys_indexesSame as pg_stat_all_indexes>, except that only
indexes on system tables are shown.pg_stat_user_indexes>pg_stat_user_indexesSame as pg_stat_all_indexes>, except that only
indexes on user tables are shown.pg_statio_all_tables>pg_statio_all_tables
One row for each table in the current database (including TOAST
tables) with information about I/O on this specific table.
See for more details.
pg_statio_sys_tables>pg_statio_sys_tablesSame as pg_statio_all_tables>, except that only
system tables are shown.pg_statio_user_tables>pg_statio_user_tablesSame as pg_statio_all_tables>, except that only
user tables are shown.pg_statio_all_indexes>pg_statio_all_indexes
One row for each index in the current database
with information about I/O on this specific index.
See for more details.
pg_statio_sys_indexes>pg_statio_sys_indexesSame as pg_statio_all_indexes>, except that only
indexes on system tables are shown.pg_statio_user_indexes>pg_statio_user_indexesSame as pg_statio_all_indexes>, except that only
indexes on user tables are shown.pg_statio_all_sequences>pg_statio_all_sequences
One row for each sequence in the current database
with information about I/O on this specific sequence.
See for more details.
pg_statio_sys_sequences>pg_statio_sys_sequencesSame 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>pg_statio_user_sequencesSame as pg_statio_all_sequences>, except that only
user sequences are shown.pg_stat_user_functions>pg_stat_user_functions
One row for each tracked function (as specified by the
parameter). See
for more details.
pg_stat_xact_user_functions>pg_stat_xact_user_functionsSimilar to pg_stat_user_functions>, but counts only
calls during the current transaction (which are not>
yet included in pg_stat_user_functions>).
The per-index statistics are particularly useful to determine which
indexes are being used and how effective they are.
Indexes can be
used either directly or via bitmap scans>. 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>.
Before PostgreSQL 8.1, the
idx_tup_read> and idx_tup_fetch> counts
were essentially always equal. Now they can be different even without
considering 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.
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.
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. These functions are listed in . The per-database access
functions 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 function-call statistics
take a function OID. (Note that only tables, indexes, and functions
in the current database can be seen with these functions.) The
per-server-process access functions take a server process
number, which ranges from one to the number of currently active
server processes.
pg_stat_activity viewColumnTypeDescriptiondatidoid>The OID of the database the backend is connected to.
This value can also be returned by directly calling
the pg_stat_get_backend_dbid function.datnamename>The name of the database the backend is connected to.pidinteger>The process ID of the backend.
This value can also be returned by directly calling
the pg_stat_get_backend_pid.usesysidoid>The id of the user logged into the backend.
This value can also be returned by directly calling
the pg_stat_get_backend_userid.usenamename>The name of the user logged into the backend.application_nametext>The name of the application that has initiated the connection
to the backend.client_addrinet>The remote IP of the client connected to the backend.
If this field is not set, it indicates that the client is either connected
via a Unix socket on the server machine or is an internal process such
as autovacuum.
This value can also be returned by directly calling
the pg_stat_get_backend_client_addr.
client_hostnametext>
If available, the hostname of the client as reported by a
reverse lookup of client_addr>. This field will
only be set when is enabled.
client_portinteger>
The remote TCP port that the client is using for communication
to the backend, or NULL> if a unix socket is used.
This value can also be returned by directly calling
the pg_stat_get_backend_client_port.
backend_starttimestamp with time zone>
The time when this process was started, i.e. when the
client connected to the server.
This value can also be returned by directly calling
the pg_stat_get_backend_start.
xact_starttimestamp with time zone>
The time when the current transaction was started. If the client is
using autocommit for transactions, this value is equal to the
query_start column.
This value can also be returned by directly calling
the pg_stat_get_backend_xact_start.
query_starttimestamp with time zone>
The time when the currently active query started, or if
state> is idle>, when the last query
was started.
This value can also be returned by directly calling
the pg_stat_get_backend_activity_start.
state_changetimestamp with time zone>The time when the state> was last changed.waitingboolean>
Boolean indicating if a backend is currently waiting on a lock.
This value can also be returned by directly calling
the pg_stat_get_backend_waiting.
statetext>
The state> of the currently running query.
Can be one of:
active
The backend is executing a query.
idle
There is no query executing in the backend.
idle in transaction
The backend is in a transaction, but is currently 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 indicates that
is disabled.
The waiting> and state> columns are
independent. If a query is in the active> state,
it may or may not be waiting>. If a query is
active> and waiting> is true, it means
that the query is being executed, but is being blocked by a lock
somewhere in the system.
querytext>
The most recent query that the backend has executed. If
state> is active> this means the currently
executing query. In all other states, it means the last query that was
executed.
The pg_stat_activity view will have one row
per server process, showing information related to each connection to
the server.
All functions used in the view are indexed by backend id number. The
function pg_stat_get_backend_idset provides a
convenient way to generate one row for each active server process. For
example, to show the PID>s and current queries of all server processes:
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;
pg_stat_bgwriter viewColumnTypeDescriptioncheckpoints_timedbigintNumber of scheduled checkpoints.
This value can also be returned by directly calling
the pg_stat_get_bgwriter_timed_checkpoints function.checkpoints_requestedbigintNumber of requested checkpoints.
This value can also be returned by directly calling
the pg_stat_get_bgwriter_requested_checkpoints function.buffers_checkpointbigintNumber of buffers written during checkpoints.
This value can also be returned by directly calling
the pg_stat_get_bgwriter_buf_written_checkpoints function.buffers_cleanbigintNumber of buffers written by the background writer.
This value can also be returned by directly calling
the pg_stat_get_bgwriter_buf_written_clean function.maxwritten_cleanbigintNumber of times the background writer stopped a cleaning
scan because it had written too many buffers.
This value can also be returned by directly calling
the pg_stat_get_bgwriter_maxwritten_clean function.buffers_backendbigintNumber of buffers written directly by a backend.
This value can also be returned by directly calling
the pg_stat_get_buf_written_backend function.buffers_backend_fsyncbigintNumber 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_allocbigintNumber of buffers allocated.
This value can also be returned by directly calling
the pg_stat_get_buf_alloc function.stats_resetbigintThe last time these statistics were reset.
This value can also be returned by directly calling
the pg_stat_get_bgwriter_stat_reset_time function.
The pg_stat_bgwriter view will always have a
single row with global data for the cluster.
pg_stat_database viewColumnTypeDescriptiondatidoid>The OID of the databasedatnamename>The name of the databasenumbackendsinteger>The number of backends currently connected to this database.
This is the only column in this view that returns a value for the
current state, all other columns return the accumulated values since
the last reset. This value can also be returned by directly calling
the pg_stat_get_db_numbackends function.xact_commitbigint>The number of transactions in this database that have been
committed. This value can also be returned by directly calling
the pg_stat_get_db_xact_commit function.xact_rollbackbigint>The number of transactions in this database that have been
rolled back. This value can also be returned by directly calling
the pg_stat_get_db_xact_rollback function.blks_readbigint>The number of disk blocks read in this database.
This value can also be returned by directly calling
the pg_stat_get_db_blocks_fetched and
pg_stat_get_db_blocks_hit functions and
subtracting the results.blks_hitsbigint>The number of disk blocks read from the buffer cache
(this only includes hits in the PostgreSQL buffer cache, and not
the operating system filesystem cache).
This value can also be returned by directly calling
the pg_stat_get_db_blocks_hit function.tup_returnedbigint>The number of rows returned by queries in this database.
This value can also be returned by directly calling
the pg_stat_get_db_tuples_returned function.tup_fetchedbigint>The number of rows fetched by queries in this database.
This value can also be returned by directly calling
the pg_stat_get_db_tuples_fetched function.tup_insertedbigint>The number of rows inserted by queries in this database.
This value can also be returned by directly calling
the pg_stat_get_db_tuples_inserted function.tup_updatedbigint>The number of rows updated by queries in this database.
This value can also be returned by directly calling
the pg_stat_get_db_tuples_updated function.tup_deletedbigint>The number of rows deleted by queries in this database.
This value can also be returned by directly calling
the pg_stat_get_db_tuples_deleted function.conflictsbigint>
The number of queries canceled due to conflict with recovery
(on standby servers) in this database. (See
for more details).
This value can also be returned by directly calling
the pg_stat_get_db_conflict_all function.
temp_filesbigint>
The number of temporary files written by queries in the database.
All temporary files are counted, regardless of why the temporary file
was created (sorting or hash) or file size, and regardless of the
setting.
This value can also be returned by directly calling
the pg_stat_get_db_temp_files function.
temp_bytesbigint>
The amount of data written to temporary files by queries in
the database. All temporary files are counted, regardless of why
the temporary file was created (sorting or hash) or file size, and
regardless of the setting.
This value can also be returned by directly calling
the pg_stat_get_db_temp_bytes function.
deadlocksbigint>Number of deadlocks detected in the database.
This value can also be returned by directly calling
the pg_stat_get_db_deadlocks function.stats_resettimestamptz>The last time the statistics were reset.
This value can also be returned by directly calling
the pg_stat_get_reset_time function.
The pg_stat_database view will contain one row
for each database in the cluster showing database-wide statistics.
pg_stat_database_conflicts viewColumnTypeDescriptiondatidoid>The OID of the databasedatnamename>The name of the databaseconfl_tablespacebigint>The number of queries that have been canceled due to
dropped tablespaces. This value can also be returned by directly calling
the pg_stat_get_db_conflict_tablespace function.confl_lockbigint>The number of queries that have been canceled due to
lock timeouts. This value can also be returned by directly calling
the pg_stat_get_db_conflict_lock function.confl_snapshotbigint>The number of queries that have been canceled due to
old snapshots. This value can also be returned by directly calling
the pg_stat_get_db_conflict_snapshot function.confl_bufferpinbigint>The number of queries that have been canceled due to
pinned buffers. This value can also be returned by directly calling
the pg_stat_get_db_conflict_bufferpin function.confl_deadlockbigint>The number of queries that have been canceled due to
deadlocks. This value can also be returned by directly calling
the pg_stat_get_db_conflict_startup_deadlock
function.
The pg_stat_database_conflicts view will contain
one row per database showing database-wide statistics about
query cancels due to conflict with recovery on standby servers.
Will only contain information on standby servers, since
conflicts do not occur on master servers.
pg_stat_replication viewColumnTypeDescriptionpidinteger>The process id of the WAL sender processusesysidoid>The OID of the user logged into this WAL sender processusenamename>The name of the user logged into this WAL sender processapplication_nametext>The name of the application that has initiated the connection
to the WAL sender.client_addrinet>The remote IP of the client connected to the WAL sender.
If this field is not set, it indicates that the client is
connected via a Unix socket on the server machine.
client_hostnametext>
If available, the hostname of the client as reported by a
reverse lookup of client_addr>. This field will
only be set when is enabled.
client_portinteger>
The remote TCP port that the client is using for communication
to the, or NULL> if a unix socket is used.
backend_starttimestamp with time zone>
The time when this process was started, i.e. when the
client connected to the WAL sender.
statetext>Current WAL sender statesent_locationtext>Last transaction log position sent on this connectionwrite_locationtext>Last transaction log position written to disk by the slaveflush_locationtext>Last transaction log position flushed to disk by the slavereplay_locationtext>Last transaction log position replayed into the database on the slavesync_priorityint>
The priority in the order which this slave will be picked as
the synchronous standby.
sync_statetext>
The synchronous state of this slave.
The pg_stat_replication view will contain one row
per WAL sender process, showing statistics about the replication to this
slave. Only directly connected standbys are listed; no information about
downstream standby servers is recorded.
pg_stat_all_tables viewColumnTypeDescriptionrelidoid>The OID of the tableschemanamename>The name of the schema that the table is inrelnamename>The name of the tableseq_scanbigint>The number of sequential scans initiated on this table.
This value can also be returned by directly calling
the pg_stat_get_numscans function.seq_tup_readbigint>The number of live rows fetch by sequential scans.
This value can also be returned by directly calling
the pg_stat_get_tuples_returned function.idx_scanbigint>The number of index scans initiated on this tableidx_tup_fetchbigint>The number of live rows fetch by index scans.n_tup_insbigint>The number of rows inserted.
This value can also be returned by directly calling
the pg_stat_get_tuples_inserted function.n_tup_updbigint>The number of rows updated.
This value can also be returned by directly calling
the pg_stat_get_tuples_updated function.n_tup_delbigint>The number of rows deleted.
This value can also be returned by directly calling
the pg_stat_get_tuples_deleted function.n_tup_hot_updbigint>The number of rows HOT (i.e., no separate index update) updated.
This value can also be returned by directly calling
the pg_stat_get_tuples_hot_updated function.n_live_tupbigint>The number of live rows.
This value can also be returned by directly calling
the pg_stat_get_live_tuples function.n_dead_tupbigint>The number of dead rows.
This value can also be returned by directly calling
the pg_stat_get_dead_tuples function.last_vacuumtimestamp with time zone>The last time the table was manually non-last_autovacuumtimestamp with time zone>The last time the table was vacuumed by the autovacuum daemon.
This value can also be returned by directly calling
the pg_stat_get_last_autovacuum_time function.last_analyzetimestamp with time zone>The last time the table was manually analyzed.
This value can also be returned by directly calling
the pg_stat_get_last_analyze_time function.last_autoanalyzetimestamp with time zone>The last time the table was analyzed by the autovacuum daemon.
This value can also be returned by directly calling
the pg_stat_get_last_autoanalyze_time function.vacuum_countbigint>The number of times this table has been manually non-autovacuum_countbigint>The number of times this table has been vacuumed by the autovacuum daemon.
This value can also be returned by directly calling
the pg_stat_get_autovacuum_count function.analyze_countbigint>The number of times this table has been manually analyzed.
This value can also be returned by directly calling
the pg_stat_get_analyze_count function.autoanalyze_countbigint>The number of times this table has been analyzed by the autovacuum daemon.
This value can also be returned by directly calling
the pg_stat_get_autoanalyze_count function.
The pg_stat_all_tables view will contain
one row for each table in the current database (including TOAST
tables) with information about accesses to this specific table. The
pg_stat_user_tables and
pg_stat_sys_tables contain the same information,
but filtered to only have rows for user and system tables.
pg_stat_all_indexes viewColumnTypeDescriptionrelidoid>The OID of the table for this indexindexrelidoid>The OID of the indexschemanamename>The name of the schema the index is inrelnamename>The name of the table for this indexindexrelnamename>The name of the indexidx_scanbigint>Number of index scans initiated on this index.
This value can also be returned by directly calling
the pg_stat_get_numscans function.idx_tup_readbigint>Number of index entries returned by scans on this index.
This value can also be returned by directly calling
the pg_stat_get_tuples_returned function.idx_tup_fetchbigint>Number of live table rows fetched by simple index scans using this index.
This value can also be returned by directly calling
the pg_stat_get_tuples_fetched function.
The pg_stat_all_indexes view will contain
one row for each index in the current database
with information about accesses to this specific index. The
pg_stat_user_indexes and
pg_stat_sys_indexes contain the same information,
but filtered to only have rows for user and system indexes.
pg_statio_all_tables viewColumnTypeDescriptionrelidoid>The OID of the tableschemanamename>The name of the schema that the table is inrelnamename>The name of the tableheap_blks_readname>Number of disk blocks read from this table.
This value can also be returned by directly calling
the pg_stat_get_blocks_fetched and
pg_stat_get_blocks_hit functions and
subtracting the results.heap_blks_hitname>Number of buffer hits in this table.
This value can also be returned by directly calling
the pg_stat_get_blocks_hit function.idx_blks_readname>Number of disk blocks read from all indexes on this tableidx_blks_hitname>Number of buffer hits in all indexes of this table.toast_blks_readname>Number of disk blocks read from this table's TOAST table (if any)toast_blks_hitname>Number of buffer hits in this table's TOAST table (if any)tidx_blks_readname>Number of disk blocks read from this table's TOAST table index (if any)tidx_blks_hitname>Number of buffer hits in this table's TOAST table index (if any)
The pg_statio_all_tables view will contain
one row for each table in the current database (including TOAST
tables) with information about I/O on this specific table. The
pg_statio_user_tables and
pg_statio_sys_tables contain the same information,
but filtered to only have rows for user and system tables.
pg_statio_all_indexes viewColumnTypeDescriptionrelidoid>The OID of the table for this indexindexrelidoid>The OID of the indexschemanamename>The name of the schema the index is inrelnamename>The name of the table for this indexindexrelnamename>The name of the indexidx_blks_readname>Number of disk blocks read from the index.
This value can also be returned by directly calling
the pg_stat_get_blocks_fetched and
pg_stat_get_blocks_hit functions and
subtracting the results.idx_blks_hitname>Number of buffer hits in the index.
This value can also be returned by directly calling
the pg_stat_get_blocks_hit function.
The pg_statio_all_indexes view will contain
one row for each index in the current database
with information about I/O on this specific index. The
pg_statio_user_indexes and
pg_statio_sys_indexes contain the same information,
but filtered to only have rows for user and system indexes.
pg_statio_all_sequences viewColumnTypeDescriptionrelidoid>The OID of the sequenceschemanamename>The name of the schema the sequence is inrelnamename>The name of the sequenceblks_readname>Number of disk blocks read from the sequenceblks_hitname>Number of buffer hits in the sequence
The pg_statio_all_indexes view will contain
one row for each sequence in the current database
with information about I/O on this specific sequence.
pg_stat_user_functions viewColumnTypeDescriptionfuncidoid>The OID of the functionschemanamename>The name of the schema the function is infuncnamename>The name of the functioncallsbigint>Number of times the function has been called.
This value can also be returned by directly calling
the pg_stat_get_function_calls function.total_timebigint>Total time spent in this functions and all other functions
called by it, in milliseconds.
This value can also be returned by directly calling
the pg_stat_get_function_time function.self_timebigint>Total time spent in this functions itself but not including
other functions called by it, in milliseconds.
This value can also be returned by directly calling
the pg_stat_get_function_self_time function.
The pg_stat_user_functions view will contain
one row for each tracked function (as specified by the
parameter).
Other Statistics Functions
Other Statistics FunctionsFunctionReturn TypeDescriptionpg_backend_pid()integer
Process ID of the server process attached to the current session
pg_stat_get_activity(integer)setof record
Returns a record of information about the backend with the specified PID, 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_backend_idset()setof integer
Set of currently active server process numbers (from 1 to the
number of active server processes). See usage example in the text.
pg_stat_get_wal_senders()setof record
One record for each active wal sender. The fields returned are a subset
of those in the pg_stat_replication view.
pg_stat_clear_snapshot()void
Discard the current statistics snapshot
pg_stat_reset()void
Reset all statistics counters for the current database to zero
(requires superuser privileges)
pg_stat_reset_shared(text)void
Reset some of the shared statistics counters for the database cluster to
zero (requires superuser privileges). Calling
pg_stat_reset_shared('bgwriter')> will zero all the values shown by
pg_stat_bgwriter>.
pg_stat_reset_single_table_counters(oid)void
Reset statistics for a single table or index in the current database to
zero (requires superuser privileges)
pg_stat_reset_single_function_counters(oid)void
Reset statistics for a single function in the current database to
zero (requires superuser privileges)
Viewing Lockslockmonitoring
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 .
Dynamic TracingDTracePostgreSQL 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, only the
DTrace
utility is supported, which is available
on OpenSolaris, Solaris 10, and Mac OS X Leopard. It is expected that
DTrace will be available in the future on FreeBSD and possibly other
operating systems. The
SystemTap project
for Linux also provides a DTrace equivalent. Supporting other dynamic
tracing utilities is theoretically possible by changing the definitions for
the macros in src/include/utils/probes.h>.
Compiling for Dynamic Tracing
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 Built-in Probes
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.
Built-in DTrace ProbesNameParametersDescriptiontransaction-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 xlog file(s) 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 bgwriter 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 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.xlog-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.xlog-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)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.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(LWLockId, LWLockMode)Probe that fires when an LWLock has been acquired.
arg0 is the LWLock's ID.
arg1 is the requested lock mode, either exclusive or shared.lwlock-release(LWLockId)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 ID.lwlock-wait-start(LWLockId, 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 ID.
arg1 is the requested lock mode, either exclusive or shared.lwlock-wait-done(LWLockId, 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 ID.
arg1 is the requested lock mode, either exclusive or shared.lwlock-condacquire(LWLockId, LWLockMode)Probe that fires when an LWLock was successfully acquired when the
caller specified no waiting.
arg0 is the LWLock's ID.
arg1 is the requested lock mode, either exclusive or shared.lwlock-condacquire-fail(LWLockId, LWLockMode)Probe that fires when an LWLock was not successfully acquired when
the caller specified no waiting.
arg0 is the LWLock's ID.
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.
Defined Types Used in Probe ParametersTypeDefinitionLocalTransactionIdunsigned intLWLockIdintLWLockModeintLOCKMODEintBlockNumberunsigned intOidunsigned intForkNumberintboolchar
Using Probes
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.
More example scripts can be found in the PgFoundry
dtrace project.
Defining New Probes
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
Example:
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