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27.2. The Statistics Collector

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.

27.2.1. 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 Chapter 19 for details about setting configuration parameters.)

The parameter track_activities enables monitoring of the current command being executed by any server process.

The parameter track_counts controls whether statistics are collected about table and index accesses.

The parameter track_functions enables tracking of usage of user-defined functions.

The parameter track_io_timing 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 SET 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 stats_temp_directory 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.

27.2.2. Viewing Statistics

Several predefined views, listed in Table 27.1, are available to show the current state of the system. There are also several other views, listed in Table 27.2, available to show the results of statistics collection. Alternatively, one can build custom views using the underlying statistics functions, as discussed in Section 27.2.20.

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 Table 27.1 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 Section 21.5) can see all the information about all sessions.

Table 27.1. Dynamic Statistics Views

View Name Description
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 pg_stat_activity for details.
pg_stat_replication One row per WAL sender process, showing statistics about replication to that sender's connected standby server. See pg_stat_replication for details.
pg_stat_wal_receiver Only one row, showing statistics about the WAL receiver from that receiver's connected server. See pg_stat_wal_receiver for details.
pg_stat_subscription At least one row per subscription, showing information about the subscription workers. See pg_stat_subscription for details.
pg_stat_ssl One row per connection (regular and replication), showing information about SSL used on this connection. See pg_stat_ssl for details.
pg_stat_gssapi One row per connection (regular and replication), showing information about GSSAPI authentication and encryption used on this connection. See pg_stat_gssapi for details.
pg_stat_progress_analyze One row for each backend (including autovacuum worker processes) running ANALYZE, showing current progress. See Section 27.4.1.
pg_stat_progress_create_index One row for each backend running CREATE INDEX or REINDEX, showing current progress. See Section 27.4.2.
pg_stat_progress_vacuum One row for each backend (including autovacuum worker processes) running VACUUM, showing current progress. See Section 27.4.3.
pg_stat_progress_cluster One row for each backend running CLUSTER or VACUUM FULL, showing current progress. See Section 27.4.4.
pg_stat_progress_basebackup One row for each WAL sender process streaming a base backup, showing current progress. See Section 27.4.5.

Table 27.2. Collected Statistics Views

View Name Description
pg_stat_archiver One row only, showing statistics about the WAL archiver process's activity. See pg_stat_archiver for details.
pg_stat_bgwriter One row only, showing statistics about the background writer process's activity. See pg_stat_bgwriter for details.
pg_stat_database One row per database, showing database-wide statistics. See pg_stat_database for details.
pg_stat_database_conflicts One row per database, showing database-wide statistics about query cancels due to conflict with recovery on standby servers. See pg_stat_database_conflicts for details.
pg_stat_all_tables One row for each table in the current database, showing statistics about accesses to that specific table. See pg_stat_all_tables for details.
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_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_tables Same as pg_stat_xact_all_tables, except that only system tables are shown.
pg_stat_xact_user_tables Same as pg_stat_xact_all_tables, except that only user tables are shown.
pg_stat_all_indexes One row for each index in the current database, showing statistics about accesses to that specific index. See pg_stat_all_indexes for details.
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 One row for each table in the current database, showing statistics about I/O on that specific table. See pg_statio_all_tables for details.
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 One row for each index in the current database, showing statistics about I/O on that specific index. See pg_statio_all_indexes for details.
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 One row for each sequence in the current database, showing statistics about I/O on that specific sequence. See pg_statio_all_sequences for details.
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.
pg_stat_user_functions One row for each tracked function, showing statistics about executions of that function. See pg_stat_user_functions for details.
pg_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_slru One row per SLRU, showing statistics of operations. See pg_stat_slru 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.

27.2.3. pg_stat_activity

The pg_stat_activity view will have one row per server process, showing information related to the current activity of that process.

Table 27.3. pg_stat_activity View

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 a parallel query worker. NULL if this process is a parallel group leader or does not participate in parallel query.

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 log_hostname 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 Table 27.4.

wait_event text

Wait event name if backend is currently waiting, otherwise NULL. See Table 27.5 through Table 27.13.

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 track_activities 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 bytes; this value can be changed via the parameter track_activity_query_size.

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.


Note

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.

Table 27.4. Wait Event Types

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 Table 27.5.
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 Table 27.6.
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 Table 27.7.
Extension The server process is waiting for some condition defined by an extension module. See Table 27.8.
IO The server process is waiting for an I/O operation to complete. wait_event will identify the specific wait point; see Table 27.9.
IPC The server process is waiting for some interaction with another server process. wait_event will identify the specific wait point; see Table 27.10.
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 Table 27.11.
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 Table 27.12.
Timeout The server process is waiting for a timeout to expire. wait_event will identify the specific wait point; see Table 27.13.

Table 27.5. Wait Events of Type Activity

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.

Table 27.6. Wait Events of Type BufferPin

BufferPin Wait Event Description
BufferPin Waiting to acquire an exclusive pin on a buffer.

Table 27.7. Wait Events of Type Client

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.

Table 27.8. Wait Events of Type Extension

Extension Wait Event Description
Extension Waiting in an extension.

Table 27.9. Wait Events of Type IO

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 pg_control file.
ControlFileSync Waiting for the pg_control file to reach durable storage.
ControlFileSyncUpdate Waiting for an update to the pg_control file to reach durable storage.
ControlFileWrite Waiting for a write to the pg_control file.
ControlFileWriteUpdate Waiting for a write to update the pg_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 fill a dynamic shared memory backing file with zeroes.
DataFileExtend Waiting for a relation data file to be extended.
DataFileFlush Waiting for a relation data file to reach durable storage.
DataFileImmediateSync Waiting for an immediate synchronization of a relation data file to durable 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 durable 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 durable 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 durable 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 durable storage during a checkpoint.
LogicalRewriteMappingSync Waiting for mapping data to reach durable 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 durable storage.
LogicalRewriteTruncate Waiting for truncate of mapping data during a logical rewrite.
LogicalRewriteWrite Waiting for a write of logical rewrite mappings.
RelationMapRead Waiting for a read of the relation map file.
RelationMapSync Waiting for the relation map file to reach durable 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 durable storage while restoring it to memory.
ReplicationSlotSync Waiting for a replication slot control file to reach durable storage.
ReplicationSlotWrite Waiting for a write to a replication slot control file.
SLRUFlushSync Waiting for SLRU data to reach durable storage during a checkpoint or database shutdown.
SLRURead Waiting for a read of an SLRU page.
SLRUSync Waiting for SLRU data to reach durable 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 durable 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 durable 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 durable 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 durable storage.
TwophaseFileWrite Waiting for a write of a two phase state file.
WALBootstrapSync Waiting for WAL to reach durable 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 durable 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 durable 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 a walsender timeline command.
WALSync Waiting for a WAL file to reach durable storage.
WALSyncMethodAssign Waiting for data to reach durable storage while assigning a new WAL sync method.
WALWrite Waiting for a write to a WAL file.

Table 27.10. Wait Events of Type IPC

IPC Wait Event Description
BackupWaitWalArchive Waiting for WAL files required for a 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.
ExecuteGather Waiting for activity from a child process while executing a Gather plan node.
HashBatchAllocate Waiting for an elected Parallel Hash participant to allocate a hash table.
HashBatchElect Waiting to elect a Parallel Hash participant to allocate a hash table.
HashBatchLoad Waiting for other Parallel Hash participants to finish loading a hash table.
HashBuildAllocate Waiting for an elected Parallel Hash participant to allocate the initial hash table.
HashBuildElect Waiting to elect a Parallel Hash participant to allocate the initial hash table.
HashBuildHashInner Waiting for other Parallel Hash participants to finish hashing the inner relation.
HashBuildHashOuter Waiting for other Parallel Hash participants to finish partitioning the outer relation.
HashGrowBatchesAllocate Waiting for an elected Parallel Hash participant to allocate more batches.
HashGrowBatchesDecide Waiting to elect a Parallel Hash participant to decide on future batch growth.
HashGrowBatchesElect Waiting to elect a Parallel Hash participant to allocate more batches.
HashGrowBatchesFinish Waiting for an elected Parallel Hash participant to decide on future batch growth.
HashGrowBatchesRepartition Waiting for other Parallel Hash participants to finish repartitioning.
HashGrowBucketsAllocate Waiting for an elected Parallel Hash participant to finish allocating more buckets.
HashGrowBucketsElect Waiting to elect a Parallel Hash participant to allocate more buckets.
HashGrowBucketsReinsert Waiting for other Parallel Hash participants to finish inserting tuples into new buckets.
LogicalSyncData Waiting for a logical replication remote server to send data for initial table synchronization.
LogicalSyncStateChange Waiting for a logical replication remote server to change state.
MessageQueueInternal Waiting for another process to be attached to a 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 the group leader to clear the transaction ID at end of a parallel operation.
ProcSignalBarrier Waiting for a barrier event to be processed by all backends.
Promote Waiting for standby promotion.
RecoveryConflictSnapshot Waiting for recovery conflict resolution for a vacuum cleanup.
RecoveryConflictTablespace Waiting for recovery conflict resolution for dropping a tablespace.
RecoveryPause Waiting for recovery to be resumed.
ReplicationOriginDrop Waiting for a replication origin to become inactive so it can be dropped.
ReplicationSlotDrop Waiting for a replication slot to become inactive so it can be dropped.
SafeSnapshot Waiting to obtain a valid snapshot for a READ ONLY DEFERRABLE transaction.
SyncRep Waiting for confirmation from a remote server during synchronous replication.
XactGroupUpdate Waiting for the group leader to update transaction status at end of a parallel operation.

Table 27.11. Wait Events of Type Lock

Lock Wait Event Description
advisory Waiting to acquire an advisory user lock.
extend Waiting to extend a relation.
frozenid Waiting to update pg_database.datfrozenxid and pg_database.datminmxid.
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.

Table 27.12. Wait Events of Type LWLock

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.
NotifyQueueTail Waiting to update limit on NOTIFY message storage.
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.
WrapLimitsVacuum Waiting to update limits on transaction id and multixact consumption.
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.

Note

Extensions can add LWLock types to the list shown in Table 27.12. 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.

Table 27.13. Wait Events of Type Timeout

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          | ProcArray
(2 rows)

27.2.4. pg_stat_replication

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.

Table 27.14. pg_stat_replication View

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 log_hostname 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 hot_standby_feedback.

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


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.

Note

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.

27.2.5. pg_stat_wal_receiver

The pg_stat_wal_receiver view will contain only one row, showing statistics about the WAL receiver from that receiver's connected server.

Table 27.15. pg_stat_wal_receiver View

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.


27.2.6. pg_stat_subscription

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.

Table 27.16. pg_stat_subscription View

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


27.2.7. pg_stat_ssl

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.

Table 27.17. pg_stat_ssl View

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.


27.2.8. pg_stat_gssapi

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.

Table 27.18. pg_stat_gssapi View

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


27.2.9. pg_stat_archiver

The pg_stat_archiver view will always have a single row, containing data about the archiver process of the cluster.

Table 27.19. pg_stat_archiver View

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


27.2.10. pg_stat_bgwriter

The pg_stat_bgwriter view will always have a single row, containing global data for the cluster.

Table 27.20. pg_stat_bgwriter View

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


27.2.11. pg_stat_database

The pg_stat_database view will contain one row for each database in the cluster, plus one for shared objects, showing database-wide statistics.

Table 27.21. pg_stat_database View

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 pg_stat_database_conflicts 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 log_temp_files 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 log_temp_files 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 (if track_io_timing is enabled, otherwise zero)

blk_write_time double precision

Time spent writing data file blocks by backends in this database, in milliseconds (if track_io_timing is enabled, otherwise zero)

stats_reset timestamp with time zone

Time at which these statistics were last reset


27.2.12. pg_stat_database_conflicts

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.

Table 27.22. pg_stat_database_conflicts View

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


27.2.13. pg_stat_all_tables

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.

Table 27.23. pg_stat_all_tables View

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


27.2.14. pg_stat_all_indexes

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.

Table 27.24. pg_stat_all_indexes View

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


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.

Note

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.

27.2.15. pg_statio_all_tables

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.

Table 27.25. pg_statio_all_tables View

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)


27.2.16. pg_statio_all_indexes

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.

Table 27.26. pg_statio_all_indexes View

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


27.2.17. pg_statio_all_sequences

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.

Table 27.27. pg_statio_all_sequences View

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


27.2.18. pg_stat_user_functions

The pg_stat_user_functions view will contain one row for each tracked function, showing statistics about executions of that function. The track_functions parameter controls exactly which functions are tracked.

Table 27.28. pg_stat_user_functions View

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


27.2.19. pg_stat_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.

Table 27.29. pg_stat_slru View

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


27.2.20. Statistics Functions

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 Table 27.30.

Table 27.30. Additional Statistics Functions

Function

Description

pg_backend_pid () → integer

Returns the 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 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 () → timestamp with time zone

Returns the timestamp of the current statistics snapshot.

pg_stat_clear_snapshot () → void

Discards the current statistics snapshot.

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 ( 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 ( 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 ( 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 ( 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 Table 27.31. 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;

Table 27.31. Per-Backend Statistics Functions

Function

Description

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 ( integer ) → text

Returns the text of this backend's most recent query.

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 ( integer ) → inet

Returns the IP address of the client connected to this backend.

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 ( integer ) → oid

Returns the OID of the database this backend is connected to.

pg_stat_get_backend_pid ( integer ) → integer

Returns the process ID of this backend.

pg_stat_get_backend_start ( integer ) → timestamp with time zone

Returns the time when this process was started.

pg_stat_get_backend_userid ( integer ) → oid

Returns the OID of the user logged into this backend.

pg_stat_get_backend_wait_event_type ( integer ) → text

Returns the wait event type name if this backend is currently waiting, otherwise NULL. See Table 27.4 for details.

pg_stat_get_backend_wait_event ( integer ) → text

Returns the wait event name if this backend is currently waiting, otherwise NULL. See Table 27.5 through Table 27.13.

pg_stat_get_backend_xact_start ( integer ) → timestamp with time zone

Returns the time when the backend's current transaction was started.