28.2. 統計資訊收集器

PostgreSQL's cumulative statistics system supports collection and reporting of information about server activity. Presently, accesses to tables and indexes in both disk-block and individual-row terms are counted. The total number of rows in each table, and information about vacuum and analyze actions for each table are also counted. If enabled, calls to user-defined functions and the total time spent in each one are counted as well.

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 cumulative statistics system.

28.2.1. 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 20 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 cumulative 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.

The parameter track_wal_io_timing enables monitoring of WAL 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.)

Cumulative statistics are collected in shared memory. Every PostgreSQL process collects statistics locally, then updates the shared data at appropriate intervals. When a server, including a physical replica, 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. In contrast, when starting from an unclean shutdown (e.g., after an immediate shutdown, a server crash, starting from a base backup, and point-in-time recovery), all statistics counters are reset.

28.2.2. Viewing Statistics

Several predefined views, listed in Table 28.1, are available to show the current state of the system. There are also several other views, listed in Table 28.2, available to show the accumulated statistics. Alternatively, one can build custom views using the underlying cumulative statistics functions, as discussed in Section 28.2.24.

When using the cumulative statistics views and functions to monitor collected data, it is important to realize that the information does not update instantaneously. Each individual server process flushes out accumulated statistics to shared memory just before going idle, but not more frequently than once per PGSTAT_MIN_INTERVAL milliseconds (1 second unless altered while building the server); so a query or transaction still in progress does not affect the displayed totals and 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 the accumulated statistics, accessed values are cached until the end of its current transaction in the default configuration. 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. When analyzing statistics interactively, or with expensive queries, the time delta between accesses to individual statistics can lead to significant skew in the cached statistics. To minimize skew, stats_fetch_consistency can be set to snapshot, at the price of increased memory usage for caching not-needed statistics data. Conversely, if it's known that statistics are only accessed once, caching accessed statistics is unnecessary and can be avoided by setting stats_fetch_consistency to none. You can invoke pg_stat_clear_snapshot() to discard the current transaction's statistics snapshot or cached values (if any). The next use of statistical information will (when in snapshot mode) cause a new snapshot to be built or (when in cache mode) accessed statistics to be cached.

A transaction can also see its own statistics (not yet flushed out to the shared memory statistics) 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 28.1 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 roles with privileges of built-in role pg_read_all_stats (see also Section 22.5) can see all the information about all sessions.

Table 28.1. 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_recovery_prefetch	Only one row, showing statistics about blocks prefetched during recovery. See pg_stat_recovery_prefetch 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 28.4.1.
pg_stat_progress_create_index	One row for each backend running CREATE INDEX or REINDEX, showing current progress. See Section 28.4.2.
pg_stat_progress_vacuum	One row for each backend (including autovacuum worker processes) running VACUUM, showing current progress. See Section 28.4.3.
pg_stat_progress_cluster	One row for each backend running CLUSTER or VACUUM FULL, showing current progress. See Section 28.4.4.
pg_stat_progress_basebackup	One row for each WAL sender process streaming a base backup, showing current progress. See Section 28.4.5.
pg_stat_progress_copy	One row for each backend running COPY, showing current progress. See Section 28.4.6.

Table 28.2. 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_wal	One row only, showing statistics about WAL activity. See pg_stat_wal 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.
pg_stat_replication_slots	One row per replication slot, showing statistics about the replication slot's usage. See pg_stat_replication_slots for details.
pg_stat_subscription_stats	One row per subscription, showing statistics about errors. See pg_stat_subscription_stats 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 views in combination with operating system utilities that allow insight into the kernel's handling of I/O.

28.2.3. 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 28.3. 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 28.4.

wait_event text Wait event name if backend is currently waiting, otherwise NULL. See Table 28.5 through Table 28.13.

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_id bigint Identifier of this backend's most recent query. If state is active this field shows the identifier of the currently executing query. In all other states, it shows the identifier of last query that was executed. Query identifiers are not computed by default so this field will be null unless compute_query_id parameter is enabled or a third-party module that computes query identifiers is configured.

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, archiver, 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 28.4. 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 28.5.
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 28.6.
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 28.7.
Extension	The server process is waiting for some condition defined by an extension module. See Table 28.8.
IO	The server process is waiting for an I/O operation to complete. wait_event will identify the specific wait point; see Table 28.9.
IPC	The server process is waiting for some interaction with another server process. wait_event will identify the specific wait point; see Table 28.10.
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 28.11.
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 28.12.
Timeout	The server process is waiting for a timeout to expire. wait_event will identify the specific wait point; see Table 28.13.

Table 28.5. 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.
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 28.6. Wait Events of Type BufferPin

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

Table 28.7. 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.
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 28.8. Wait Events of Type Extension

Extension Wait Event	Description
Extension	Waiting in an extension.

Table 28.9. Wait Events of Type IO

IO Wait Event	Description
BaseBackupRead	Waiting for base backup to read from a file.
BufFileRead	Waiting for a read from a buffered file.
BufFileWrite	Waiting for a write to a buffered file.
BufFileTruncate	Waiting for a buffered file to be truncated.
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.
VersionFileWrite	Waiting for the version file to be written while creating a database.
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 28.10. Wait Events of Type IPC

IPC Wait Event	Description
AppendReady	Waiting for subplan nodes of an Append plan node to be ready.
ArchiveCleanupCommand	Waiting for archive_cleanup_command to complete.
ArchiveCommand	Waiting for archive_command to complete.
BackendTermination	Waiting for the termination of another backend.
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.
BufferIO	Waiting for buffer I/O to complete.
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.
RecoveryEndCommand	Waiting for recovery_end_command to complete.
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.
RestoreCommand	Waiting for restore_command to complete.
SafeSnapshot	Waiting to obtain a valid snapshot for a READ ONLY DEFERRABLE transaction.
SyncRep	Waiting for confirmation from a remote server during synchronous replication.
WalReceiverExit	Waiting for the WAL receiver to exit.
WalReceiverWaitStart	Waiting for startup process to send initial data for streaming replication.
XactGroupUpdate	Waiting for the group leader to update transaction status at end of a parallel operation.

Table 28.11. 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 28.12. 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.
BufferMapping	Waiting to associate a data block with a buffer in the buffer pool.
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.
PgStatsDSA	Waiting for stats dynamic shared memory allocator access
PgStatsHash	Waiting for stats shared memory hash table access
PgStatsData	Waiting for shared memory stats data access
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 28.12. 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 28.13. Wait Events of Type Timeout

Timeout Wait Event	Description
BaseBackupThrottle	Waiting during base backup when throttling activity.
CheckpointWriteDelay	Waiting between writes while performing a checkpoint.
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).
RegisterSyncRequest	Waiting while sending synchronization requests to the checkpointer, because the request queue is full.
VacuumDelay	Waiting in a cost-based vacuum delay point.
VacuumTruncate	Waiting to acquire an exclusive lock to truncate off any empty pages at the end of a table vacuumed.

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)

28.2.4. 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 28.14. 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.

28.2.5. pg_stat_replication_slots

The pg_stat_replication_slots view will contain one row per logical replication slot, showing statistics about its usage.

Table 28.15. pg_stat_replication_slots View

Column Type Description

slot_name text A unique, cluster-wide identifier for the replication slot

spill_txns bigint Number of transactions spilled to disk once the memory used by logical decoding to decode changes from WAL has exceeded logical_decoding_work_mem. The counter gets incremented for both top-level transactions and subtransactions.

spill_count bigint Number of times transactions were spilled to disk while decoding changes from WAL for this slot. This counter is incremented each time a transaction is spilled, and the same transaction may be spilled multiple times.

spill_bytes bigint Amount of decoded transaction data spilled to disk while performing decoding of changes from WAL for this slot. This and other spill counters can be used to gauge the I/O which occurred during logical decoding and allow tuning logical_decoding_work_mem.

stream_txns bigint Number of in-progress transactions streamed to the decoding output plugin after the memory used by logical decoding to decode changes from WAL for this slot has exceeded logical_decoding_work_mem. Streaming only works with top-level transactions (subtransactions can't be streamed independently), so the counter is not incremented for subtransactions.

stream_countbigint Number of times in-progress transactions were streamed to the decoding output plugin while decoding changes from WAL for this slot. This counter is incremented each time a transaction is streamed, and the same transaction may be streamed multiple times.

stream_bytesbigint Amount of transaction data decoded for streaming in-progress transactions to the decoding output plugin while decoding changes from WAL for this slot. This and other streaming counters for this slot can be used to tune logical_decoding_work_mem.

total_txns bigint Number of decoded transactions sent to the decoding output plugin for this slot. This counts top-level transactions only, and is not incremented for subtransactions. Note that this includes the transactions that are streamed and/or spilled.

total_bytesbigint Amount of transaction data decoded for sending transactions to the decoding output plugin while decoding changes from WAL for this slot. Note that this includes data that is streamed and/or spilled.

stats_reset timestamp with time zone Time at which these statistics were last reset

28.2.6. 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 28.16. 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.

28.2.7. pg_stat_recovery_prefetch

The pg_stat_recovery_prefetch view will contain only one row. The columns wal_distance, block_distance and io_depth show current values, and the other columns show cumulative counters that can be reset with the pg_stat_reset_shared function.

Table 28.17. pg_stat_recovery_prefetch View

Column Type Description

stats_reset timestamp with time zone Time at which these statistics were last reset

prefetch bigint Number of blocks prefetched because they were not in the buffer pool

hit bigint Number of blocks not prefetched because they were already in the buffer pool

skip_init bigint Number of blocks not prefetched because they would be zero-initialized

skip_new bigint Number of blocks not prefetched because they didn't exist yet

skip_fpw bigint Number of blocks not prefetched because a full page image was included in the WAL

skip_rep bigint Number of blocks not prefetched because they were already recently prefetched

wal_distance int How many bytes ahead the prefetcher is looking

block_distance int How many blocks ahead the prefetcher is looking

io_depth int How many prefetches have been initiated but are not yet known to have completed

28.2.8. pg_stat_subscription

Table 28.18. 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

28.2.9. pg_stat_subscription_stats

The pg_stat_subscription_stats view will contain one row per subscription.

Table 28.19. pg_stat_subscription_stats View

Column Type Description

subid oid OID of the subscription

subname name Name of the subscription

apply_error_count bigint Number of times an error occurred while applying changes

sync_error_count bigint Number of times an error occurred during the initial table synchronization

stats_reset timestamp with time zone Time at which these statistics were last reset

28.2.10. 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 28.20. 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

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.

28.2.11. 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 28.21. 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

28.2.12. pg_stat_archiver

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

Table 28.22. 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 WAL file most recently successfully archived

last_archived_time timestamp with time zone Time of the most recent 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 most recent failed archival operation

last_failed_time timestamp with time zone Time of the most recent failed archival operation

stats_reset timestamp with time zone Time at which these statistics were last reset

Normally, WAL files are archived in order, oldest to newest, but that is not guaranteed, and does not hold under special circumstances like when promoting a standby or after crash recovery. Therefore it is not safe to assume that all files older than last_archived_wal have also been successfully archived.

28.2.13. pg_stat_bgwriter

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

Table 28.23. 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

28.2.14. pg_stat_wal

The pg_stat_wal view will always have a single row, containing data about WAL activity of the cluster.

Table 28.24. pg_stat_wal View

Column Type Description

wal_records bigint Total number of WAL records generated

wal_fpi bigint Total number of WAL full page images generated

wal_bytes numeric Total amount of WAL generated in bytes

wal_buffers_full bigint Number of times WAL data was written to disk because WAL buffers became full

wal_write bigint Number of times WAL buffers were written out to disk via XLogWrite request. See Section 30.5 for more information about the internal WAL function XLogWrite.

wal_sync bigint Number of times WAL files were synced to disk via issue_xlog_fsync request (if fsync is on and wal_sync_method is either fdatasync, fsync or fsync_writethrough, otherwise zero). See Section 30.5 for more information about the internal WAL function issue_xlog_fsync.

wal_write_time double precision Total amount of time spent writing WAL buffers to disk via XLogWrite request, in milliseconds (if track_wal_io_timing is enabled, otherwise zero). This includes the sync time when wal_sync_method is either open_datasync or open_sync.

wal_sync_time double precision Total amount of time spent syncing WAL files to disk via issue_xlog_fsync request, in milliseconds (if track_wal_io_timing is enabled, fsync is on, and wal_sync_method is either fdatasync, fsync or fsync_writethrough, otherwise zero).

stats_reset timestamp with time zone Time at which these statistics were last reset

28.2.15. 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 28.25. 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 live rows fetched by sequential scans and index entries returned by index scans in this database

tup_fetched bigint Number of live rows fetched by index scans 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)

session_time double precision Time spent by database sessions in this database, in milliseconds (note that statistics are only updated when the state of a session changes, so if sessions have been idle for a long time, this idle time won't be included)

active_time double precision Time spent executing SQL statements in this database, in milliseconds (this corresponds to the states active and fastpath function call in pg_stat_activity)

idle_in_transaction_time double precision Time spent idling while in a transaction in this database, in milliseconds (this corresponds to the states idle in transaction and idle in transaction (aborted) in pg_stat_activity)

sessions bigint Total number of sessions established to this database

sessions_abandoned bigint Number of database sessions to this database that were terminated because connection to the client was lost

sessions_fatal bigint Number of database sessions to this database that were terminated by fatal errors

sessions_killed bigint Number of database sessions to this database that were terminated by operator intervention

stats_reset timestamp with time zone Time at which these statistics were last reset

28.2.16. 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 primary servers.

Table 28.26. 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

28.2.17. 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 28.27. 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

28.2.18. 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 28.28. 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.

28.2.19. 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 28.29. 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)

28.2.20. 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 28.30. 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

28.2.21. 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 28.31. 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

28.2.22. 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 28.32. 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

28.2.23. 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 28.33. 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

28.2.24. 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 the cumulative statistics system are listed in Table 28.34.

Table 28.34. 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, or NULL if no statistics snapshot has been taken. A snapshot is taken the first time cumulative statistics are accessed in a transaction if stats_fetch_consistency is set to snapshot

pg_stat_clear_snapshot () → void Discards the current statistics snapshot or cached information.

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, archiver to reset all the counters shown in the pg_stat_archiver view, wal to reset all the counters shown in the pg_stat_wal view or recovery_prefetch to reset all the counters shown in the pg_stat_recovery_prefetch 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 or shared across all databases in the cluster 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_reset_replication_slot ( text ) → void Resets statistics of the replication slot defined by the argument. If the argument is NULL, resets statistics for all the replication slots. This function is restricted to superusers by default, but other users can be granted EXECUTE to run the function.

pg_stat_reset_subscription_stats ( oid ) → void Resets statistics for a single subscription shown in the pg_stat_subscription_stats view to zero. If the argument is NULL, reset statistics for all subscriptions. This function is restricted to superusers by default, but other users can be granted EXECUTE to run the function.

Warning

Using pg_stat_reset() also resets counters that autovacuum uses to determine when to trigger a vacuum or an analyze. Resetting these counters can cause autovacuum to not perform necessary work, which can cause problems such as table bloat or out-dated table statistics. A database-wide ANALYZE is recommended after the statistics have been reset.

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 28.35. 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 28.35. 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 28.4 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 28.5 through Table 28.13.

pg_stat_get_backend_xact_start ( integer ) → timestamp with time zone Returns the time when the backend's current transaction was started.