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28.2. 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.

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 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.

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 results of statistics collection. Alternatively, one can build custom views using the underlying statistics functions, as discussed in Section 28.2.3.

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.

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_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_progress_vacuum One row for each backend (including autovacuum worker processes) running VACUUM, showing current progress. See Section 28.4.1.

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_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).

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.

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
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
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. Possible values are:
  • LWLock: The backend is waiting for a lightweight lock. Each such lock protects 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.)

  • Lock: The backend 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.

  • BufferPin: The server process is waiting to access to a data buffer during a period when no other process can be examining that buffer. Buffer pin waits can be protracted if another process holds an open cursor which last read data from the buffer in question.

  • Activity: The server process is idle. This is used by system processes waiting for activity in their main processing loop. wait_event will identify the specific wait point.

  • Extension: The server process is waiting for activity in an extension module. This category is useful for modules to track custom waiting points.

  • Client: The server process is waiting for some activity on a socket from user applications, and that the server expects something to happen that is independent from its internal processes. wait_event will identify the specific wait point.

  • IPC: The server process is waiting for some activity from another process in the server. wait_event will identify the specific wait point.

  • Timeout: The server process is waiting for a timeout to expire. wait_event will identify the specific wait point.

  • IO: The server process is waiting for a IO to complete. wait_event will identify the specific wait point.

wait_event text Wait event name if backend is currently waiting, otherwise NULL. See Table 28.4 for details.
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 characters; 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, background worker, background writer, client backend, checkpointer, startup, walreceiver, walsender and walwriter.

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

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 Description

Wait Event Type Wait Event Name Description
LWLock ShmemIndexLock Waiting to find or allocate space in shared memory.
OidGenLock Waiting to allocate or assign an OID.
XidGenLock Waiting to allocate or assign a transaction id.
ProcArrayLock Waiting to get a snapshot or clearing a transaction id at transaction end.
SInvalReadLock Waiting to retrieve or remove messages from shared invalidation queue.
SInvalWriteLock Waiting to add a message in shared invalidation queue.
WALBufMappingLock Waiting to replace a page in WAL buffers.
WALWriteLock Waiting for WAL buffers to be written to disk.
ControlFileLock Waiting to read or update the control file or creation of a new WAL file.
CheckpointLock Waiting to perform checkpoint.
CLogControlLock Waiting to read or update transaction status.
SubtransControlLock Waiting to read or update sub-transaction information.
MultiXactGenLock Waiting to read or update shared multixact state.
MultiXactOffsetControlLock Waiting to read or update multixact offset mappings.
MultiXactMemberControlLock Waiting to read or update multixact member mappings.
RelCacheInitLock Waiting to read or write relation cache initialization file.
CheckpointerCommLock Waiting to manage fsync requests.
TwoPhaseStateLock Waiting to read or update the state of prepared transactions.
TablespaceCreateLock Waiting to create or drop the tablespace.
BtreeVacuumLock Waiting to read or update vacuum-related information for a B-tree index.
AddinShmemInitLock Waiting to manage space allocation in shared memory.
AutovacuumLock Autovacuum worker or launcher waiting to update or read the current state of autovacuum workers.
AutovacuumScheduleLock Waiting to ensure that the table it has selected for a vacuum still needs vacuuming.
SyncScanLock Waiting to get the start location of a scan on a table for synchronized scans.
RelationMappingLock Waiting to update the relation map file used to store catalog to filenode mapping.
AsyncCtlLock Waiting to read or update shared notification state.
AsyncQueueLock Waiting to read or update notification messages.
SerializableXactHashLock Waiting to retrieve or store information about serializable transactions.
SerializableFinishedListLock Waiting to access the list of finished serializable transactions.
SerializablePredicateLockListLock Waiting to perform an operation on a list of locks held by serializable transactions.
OldSerXidLock Waiting to read or record conflicting serializable transactions.
SyncRepLock Waiting to read or update information about synchronous replicas.
BackgroundWorkerLock Waiting to read or update background worker state.
DynamicSharedMemoryControlLock Waiting to read or update dynamic shared memory state.
AutoFileLock Waiting to update the postgresql.auto.conf file.
ReplicationSlotAllocationLock Waiting to allocate or free a replication slot.
ReplicationSlotControlLock Waiting to read or update replication slot state.
CommitTsControlLock Waiting to read or update transaction commit timestamps.
CommitTsLock Waiting to read or update the last value set for the transaction timestamp.
ReplicationOriginLock Waiting to setup, drop or use replication origin.
MultiXactTruncationLock Waiting to read or truncate multixact information.
OldSnapshotTimeMapLock Waiting to read or update old snapshot control information.
CLogTruncationLock Waiting to truncate the write-ahead log or waiting for write-ahead log truncation to finish.
clog Waiting for I/O on a clog (transaction status) buffer.
commit_timestamp Waiting for I/O on commit timestamp buffer.
subtrans Waiting for I/O a subtransaction buffer.
multixact_offset Waiting for I/O on a multixact offset buffer.
multixact_member Waiting for I/O on a multixact_member buffer.
async Waiting for I/O on an async (notify) buffer.
oldserxid Waiting to I/O on an oldserxid buffer.
wal_insert Waiting to insert WAL into a memory buffer.
buffer_content Waiting to read or write a data page in memory.
buffer_io Waiting for I/O on a data page.
replication_origin Waiting to read or update the replication progress.
replication_slot_io Waiting for I/O on a replication slot.
proc Waiting to read or update the fast-path lock information.
buffer_mapping Waiting to associate a data block with a buffer in the buffer pool.
lock_manager Waiting to add or examine locks for backends, or waiting to join or exit a locking group (used by parallel query).
predicate_lock_manager Waiting to add or examine predicate lock information.
parallel_query_dsa Waiting for parallel query dynamic shared memory allocation lock.
tbm Waiting for TBM shared iterator lock.
Lock relation Waiting to acquire a lock on a relation.
extend Waiting to extend a relation.
page Waiting to acquire a lock on page of a relation.
tuple Waiting to acquire a lock on a tuple.
transactionid Waiting for a transaction to finish.
virtualxid Waiting to acquire a virtual xid lock.
speculative token Waiting to acquire a speculative insertion lock.
object Waiting to acquire a lock on a non-relation database object.
userlock Waiting to acquire a user lock.
advisory Waiting to acquire an advisory user lock.
BufferPin BufferPin Waiting to acquire a pin on a buffer.
Activity ArchiverMain Waiting in main loop of the 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 background worker.
CheckpointerMain Waiting in main loop of checkpointer process.
LogicalLauncherMain Waiting in main loop of logical launcher process.
LogicalApplyMain Waiting in main loop of logical apply process.
PgStatMain Waiting in main loop of the statistics collector process.
RecoveryWalAll Waiting for WAL from any kind of source (local, archive or stream) at recovery.
RecoveryWalStream Waiting for WAL from a stream at 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.
Client ClientRead Waiting to read data from the client.
ClientWrite Waiting to write data from the client.
LibPQWalReceiverConnect Waiting in WAL receiver to establish connection to remote server.
LibPQWalReceiverReceive Waiting in WAL receiver to receive data from remote server.
SSLOpenServer Waiting for SSL while attempting connection.
WalReceiverWaitStart Waiting for startup process to send initial data for streaming replication.
WalSenderWaitForWAL Waiting for WAL to be flushed in WAL sender process.
WalSenderWriteData Waiting for any activity when processing replies from WAL receiver in WAL sender process.
Extension Extension Waiting in an extension.
IPC 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.
ExecuteGather Waiting for activity from child process when executing Gather node.
LogicalSyncData Waiting for logical replication remote server to send data for initial table synchronization.
LogicalSyncStateChange Waiting for logical replication remote server to change state.
MessageQueueInternal Waiting for other process to be attached in shared message queue.
MessageQueuePutMessage Waiting to write a protocol message to a shared message queue.
MessageQueueReceive Waiting to receive bytes from a shared message queue.
MessageQueueSend Waiting to send bytes to a shared message queue.
ParallelFinish Waiting for parallel workers to finish computing.
ParallelBitmapScan Waiting for parallel bitmap scan to become initialized.
ProcArrayGroupUpdate Waiting for group leader to clear transaction id at transaction end.
ReplicationOriginDrop Waiting for a replication origin to become inactive to be dropped.
ReplicationSlotDrop Waiting for a replication slot to become inactive to be dropped.
SafeSnapshot Waiting for a snapshot for a READ ONLY DEFERRABLE transaction.
SyncRep Waiting for confirmation from remote server during synchronous replication.
Timeout BaseBackupThrottle Waiting during base backup when throttling activity.
PgSleep Waiting in process that called pg_sleep.
RecoveryApplyDelay Waiting to apply WAL at recovery because it is delayed.
IO BufFileRead Waiting for a read from a buffered file.
BufFileWrite Waiting for a write to a buffered file.
ControlFileRead Waiting for a read from the control file.
ControlFileSync Waiting for the control file to reach stable storage.
ControlFileSyncUpdate Waiting for an update to the control file to reach stable storage.
ControlFileWrite Waiting for a write to the control file.
ControlFileWriteUpdate Waiting for a write to update the control file.
CopyFileRead Waiting for a read during a file copy operation.
CopyFileWrite Waiting for a write during a file copy operation.
DataFileExtend Waiting for a relation data file to be extended.
DataFileFlush Waiting for a relation data file to reach stable storage.
DataFileImmediateSync Waiting for an immediate synchronization of a relation data file to stable storage.
DataFilePrefetch Waiting for an asynchronous prefetch from a relation data file.
DataFileRead Waiting for a read from a relation data file.
DataFileSync Waiting for changes to a relation data file to reach stable storage.
DataFileTruncate Waiting for a relation data file to be truncated.
DataFileWrite Waiting for a write to a relation data file.
DSMFillZeroWrite Waiting to write zero bytes to a dynamic shared memory backing file.
LockFileAddToDataDirRead Waiting for a read while adding a line to the data directory lock file.
LockFileAddToDataDirSync Waiting for data to reach stable storage while adding a line to the data directory lock file.
LockFileAddToDataDirWrite Waiting for a write while adding a line to the data directory lock file.
LockFileCreateRead Waiting to read while creating the data directory lock file.
LockFileCreateSync Waiting for data to reach stable storage while creating the data directory lock file.
LockFileCreateWrite Waiting for a write while creating the data directory lock file.
LockFileReCheckDataDirRead Waiting for a read during recheck of the data directory lock file.
LogicalRewriteCheckpointSync Waiting for logical rewrite mappings to reach stable storage during a checkpoint.
LogicalRewriteMappingSync Waiting for mapping data to reach stable storage during a logical rewrite.
LogicalRewriteMappingWrite Waiting for a write of mapping data during a logical rewrite.
LogicalRewriteSync Waiting for logical rewrite mappings to reach stable storage.
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 stable storage.
RelationMapWrite Waiting for a write to the relation map file.
ReorderBufferRead Waiting for a read during reorder buffer management.
ReorderBufferWrite Waiting for a write during reorder buffer management.
ReorderLogicalMappingRead Waiting for a read of a logical mapping during reorder buffer management.
ReplicationSlotRead Waiting for a read from a replication slot control file.
ReplicationSlotRestoreSync Waiting for a replication slot control file to reach stable storage while restoring it to memory.
ReplicationSlotSync Waiting for a replication slot control file to reach stable storage.
ReplicationSlotWrite Waiting for a write to a replication slot control file.
SLRUFlushSync Waiting for SLRU data to reach stable storage during a checkpoint or database shutdown.
SLRURead Waiting for a read of an SLRU page.
SLRUSync Waiting for SLRU data to reach stable storage following a page write.
SLRUWrite Waiting for a write of an SLRU page.
SnapbuildRead Waiting for a read of a serialized historical catalog snapshot.
SnapbuildSync Waiting for a serialized historical catalog snapshot to reach stable storage.
SnapbuildWrite Waiting for a write of a serialized historical catalog snapshot.
TimelineHistoryFileSync Waiting for a timeline history file received via streaming replication to reach stable storage.
TimelineHistoryFileWrite Waiting for a write of a timeline history file received via streaming replication.
TimelineHistoryRead Waiting for a read of a timeline history file.
TimelineHistorySync Waiting for a newly created timeline history file to reach stable storage.
TimelineHistoryWrite Waiting for a write of a newly created timeline history file.
TwophaseFileRead Waiting for a read of a two phase state file.
TwophaseFileSync Waiting for a two phase state file to reach stable storage.
TwophaseFileWrite Waiting for a write of a two phase state file.
WALBootstrapSync Waiting for WAL to reach stable storage during bootstrapping.
WALBootstrapWrite Waiting for a write of a WAL page during bootstrapping.
WALCopyRead Waiting for a read when creating a new WAL segment by copying an existing one.
WALCopySync Waiting a new WAL segment created by copying an existing one to reach stable storage.
WALCopyWrite Waiting for a write when creating a new WAL segment by copying an existing one.
WALInitSync Waiting for a newly initialized WAL file to reach stable storage.
WALInitWrite Waiting for a write while initializing a new WAL file.
WALRead Waiting for a read from a WAL file.
WALSenderTimelineHistoryRead Waiting for a read from a timeline history file during walsender timeline command.
WALSyncMethodAssign Waiting for data to reach stable storage while assigning WAL sync method.
WALWrite Waiting for a write to a WAL file.

Note

For tranches registered by extensions, the name is specified by extension and this will be displayed as wait_event. It is quite possible that user has registered the tranche in one of the backends (by having allocation in dynamic shared memory) in which case other backends won't have that information, so we display extension for such cases.

Here is an example of how wait events can be viewed

SELECT pid, wait_event_type, wait_event FROM pg_stat_activity WHERE wait_event is NOT NULL;
 pid  | wait_event_type |  wait_event
------+-----------------+---------------
 2540 | Lock            | relation
 6644 | LWLock          | ProcArrayLock
(2 rows)

Table 28.5. 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 remote_flush 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.


The pg_stat_replication view will contain one row per WAL sender process, showing statistics about replication to that sender's connected standby server. Only directly connected standbys are listed; no information is available about downstream standby servers.

The lag times reported in the pg_stat_replication view are measurements of the time taken for recent WAL to be written, flushed and replayed and for the sender to know about it. These times represent the commit delay that was (or would have been) introduced by each synchronous commit level, if the remote server was configured as a synchronous standby. For an asynchronous standby, the replay_lag column approximates the delay before recent transactions became visible to queries. If the standby server has entirely caught up with the sending server and there is no more WAL activity, the most recently measured lag times will continue to be displayed for a short time and then show NULL.

Lag times work automatically for physical replication. Logical decoding plugins may optionally emit tracking messages; if they do not, the tracking mechanism will simply display NULL lag.

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.

Table 28.6. 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
received_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
conninfo text Connection string used by this WAL receiver, with security-sensitive fields obfuscated.

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

Table 28.7. pg_stat_subscription View

Column Type Description
subid oid OID of the subscription
subname text Name of the subscription
pid integer Process ID of the subscription worker process
relid Oid OID of the relation that the worker is synchronizing; null for the main apply worker
received_lsn pg_lsn Last write-ahead log location received, the initial value of this field being 0
last_msg_send_time timestamp with time zone Send time of last message received from origin WAL sender
last_msg_receipt_time timestamp with time zone Receipt time of last message received from origin WAL sender
latest_end_lsn pg_lsn Last write-ahead log location reported to origin WAL sender
latest_end_time timestamp with time zone Time of last write-ahead log location reported to origin WAL sender

The pg_stat_subscription view will contain one row per subscription for main worker (with null PID if the worker is not running), and additional rows for workers handling the initial data copy of the subscribed tables.

Table 28.8. 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
clientdn 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)

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 28.9. 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

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

Table 28.10. 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

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

Table 28.11. pg_stat_database View

Column Type Description
datid oid OID of a database
datname name Name of this database
numbackends integer Number of backends currently connected to this database. 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
blk_read_time double precision Time spent reading data file blocks by backends in this database, in milliseconds
blk_write_time double precision Time spent writing data file blocks by backends in this database, in milliseconds
stats_reset timestamp with time zone Time at which these statistics were last reset

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

Table 28.12. 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

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 28.13. 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
last_vacuum timestamp with time zone Last time at which this table was manually vacuumed (not counting VACUUM FULL)
last_autovacuum timestamp with time zone Last time at which this table was vacuumed by the autovacuum daemon
last_analyze timestamp with time zone Last time at which this table was manually analyzed
last_autoanalyze timestamp with time zone Last time at which this table was analyzed by the autovacuum daemon
vacuum_count bigint Number of times this table has been manually vacuumed (not counting VACUUM FULL)
autovacuum_count bigint Number of times this table has been vacuumed by the autovacuum daemon
analyze_count bigint Number of times this table has been manually analyzed
autoanalyze_count bigint Number of times this table has been analyzed by the autovacuum daemon

The pg_stat_all_tables view will contain one row for each table in the current database (including TOAST tables), showing statistics about accesses to that specific table. The pg_stat_user_tables and pg_stat_sys_tables views contain the same information, but filtered to only show user and system tables respectively.

Table 28.14. 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

The pg_stat_all_indexes view will contain one row for each index in the current database, showing statistics about accesses to that specific index. The pg_stat_user_indexes and pg_stat_sys_indexes views contain the same information, but filtered to only show user and system indexes respectively.

Indexes can be used by simple index scans, bitmap index scans, and the optimizer. In a bitmap scan the output of several indexes can be combined via AND or OR rules, so it is difficult to associate individual heap row fetches with specific indexes when a bitmap scan is used. Therefore, a bitmap scan increments the pg_stat_all_indexes.idx_tup_read count(s) for the index(es) it uses, and it increments the pg_stat_all_tables.idx_tup_fetch count for the table, but it does not affect pg_stat_all_indexes.idx_tup_fetch. The optimizer also accesses indexes to check for supplied constants whose values are outside the recorded range of the optimizer statistics because the optimizer statistics might be stale.

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.

Table 28.15. 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)

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 28.16. 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

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 28.17. 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

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 28.18. 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

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.

28.2.3. 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 28.19.

Table 28.19. Additional Statistics Functions

Function Return Type Description
pg_backend_pid() integer Process ID of the server process handling the current session
pg_stat_get_activity(integer) setof record Returns a record of information about the backend with the specified PID, or one record for each active backend in the system if NULL is specified. The fields returned are a subset of those in the pg_stat_activity view.
pg_stat_get_snapshot_timestamp() timestamp with time zone Returns the timestamp of the current statistics snapshot
pg_stat_clear_snapshot() void Discard the current statistics snapshot
pg_stat_reset() void Reset all statistics counters for the current database to zero (requires superuser privileges by default, but EXECUTE for this function can be granted to others.)
pg_stat_reset_shared(text) void Reset some cluster-wide statistics counters to zero, depending on the argument (requires superuser privileges by default, but EXECUTE for this function can be granted to others). Calling pg_stat_reset_shared('bgwriter') will zero all the counters shown in the pg_stat_bgwriter view. Calling pg_stat_reset_shared('archiver') will zero all the counters shown in the pg_stat_archiver view.
pg_stat_reset_single_table_counters(oid) void Reset statistics for a single table or index in the current database to zero (requires superuser privileges by default, but EXECUTE for this function can be granted to others)
pg_stat_reset_single_function_counters(oid) void Reset statistics for a single function in the current database to zero (requires superuser privileges by default, but EXECUTE for this function can be granted to others)

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 28.20. 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 28.20. Per-Backend Statistics Functions

Function Return Type Description
pg_stat_get_backend_idset() setof integer Set of currently active backend ID numbers (from 1 to the number of active backends)
pg_stat_get_backend_activity(integer) text Text of this backend's most recent query
pg_stat_get_backend_activity_start(integer) timestamp with time zone Time when the most recent query was started
pg_stat_get_backend_client_addr(integer) inet IP address of the client connected to this backend
pg_stat_get_backend_client_port(integer) integer TCP port number that the client is using for communication
pg_stat_get_backend_dbid(integer) oid OID of the database this backend is connected to
pg_stat_get_backend_pid(integer) integer Process ID of this backend
pg_stat_get_backend_start(integer) timestamp with time zone Time when this process was started
pg_stat_get_backend_userid(integer) oid OID of the user logged into this backend
pg_stat_get_backend_wait_event_type(integer) text Wait event type name if backend is currently waiting, otherwise NULL. See Table 28.4 for details.
pg_stat_get_backend_wait_event(integer) text Wait event name if backend is currently waiting, otherwise NULL. See Table 28.4 for details.
pg_stat_get_backend_xact_start(integer) timestamp with time zone Time when the current transaction was started

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