27.2. 統計資訊收集器

版本：11

PostgreSQL's statistics collector is a subsystem that supports collection and reporting of information about server activity. Presently, the collector can count accesses to tables and indexes in both disk-block and individual-row terms. It also tracks the total number of rows in each table, and information about vacuum and analyze actions for each table. It can also count calls to user-defined functions and the total time spent in each one.

PostgreSQL also supports reporting dynamic information about exactly what is going on in the system right now, such as the exact command currently being executed by other server processes, and which other connections exist in the system. This facility is independent of the collector process.

27.2.1. Statistics Collection Configuration

Since collection of statistics adds some overhead to query execution, the system can be configured to collect or not collect information. This is controlled by configuration parameters that are normally set in postgresql.conf. (See Chapter 19 for details about setting configuration parameters.)

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

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

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

The parameter track_io_timing enables monitoring of block read and write times.

Normally these parameters are set in postgresql.conf so that they apply to all server processes, but it is possible to turn them on or off in individual sessions using the SET command. (To prevent ordinary users from hiding their activity from the administrator, only superusers are allowed to change these parameters with SET.)

The statistics collector transmits the collected information to other PostgreSQL processes through temporary files. These files are stored in the directory named by the stats_temp_directory parameter, pg_stat_tmp by default. For better performance, stats_temp_directory can be pointed at a RAM-based file system, decreasing physical I/O requirements. When the server shuts down cleanly, a permanent copy of the statistics data is stored in the pg_stat subdirectory, so that statistics can be retained across server restarts. When recovery is performed at server start (e.g., after immediate shutdown, server crash, and point-in-time recovery), all statistics counters are reset.

27.2.2. Viewing Statistics

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

When using the statistics to monitor collected data, it is important to realize that the information does not update instantaneously. Each individual server process transmits new statistical counts to the collector just before going idle; so a query or transaction still in progress does not affect the displayed totals. Also, the collector itself emits a new report at most once per PGSTAT_STAT_INTERVAL milliseconds (500 ms unless altered while building the server). So the displayed information lags behind actual activity. However, current-query information collected by track_activities is always up-to-date.

Another important point is that when a server process is asked to display any of these statistics, it first fetches the most recent report emitted by the collector process and then continues to use this snapshot for all statistical views and functions until the end of its current transaction. So the statistics will show static information as long as you continue the current transaction. Similarly, information about the current queries of all sessions is collected when any such information is first requested within a transaction, and the same information will be displayed throughout the transaction. This is a feature, not a bug, because it allows you to perform several queries on the statistics and correlate the results without worrying that the numbers are changing underneath you. But if you want to see new results with each query, be sure to do the queries outside any transaction block. Alternatively, you can invoke pg_stat_clear_snapshot(), which will discard the current transaction's statistics snapshot (if any). The next use of statistical information will cause a new snapshot to be fetched.

A transaction can also see its own statistics (as yet untransmitted to the collector) in the views pg_stat_xact_all_tables, pg_stat_xact_sys_tables, pg_stat_xact_user_tables, and pg_stat_xact_user_functions. These numbers do not act as stated above; instead they update continuously throughout the transaction.

Some of the information in the dynamic statistics views shown in Table 27.1 is security restricted. Ordinary users can only see all the information about their own sessions (sessions belonging to a role that they are a member of). In rows about other sessions, many columns will be null. Note, however, that the existence of a session and its general properties such as its sessions user and database are visible to all users. Superusers and members of the built-in role pg_read_all_stats (see also Section 21.5) can see all the information about all sessions.

Table 27.1. Dynamic Statistics Views

View Name

Description

pg_stat_activity

One row per server process, showing information related to the current activity of that process, such as state and current query. See for details.

pg_stat_replication

One row per WAL sender process, showing statistics about replication to that sender's connected standby server. See for details.

pg_stat_wal_receiver

Only one row, showing statistics about the WAL receiver from that receiver's connected server. See for details.

pg_stat_subscription

At least one row per subscription, showing information about the subscription workers. See for details.

pg_stat_ssl

One row per connection (regular and replication), showing information about SSL used on this connection. See for details.

pg_stat_gssapi

One row per connection (regular and replication), showing information about GSSAPI authentication and encryption used on this connection. See for details.

pg_stat_progress_analyze

One row for each backend (including autovacuum worker processes) running ANALYZE, showing current progress. See .

pg_stat_progress_create_index

One row for each backend running CREATE INDEX or REINDEX, showing current progress. See .

pg_stat_progress_vacuum

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

pg_stat_progress_cluster

One row for each backend running CLUSTER or VACUUM FULL, showing current progress. See .

pg_stat_progress_basebackup

One row for each WAL sender process streaming a base backup, showing current progress. See .

Table 27.2. Collected Statistics Views

View Name

Description

pg_stat_archiver

One row only, showing statistics about the WAL archiver process's activity. See for details.

pg_stat_bgwriter

One row only, showing statistics about the background writer process's activity. See for details.

pg_stat_database

One row per database, showing database-wide statistics. See 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 for details.

pg_stat_all_tables

目前資料庫中每個資料表為一筆資料，顯示有關對該資料表的存取統計數據。有關詳細資訊，請參閱。

pg_stat_sys_tables

除了僅顯示系統資料表之外，與 pg_stat_all_tables 相同。

pg_stat_user_tables

除了僅顯示使用者資料表之外，與 pg_stat_all_tables 相同。

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 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 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 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 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 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 for details.

The per-index statistics are particularly useful to determine which indexes are being used and how effective they are.

The pg_statio_ views are primarily useful to determine the effectiveness of the buffer cache. When the number of actual disk reads is much smaller than the number of buffer hits, then the cache is satisfying most read requests without invoking a kernel call. However, these statistics do not give the entire story: due to the way in which PostgreSQL handles disk I/O, data that is not in the PostgreSQL buffer cache might still reside in the kernel's I/O cache, and might therefore still be fetched without requiring a physical read. Users interested in obtaining more detailed information on PostgreSQL I/O behavior are advised to use the PostgreSQL statistics collector in combination with operating system utilities that allow insight into the kernel's handling of I/O.

27.2.3. `pg_stat_activity`

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

Table 27.3. `pg_stat_activity` View

Column Type

Description

datid oid

OID of the database this backend is connected to

datname name

Name of the database this backend is connected to

pid integer

Process ID of this backend

leader_pid integer

Process ID of the parallel group leader, if this process is a parallel query worker. NULL if this process is a parallel group leader or does not participate in parallel query.

usesysid oid

OID of the user logged into this backend

usename name

Name of the user logged into this backend

application_name text

Name of the application that is connected to this backend

client_addr inet

IP address of the client connected to this backend. If this field is null, it indicates either that the client is connected via a Unix socket on the server machine or that this is an internal process such as autovacuum.

client_hostname text

Host name of the connected client, as reported by a reverse DNS lookup of client_addr. This field will only be non-null for IP connections, and only when 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 .

wait_event text

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

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 is disabled in this backend.

backend_xid xid

Top-level transaction identifier of this backend, if any.

backend_xmin xid

The current backend's xmin horizon.

query text

Text of this backend's most recent query. If state is active this field shows the currently executing query. In all other states, it shows the last query that was executed. By default the query text is truncated at 1024 bytes; this value can be changed via the parameter .

backend_type text

Type of current backend. Possible types are autovacuum launcher, autovacuum worker, logical replication launcher, logical replication worker, parallel worker, background writer, client backend, checkpointer, startup, walreceiver, walsender and walwriter. In addition, background workers registered by extensions may have additional types.

Note

The wait_event and state columns are independent. If a backend is in the active state, it may or may not be waiting on some event. If the state is active and wait_event is non-null, it means that a query is being executed, but is being blocked somewhere in the system.

Table 27.4. Wait Event Types

Wait Event Type

Description

Activity

The server process is idle. This event type indicates a process waiting for activity in its main processing loop. wait_event will identify the specific wait point; see .

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 .

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 .

Extension

The server process is waiting for some condition defined by an extension module. See .

IO

The server process is waiting for an I/O operation to complete. wait_event will identify the specific wait point; see .

IPC

The server process is waiting for some interaction with another server process. wait_event will identify the specific wait point; see .

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 .

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 .

Timeout

The server process is waiting for a timeout to expire. wait_event will identify the specific wait point; see .

Table 27.5. Wait Events of Type `Activity`

Activity Wait Event

Description

ArchiverMain

Waiting in main loop of archiver process.

AutoVacuumMain

Waiting in main loop of autovacuum launcher process.

BgWriterHibernate

Waiting in background writer process, hibernating.

BgWriterMain

Waiting in main loop of background writer process.

CheckpointerMain

Waiting in main loop of checkpointer process.

LogicalApplyMain

Waiting in main loop of logical replication apply process.

LogicalLauncherMain

Waiting in main loop of logical replication launcher process.

PgStatMain

Waiting in main loop of statistics collector process.

RecoveryWalStream

Waiting in main loop of startup process for WAL to arrive, during streaming recovery.

SysLoggerMain

Waiting in main loop of syslogger process.

WalReceiverMain

Waiting in main loop of WAL receiver process.

WalSenderMain

Waiting in main loop of WAL sender process.

WalWriterMain

Waiting in main loop of WAL writer process.

Table 27.6. Wait Events of Type `BufferPin`

BufferPin Wait Event

Description

BufferPin

Waiting to acquire an exclusive pin on a buffer.

Table 27.7. Wait Events of Type `Client`

Client Wait Event

Description

ClientRead

Waiting to read data from the client.

ClientWrite

Waiting to write data to the client.

GSSOpenServer

Waiting to read data from the client while establishing a GSSAPI session.

LibPQWalReceiverConnect

Waiting in WAL receiver to establish connection to remote server.

LibPQWalReceiverReceive

Waiting in WAL receiver to receive data from remote server.

SSLOpenServer

Waiting for SSL while attempting connection.

WalReceiverWaitStart

Waiting for startup process to send initial data for streaming replication.

WalSenderWaitForWAL

Waiting for WAL to be flushed in WAL sender process.

WalSenderWriteData

Waiting for any activity when processing replies from WAL receiver in WAL sender process.

Table 27.8. Wait Events of Type `Extension`

Extension Wait Event

Description

Extension

Waiting in an extension.

Table 27.9. Wait Events of Type `IO`

IO Wait Event

Description

BufFileRead

Waiting for a read from a buffered file.

BufFileWrite

Waiting for a write to a buffered file.

ControlFileRead

Waiting for a read from the pg_control file.

ControlFileSync

Waiting for the pg_control file to reach durable storage.

ControlFileSyncUpdate

Waiting for an update to the pg_control file to reach durable storage.

ControlFileWrite

Waiting for a write to the pg_control file.

ControlFileWriteUpdate

Waiting for a write to update the pg_control file.

CopyFileRead

Waiting for a read during a file copy operation.

CopyFileWrite

Waiting for a write during a file copy operation.

DSMFillZeroWrite

Waiting to fill a dynamic shared memory backing file with zeroes.

DataFileExtend

Waiting for a relation data file to be extended.

DataFileFlush

Waiting for a relation data file to reach durable storage.

DataFileImmediateSync

Waiting for an immediate synchronization of a relation data file to durable storage.

DataFilePrefetch

Waiting for an asynchronous prefetch from a relation data file.

DataFileRead

Waiting for a read from a relation data file.

DataFileSync

Waiting for changes to a relation data file to reach durable storage.

DataFileTruncate

Waiting for a relation data file to be truncated.

DataFileWrite

Waiting for a write to a relation data file.

LockFileAddToDataDirRead

Waiting for a read while adding a line to the data directory lock file.

LockFileAddToDataDirSync

Waiting for data to reach durable storage while adding a line to the data directory lock file.

LockFileAddToDataDirWrite

Waiting for a write while adding a line to the data directory lock file.

LockFileCreateRead

Waiting to read while creating the data directory lock file.

LockFileCreateSync

Waiting for data to reach durable storage while creating the data directory lock file.

LockFileCreateWrite

Waiting for a write while creating the data directory lock file.

LockFileReCheckDataDirRead

Waiting for a read during recheck of the data directory lock file.

LogicalRewriteCheckpointSync

Waiting for logical rewrite mappings to reach durable storage during a checkpoint.

LogicalRewriteMappingSync

Waiting for mapping data to reach durable storage during a logical rewrite.

LogicalRewriteMappingWrite

Waiting for a write of mapping data during a logical rewrite.

LogicalRewriteSync

Waiting for logical rewrite mappings to reach durable storage.

LogicalRewriteTruncate

Waiting for truncate of mapping data during a logical rewrite.

LogicalRewriteWrite

Waiting for a write of logical rewrite mappings.

RelationMapRead

Waiting for a read of the relation map file.

RelationMapSync

Waiting for the relation map file to reach durable storage.

RelationMapWrite

Waiting for a write to the relation map file.

ReorderBufferRead

Waiting for a read during reorder buffer management.

ReorderBufferWrite

Waiting for a write during reorder buffer management.

ReorderLogicalMappingRead

Waiting for a read of a logical mapping during reorder buffer management.

ReplicationSlotRead

Waiting for a read from a replication slot control file.

ReplicationSlotRestoreSync

Waiting for a replication slot control file to reach durable storage while restoring it to memory.

ReplicationSlotSync

Waiting for a replication slot control file to reach durable storage.

ReplicationSlotWrite

Waiting for a write to a replication slot control file.

SLRUFlushSync

Waiting for SLRU data to reach durable storage during a checkpoint or database shutdown.

SLRURead

Waiting for a read of an SLRU page.

SLRUSync

Waiting for SLRU data to reach durable storage following a page write.

SLRUWrite

Waiting for a write of an SLRU page.

SnapbuildRead

Waiting for a read of a serialized historical catalog snapshot.

SnapbuildSync

Waiting for a serialized historical catalog snapshot to reach durable storage.

SnapbuildWrite

Waiting for a write of a serialized historical catalog snapshot.

TimelineHistoryFileSync

Waiting for a timeline history file received via streaming replication to reach durable storage.

TimelineHistoryFileWrite

Waiting for a write of a timeline history file received via streaming replication.

TimelineHistoryRead

Waiting for a read of a timeline history file.

TimelineHistorySync

Waiting for a newly created timeline history file to reach durable storage.

TimelineHistoryWrite

Waiting for a write of a newly created timeline history file.

TwophaseFileRead

Waiting for a read of a two phase state file.

TwophaseFileSync

Waiting for a two phase state file to reach durable storage.

TwophaseFileWrite

Waiting for a write of a two phase state file.

WALBootstrapSync

Waiting for WAL to reach durable storage during bootstrapping.

WALBootstrapWrite

Waiting for a write of a WAL page during bootstrapping.

WALCopyRead

Waiting for a read when creating a new WAL segment by copying an existing one.

WALCopySync

Waiting for a new WAL segment created by copying an existing one to reach durable storage.

WALCopyWrite

Waiting for a write when creating a new WAL segment by copying an existing one.

WALInitSync

Waiting for a newly initialized WAL file to reach durable storage.

WALInitWrite

Waiting for a write while initializing a new WAL file.

WALRead

Waiting for a read from a WAL file.

WALSenderTimelineHistoryRead

Waiting for a read from a timeline history file during a walsender timeline command.

WALSync

Waiting for a WAL file to reach durable storage.

WALSyncMethodAssign

Waiting for data to reach durable storage while assigning a new WAL sync method.

WALWrite

Waiting for a write to a WAL file.

Table 27.10. Wait Events of Type `IPC`

IPC Wait Event

Description

BackupWaitWalArchive

Waiting for WAL files required for a backup to be successfully archived.

BgWorkerShutdown

Waiting for background worker to shut down.

BgWorkerStartup

Waiting for background worker to start up.

BtreePage

Waiting for the page number needed to continue a parallel B-tree scan to become available.

CheckpointDone

Waiting for a checkpoint to complete.

CheckpointStart

Waiting for a checkpoint to start.

ExecuteGather

Waiting for activity from a child process while executing a Gather plan node.

HashBatchAllocate

Waiting for an elected Parallel Hash participant to allocate a hash table.

HashBatchElect

Waiting to elect a Parallel Hash participant to allocate a hash table.

HashBatchLoad

Waiting for other Parallel Hash participants to finish loading a hash table.

HashBuildAllocate

Waiting for an elected Parallel Hash participant to allocate the initial hash table.

HashBuildElect

Waiting to elect a Parallel Hash participant to allocate the initial hash table.

HashBuildHashInner

Waiting for other Parallel Hash participants to finish hashing the inner relation.

HashBuildHashOuter

Waiting for other Parallel Hash participants to finish partitioning the outer relation.

HashGrowBatchesAllocate

Waiting for an elected Parallel Hash participant to allocate more batches.

HashGrowBatchesDecide

Waiting to elect a Parallel Hash participant to decide on future batch growth.

HashGrowBatchesElect

Waiting to elect a Parallel Hash participant to allocate more batches.

HashGrowBatchesFinish

Waiting for an elected Parallel Hash participant to decide on future batch growth.

HashGrowBatchesRepartition

Waiting for other Parallel Hash participants to finish repartitioning.

HashGrowBucketsAllocate

Waiting for an elected Parallel Hash participant to finish allocating more buckets.

HashGrowBucketsElect

Waiting to elect a Parallel Hash participant to allocate more buckets.

HashGrowBucketsReinsert

Waiting for other Parallel Hash participants to finish inserting tuples into new buckets.

LogicalSyncData

Waiting for a logical replication remote server to send data for initial table synchronization.

LogicalSyncStateChange

Waiting for a logical replication remote server to change state.

MessageQueueInternal

Waiting for another process to be attached to a shared message queue.

MessageQueuePutMessage

Waiting to write a protocol message to a shared message queue.

MessageQueueReceive

Waiting to receive bytes from a shared message queue.

MessageQueueSend

Waiting to send bytes to a shared message queue.

ParallelBitmapScan

Waiting for parallel bitmap scan to become initialized.

ParallelCreateIndexScan

Waiting for parallel CREATE INDEX workers to finish heap scan.

ParallelFinish

Waiting for parallel workers to finish computing.

ProcArrayGroupUpdate

Waiting for the group leader to clear the transaction ID at end of a parallel operation.

ProcSignalBarrier

Waiting for a barrier event to be processed by all backends.

Promote

Waiting for standby promotion.

RecoveryConflictSnapshot

Waiting for recovery conflict resolution for a vacuum cleanup.

RecoveryConflictTablespace

Waiting for recovery conflict resolution for dropping a tablespace.

RecoveryPause

Waiting for recovery to be resumed.

ReplicationOriginDrop

Waiting for a replication origin to become inactive so it can be dropped.

ReplicationSlotDrop

Waiting for a replication slot to become inactive so it can be dropped.

SafeSnapshot

Waiting to obtain a valid snapshot for a READ ONLY DEFERRABLE transaction.

SyncRep

Waiting for confirmation from a remote server during synchronous replication.

XactGroupUpdate

Waiting for the group leader to update transaction status at end of a parallel operation.

Table 27.11. Wait Events of Type `Lock`

Lock Wait Event

Description

advisory

Waiting to acquire an advisory user lock.

extend

Waiting to extend a relation.

frozenid

Waiting to update pg_database.datfrozenxid and pg_database.datminmxid.

object

Waiting to acquire a lock on a non-relation database object.

page

Waiting to acquire a lock on a page of a relation.

relation

Waiting to acquire a lock on a relation.

spectoken

Waiting to acquire a speculative insertion lock.

transactionid

Waiting for a transaction to finish.

tuple

Waiting to acquire a lock on a tuple.

userlock

Waiting to acquire a user lock.

virtualxid

Waiting to acquire a virtual transaction ID lock.

Table 27.12. Wait Events of Type `LWLock`

LWLock Wait Event

Description

AddinShmemInit

Waiting to manage an extension's space allocation in shared memory.

AutoFile

Waiting to update the postgresql.auto.conf file.

Autovacuum

Waiting to read or update the current state of autovacuum workers.

AutovacuumSchedule

Waiting to ensure that a table selected for autovacuum still needs vacuuming.

BackgroundWorker

Waiting to read or update background worker state.

BtreeVacuum

Waiting to read or update vacuum-related information for a B-tree index.

BufferContent

Waiting to access a data page in memory.

BufferIO

Waiting for I/O on a data page.

BufferMapping

Waiting to associate a data block with a buffer in the buffer pool.

Checkpoint

Waiting to begin a checkpoint.

CheckpointerComm

Waiting to manage fsync requests.

CommitTs

Waiting to read or update the last value set for a transaction commit timestamp.

CommitTsBuffer

Waiting for I/O on a commit timestamp SLRU buffer.

CommitTsSLRU

Waiting to access the commit timestamp SLRU cache.

ControlFile

Waiting to read or update the pg_control file or create a new WAL file.

DynamicSharedMemoryControl

Waiting to read or update dynamic shared memory allocation information.

LockFastPath

Waiting to read or update a process' fast-path lock information.

LockManager

Waiting to read or update information about “heavyweight” locks.

LogicalRepWorker

Waiting to read or update the state of logical replication workers.

MultiXactGen

Waiting to read or update shared multixact state.

MultiXactMemberBuffer

Waiting for I/O on a multixact member SLRU buffer.

MultiXactMemberSLRU

Waiting to access the multixact member SLRU cache.

MultiXactOffsetBuffer

Waiting for I/O on a multixact offset SLRU buffer.

MultiXactOffsetSLRU

Waiting to access the multixact offset SLRU cache.

MultiXactTruncation

Waiting to read or truncate multixact information.

NotifyBuffer

Waiting for I/O on a NOTIFY message SLRU buffer.

NotifyQueue

Waiting to read or update NOTIFY messages.

NotifyQueueTail

Waiting to update limit on NOTIFY message storage.

NotifySLRU

Waiting to access the NOTIFY message SLRU cache.

OidGen

Waiting to allocate a new OID.

OldSnapshotTimeMap

Waiting to read or update old snapshot control information.

ParallelAppend

Waiting to choose the next subplan during Parallel Append plan execution.

ParallelHashJoin

Waiting to synchronize workers during Parallel Hash Join plan execution.

ParallelQueryDSA

Waiting for parallel query dynamic shared memory allocation.

PerSessionDSA

Waiting for parallel query dynamic shared memory allocation.

PerSessionRecordType

Waiting to access a parallel query's information about composite types.

PerSessionRecordTypmod

Waiting to access a parallel query's information about type modifiers that identify anonymous record types.

PerXactPredicateList

Waiting to access the list of predicate locks held by the current serializable transaction during a parallel query.

PredicateLockManager

Waiting to access predicate lock information used by serializable transactions.

ProcArray

Waiting to access the shared per-process data structures (typically, to get a snapshot or report a session's transaction ID).

RelationMapping

Waiting to read or update a pg_filenode.map file (used to track the filenode assignments of certain system catalogs).

RelCacheInit

Waiting to read or update a pg_internal.init relation cache initialization file.

ReplicationOrigin

Waiting to create, drop or use a replication origin.

ReplicationOriginState

Waiting to read or update the progress of one replication origin.

ReplicationSlotAllocation

Waiting to allocate or free a replication slot.

ReplicationSlotControl

Waiting to read or update replication slot state.

ReplicationSlotIO

Waiting for I/O on a replication slot.

SerialBuffer

Waiting for I/O on a serializable transaction conflict SLRU buffer.

SerializableFinishedList

Waiting to access the list of finished serializable transactions.

SerializablePredicateList

Waiting to access the list of predicate locks held by serializable transactions.

SerializableXactHash

Waiting to read or update information about serializable transactions.

SerialSLRU

Waiting to access the serializable transaction conflict SLRU cache.

SharedTidBitmap

Waiting to access a shared TID bitmap during a parallel bitmap index scan.

SharedTupleStore

Waiting to access a shared tuple store during parallel query.

ShmemIndex

Waiting to find or allocate space in shared memory.

SInvalRead

Waiting to retrieve messages from the shared catalog invalidation queue.

SInvalWrite

Waiting to add a message to the shared catalog invalidation queue.

SubtransBuffer

Waiting for I/O on a sub-transaction SLRU buffer.

SubtransSLRU

Waiting to access the sub-transaction SLRU cache.

SyncRep

Waiting to read or update information about the state of synchronous replication.

SyncScan

Waiting to select the starting location of a synchronized table scan.

TablespaceCreate

Waiting to create or drop a tablespace.

TwoPhaseState

Waiting to read or update the state of prepared transactions.

WALBufMapping

Waiting to replace a page in WAL buffers.

WALInsert

Waiting to insert WAL data into a memory buffer.

WALWrite

Waiting for WAL buffers to be written to disk.

WrapLimitsVacuum

Waiting to update limits on transaction id and multixact consumption.

XactBuffer

Waiting for I/O on a transaction status SLRU buffer.

XactSLRU

Waiting to access the transaction status SLRU cache.

XactTruncation

Waiting to execute pg_xact_status or update the oldest transaction ID available to it.

XidGen

Waiting to allocate a new transaction ID.

Note

Extensions can add LWLock types to the list shown in Table 27.12. In some cases, the name assigned by an extension will not be available in all server processes; so an LWLock wait event might be reported as just “extension” rather than the extension-assigned name.

Table 27.13. Wait Events of Type `Timeout`

Timeout Wait Event

Description

BaseBackupThrottle

Waiting during base backup when throttling activity.

PgSleep

Waiting due to a call to pg_sleep or a sibling function.

RecoveryApplyDelay

Waiting to apply WAL during recovery because of a delay setting.

RecoveryRetrieveRetryInterval

Waiting during recovery when WAL data is not available from any source (pg_wal, archive or stream).

VacuumDelay

Waiting in a cost-based vacuum delay point.

Here is an example of how wait events can be viewed:

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

27.2.4. `pg_stat_replication`

pg_stat_replication 檢視表中的每一筆資料表示每個 WAL 發送者的執行程序，顯示有關複製到該發送者的備用伺服器統計資訊。僅列出直接連接的備用資料庫，不會包含其下游備用伺服器的資訊。

Table 27.14. `pg_stat_replication` View

Column Type

Description

pid integer

Process ID of a WAL sender process

usesysid oid

OID of the user logged into this WAL sender process

usename name

Name of the user logged into this WAL sender process

application_name text

Name of the application that is connected to this WAL sender

client_addr inet

IP address of the client connected to this WAL sender. If this field is null, it indicates that the client is connected via a Unix socket on the server machine.

client_hostname text

Host name of the connected client, as reported by a reverse DNS lookup of client_addr. This field will only be non-null for IP connections, and only when 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 .

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

最後從備用伺服器收到回覆訊息的發送時間

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

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

Note

The reported lag times are not predictions of how long it will take for the standby to catch up with the sending server assuming the current rate of replay. Such a system would show similar times while new WAL is being generated, but would differ when the sender becomes idle. In particular, when the standby has caught up completely, pg_stat_replication shows the time taken to write, flush and replay the most recent reported WAL location rather than zero as some users might expect. This is consistent with the goal of measuring synchronous commit and transaction visibility delays for recent write transactions. To reduce confusion for users expecting a different model of lag, the lag columns revert to NULL after a short time on a fully replayed idle system. Monitoring systems should choose whether to represent this as missing data, zero or continue to display the last known value.

27.2.5. `pg_stat_wal_receiver`

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

Table 27.15. `pg_stat_wal_receiver` View

Column Type

Description

pid integer

Process ID of the WAL receiver process

status text

Activity status of the WAL receiver process

receive_start_lsn pg_lsn

First write-ahead log location used when WAL receiver is started

receive_start_tli integer

First timeline number used when WAL receiver is started

written_lsn pg_lsn

Last write-ahead log location already received and written to disk, but not flushed. This should not be used for data integrity checks.

flushed_lsn pg_lsn

Last write-ahead log location already received and flushed to disk, the initial value of this field being the first log location used when WAL receiver is started

received_tli integer

Timeline number of last write-ahead log location received and flushed to disk, the initial value of this field being the timeline number of the first log location used when WAL receiver is started

last_msg_send_time timestamp with time zone

Send time of last message received from origin WAL sender

last_msg_receipt_time timestamp with time zone

Receipt time of last message received from origin WAL sender

latest_end_lsn pg_lsn

Last write-ahead log location reported to origin WAL sender

latest_end_time timestamp with time zone

Time of last write-ahead log location reported to origin WAL sender

slot_name text

Replication slot name used by this WAL receiver

sender_host text

Host of the PostgreSQL instance this WAL receiver is connected to. This can be a host name, an IP address, or a directory path if the connection is via Unix socket. (The path case can be distinguished because it will always be an absolute path, beginning with /.)

sender_port integer

Port number of the PostgreSQL instance this WAL receiver is connected to.

conninfo text

Connection string used by this WAL receiver, with security-sensitive fields obfuscated.

27.2.6. `pg_stat_subscription`

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

Table 27.16. `pg_stat_subscription` View

Column Type

Description

subid oid

OID of the subscription

subname name

Name of the subscription

pid integer

Process ID of the subscription worker process

relid oid

OID of the relation that the worker is synchronizing; null for the main apply worker

received_lsn pg_lsn

Last write-ahead log location received, the initial value of this field being 0

last_msg_send_time timestamp with time zone

Send time of last message received from origin WAL sender

last_msg_receipt_time timestamp with time zone

Receipt time of last message received from origin WAL sender

latest_end_lsn pg_lsn

Last write-ahead log location reported to origin WAL sender

latest_end_time timestamp with time zone

Time of last write-ahead log location reported to origin WAL sender

27.2.7. `pg_stat_ssl`

The pg_stat_ssl view will contain one row per backend or WAL sender process, showing statistics about SSL usage on this connection. It can be joined to pg_stat_activity or pg_stat_replication on the pid column to get more details about the connection.

Table 27.17. `pg_stat_ssl` View

Column Type

Description

pid integer

Process ID of a backend or WAL sender process

ssl boolean

True if SSL is used on this connection

version text

Version of SSL in use, or NULL if SSL is not in use on this connection

cipher text

Name of SSL cipher in use, or NULL if SSL is not in use on this connection

bits integer

Number of bits in the encryption algorithm used, or NULL if SSL is not used on this connection

compression boolean

True if SSL compression is in use, false if not, or NULL if SSL is not in use on this connection

client_dn text

Distinguished Name (DN) field from the client certificate used, or NULL if no client certificate was supplied or if SSL is not in use on this connection. This field is truncated if the DN field is longer than NAMEDATALEN (64 characters in a standard build).

client_serial numeric

Serial number of the client certificate, or NULL if no client certificate was supplied or if SSL is not in use on this connection. The combination of certificate serial number and certificate issuer uniquely identifies a certificate (unless the issuer erroneously reuses serial numbers).

issuer_dn text

DN of the issuer of the client certificate, or NULL if no client certificate was supplied or if SSL is not in use on this connection. This field is truncated like client_dn.

27.2.8. `pg_stat_gssapi`

The pg_stat_gssapi view will contain one row per backend, showing information about GSSAPI usage on this connection. It can be joined to pg_stat_activity or pg_stat_replication on the pid column to get more details about the connection.

Table 27.18. `pg_stat_gssapi` View

Column Type

Description

pid integer

Process ID of a backend

gss_authenticated boolean

True if GSSAPI authentication was used for this connection

principal text

Principal used to authenticate this connection, or NULL if GSSAPI was not used to authenticate this connection. This field is truncated if the principal is longer than NAMEDATALEN (64 characters in a standard build).

encrypted boolean

True if GSSAPI encryption is in use on this connection

27.2.9. `pg_stat_archiver`

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

Table 27.19. `pg_stat_archiver` View

Column Type

Description

archived_count bigint

Number of WAL files that have been successfully archived

last_archived_wal text

Name of the last WAL file successfully archived

last_archived_time timestamp with time zone

Time of the last successful archive operation

failed_count bigint

Number of failed attempts for archiving WAL files

last_failed_wal text

Name of the WAL file of the last failed archival operation

last_failed_time timestamp with time zone

Time of the last failed archival operation

stats_reset timestamp with time zone

Time at which these statistics were last reset

27.2.10. `pg_stat_bgwriter`

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

Table 27.20. `pg_stat_bgwriter` View

Column Type

Description

checkpoints_timed bigint

Number of scheduled checkpoints that have been performed

checkpoints_req bigint

Number of requested checkpoints that have been performed

checkpoint_write_time double precision

Total amount of time that has been spent in the portion of checkpoint processing where files are written to disk, in milliseconds

checkpoint_sync_time double precision

Total amount of time that has been spent in the portion of checkpoint processing where files are synchronized to disk, in milliseconds

buffers_checkpoint bigint

Number of buffers written during checkpoints

buffers_clean bigint

Number of buffers written by the background writer

maxwritten_clean bigint

Number of times the background writer stopped a cleaning scan because it had written too many buffers

buffers_backend bigint

Number of buffers written directly by a backend

buffers_backend_fsync bigint

Number of times a backend had to execute its own fsync call (normally the background writer handles those even when the backend does its own write)

buffers_alloc bigint

Number of buffers allocated

stats_reset timestamp with time zone

Time at which these statistics were last reset

27.2.11. `pg_stat_database`

pg_stat_database 檢視表將為叢集中的每個資料庫綜合為一筆資料，再加上每個共享物件，列出資料庫層級的統計資訊。

Table 27.21. `pg_stat_database` View

Column Type

Description

datid oid

OID of this database, or 0 for objects belonging to a shared relation

datname name

Name of this database, or NULL for shared objects.

numbackends integer

Number of backends currently connected to this database, or NULL for shared objects. This is the only column in this view that returns a value reflecting current state; all other columns return the accumulated values since the last reset.

xact_commit bigint

Number of transactions in this database that have been committed

xact_rollback bigint

Number of transactions in this database that have been rolled back

blks_read bigint

Number of disk blocks read in this database

blks_hit bigint

Number of times disk blocks were found already in the buffer cache, so that a read was not necessary (this only includes hits in the PostgreSQL buffer cache, not the operating system's file system cache)

tup_returned bigint

Number of rows returned by queries in this database

tup_fetched bigint

Number of rows fetched by queries in this database

tup_inserted bigint

Number of rows inserted by queries in this database

tup_updated bigint

Number of rows updated by queries in this database

tup_deleted bigint

Number of rows deleted by queries in this database

conflicts bigint

Number of queries canceled due to conflicts with recovery in this database. (Conflicts occur only on standby servers; see for details.)

temp_files bigint

由此資料庫的查詢所建立的暫存檔案數量。無論建立暫存檔案的原因（例如排序或 Hash）如何，以及如何設定，都會對所有的暫存檔案進行計數。

temp_bytes bigint

此資料庫中的查詢寫入暫存檔案的資料總量。無論建立暫時檔案的原因以及設定為何，都將對所有的暫存檔案進行統計。

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 is enabled, otherwise zero)

blk_write_time double precision

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

stats_reset timestamp with time zone

上次重置這個統計資訊的時間

27.2.12. `pg_stat_database_conflicts`

The pg_stat_database_conflicts view will contain one row per database, showing database-wide statistics about query cancels occurring due to conflicts with recovery on standby servers. This view will only contain information on standby servers, since conflicts do not occur on master servers.

Table 27.22. `pg_stat_database_conflicts` View

Column Type

Description

datid oid

OID of a database

datname name

Name of this database

confl_tablespace bigint

Number of queries in this database that have been canceled due to dropped tablespaces

confl_lock bigint

Number of queries in this database that have been canceled due to lock timeouts

confl_snapshot bigint

Number of queries in this database that have been canceled due to old snapshots

confl_bufferpin bigint

Number of queries in this database that have been canceled due to pinned buffers

confl_deadlock bigint

Number of queries in this database that have been canceled due to deadlocks

27.2.13. `pg_stat_all_tables`

pg_stat_all_tables 檢視表將為目前資料庫中的每個資料表（包括 TOAST 資料表）為一筆資料，顯示有關對該資料表的存取統計數據。 pg_stat_user_tables 和 pg_stat_sys_tables 檢視表包含相同的資訊，差別是過濾之後僅顯示使用者表和系統資料表。

Table 27.23. `pg_stat_all_tables` View

Column Type

Description

relid oid

OID of a table

schemaname name

Name of the schema that this table is in

relname name

Name of this table

seq_scan bigint

Number of sequential scans initiated on this table

seq_tup_read bigint

Number of live rows fetched by sequential scans

idx_scan bigint

Number of index scans initiated on this table

idx_tup_fetch bigint

Number of live rows fetched by index scans

n_tup_ins bigint

Number of rows inserted

n_tup_upd bigint

Number of rows updated (includes HOT updated rows)

n_tup_del bigint

Number of rows deleted

n_tup_hot_upd bigint

Number of rows HOT updated (i.e., with no separate index update required)

n_live_tup bigint

Estimated number of live rows

n_dead_tup bigint

Estimated number of dead rows

n_mod_since_analyze bigint

Estimated number of rows modified since this table was last analyzed

n_ins_since_vacuum bigint

Estimated number of rows inserted since this table was last vacuumed

last_vacuum timestamp with time zone

Last time at which this table was manually vacuumed (not counting VACUUM FULL)

last_autovacuum timestamp with time zone

Last time at which this table was vacuumed by the autovacuum daemon

last_analyze timestamp with time zone

Last time at which this table was manually analyzed

last_autoanalyze timestamp with time zone

Last time at which this table was analyzed by the autovacuum daemon

vacuum_count bigint

Number of times this table has been manually vacuumed (not counting VACUUM FULL)

autovacuum_count bigint

Number of times this table has been vacuumed by the autovacuum daemon

analyze_count bigint

Number of times this table has been manually analyzed

autoanalyze_count bigint

Number of times this table has been analyzed by the autovacuum daemon

27.2.14. `pg_stat_all_indexes`

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

Table 27.24. `pg_stat_all_indexes` View

Column Type

Description

relid oid

OID of the table for this index

indexrelid oid

OID of this index

schemaname name

Name of the schema this index is in

relname name

Name of the table for this index

indexrelname name

Name of this index

idx_scan bigint

Number of index scans initiated on this index

idx_tup_read bigint

Number of index entries returned by scans on this index

idx_tup_fetch bigint

Number of live table rows fetched by simple index scans using this index

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

Note

The idx_tup_read and idx_tup_fetch counts can be different even without any use of bitmap scans, because idx_tup_read counts index entries retrieved from the index while idx_tup_fetch counts live rows fetched from the table. The latter will be less if any dead or not-yet-committed rows are fetched using the index, or if any heap fetches are avoided by means of an index-only scan.

27.2.15. `pg_statio_all_tables`

The pg_statio_all_tables view will contain one row for each table in the current database (including TOAST tables), showing statistics about I/O on that specific table. The pg_statio_user_tables and pg_statio_sys_tables views contain the same information, but filtered to only show user and system tables respectively.

Table 27.25. `pg_statio_all_tables` View

Column Type

Description

relid oid

OID of a table

schemaname name

Name of the schema that this table is in

relname name

Name of this table

heap_blks_read bigint

Number of disk blocks read from this table

heap_blks_hit bigint

Number of buffer hits in this table

idx_blks_read bigint

Number of disk blocks read from all indexes on this table

idx_blks_hit bigint

Number of buffer hits in all indexes on this table

toast_blks_read bigint

Number of disk blocks read from this table's TOAST table (if any)

toast_blks_hit bigint

Number of buffer hits in this table's TOAST table (if any)

tidx_blks_read bigint

Number of disk blocks read from this table's TOAST table indexes (if any)

tidx_blks_hit bigint

Number of buffer hits in this table's TOAST table indexes (if any)

27.2.16. `pg_statio_all_indexes`

The pg_statio_all_indexes view will contain one row for each index in the current database, showing statistics about I/O on that specific index. The pg_statio_user_indexes and pg_statio_sys_indexes views contain the same information, but filtered to only show user and system indexes respectively.

Table 27.26. `pg_statio_all_indexes` View

Column Type

Description

relid oid

OID of the table for this index

indexrelid oid

OID of this index

schemaname name

Name of the schema this index is in

relname name

Name of the table for this index

indexrelname name

Name of this index

idx_blks_read bigint

Number of disk blocks read from this index

idx_blks_hit bigint

Number of buffer hits in this index

27.2.17. `pg_statio_all_sequences`

The pg_statio_all_sequences view will contain one row for each sequence in the current database, showing statistics about I/O on that specific sequence.

Table 27.27. `pg_statio_all_sequences` View

Column Type

Description

relid oid

OID of a sequence

schemaname name

Name of the schema this sequence is in

relname name

Name of this sequence

blks_read bigint

Number of disk blocks read from this sequence

blks_hit bigint

Number of buffer hits in this sequence

27.2.18. `pg_stat_user_functions`

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

Table 27.28. `pg_stat_user_functions` View

Column Type

Description

funcid oid

OID of a function

schemaname name

Name of the schema this function is in

funcname name

Name of this function

calls bigint

Number of times this function has been called

total_time double precision

Total time spent in this function and all other functions called by it, in milliseconds

self_time double precision

Total time spent in this function itself, not including other functions called by it, in milliseconds

27.2.19. `pg_stat_slru`

PostgreSQL accesses certain on-disk information via SLRU (simple least-recently-used) caches. The pg_stat_slru view will contain one row for each tracked SLRU cache, showing statistics about access to cached pages.

Table 27.29. `pg_stat_slru` View

Column Type

Description

name text

Name of the SLRU

blks_zeroed bigint

Number of blocks zeroed during initializations

blks_hit bigint

Number of times disk blocks were found already in the SLRU, so that a read was not necessary (this only includes hits in the SLRU, not the operating system's file system cache)

blks_read bigint

Number of disk blocks read for this SLRU

blks_written bigint

Number of disk blocks written for this SLRU

blks_exists bigint

Number of blocks checked for existence for this SLRU

flushes bigint

Number of flushes of dirty data for this SLRU

truncates bigint

Number of truncates for this SLRU

stats_reset timestamp with time zone

Time at which these statistics were last reset

27.2.20. Statistics Functions

Other ways of looking at the statistics can be set up by writing queries that use the same underlying statistics access functions used by the standard views shown above. For details such as the functions' names, consult the definitions of the standard views. (For example, in psql you could issue \d+ pg_stat_activity.) The access functions for per-database statistics take a database OID as an argument to identify which database to report on. The per-table and per-index functions take a table or index OID. The functions for per-function statistics take a function OID. Note that only tables, indexes, and functions in the current database can be seen with these functions.

Additional functions related to statistics collection are listed in Table 27.30.

Table 27.30. Additional Statistics Functions

Function

Description

pg_backend_pid () → integer

Returns the process ID of the server process attached to the current session.

pg_stat_get_activity ( integer ) → setof record

Returns a record of information about the backend with the specified process ID, or one record for each active backend in the system if NULL is specified. The fields returned are a subset of those in the pg_stat_activity view.

pg_stat_get_snapshot_timestamp () → timestamp with time zone

Returns the timestamp of the current statistics snapshot.

pg_stat_clear_snapshot () → void

Discards the current statistics snapshot.

pg_stat_reset () → void

Resets all statistics counters for the current database to zero.