27.2. 統計資訊收集器

版本：11

PostgreSQL 的統計資訊收集器是一個子系統，支援收集和回報有關伺服器活動的資訊。目前，收集器可以以磁區和單個資料列的方式計算對資料表和索引的存取。它也追踪著每個資料表中的總資料筆數，以及有關每個資料表的清理和分析操作的資訊。它還可以計算對使用者定義函數的呼叫以及每個函數所花費的總時間。

PostgreSQL 也支援回報有關目前系統中確切正在發生情況的動態資訊，例如目前由其他伺服器程序執行的確切指令以及系統中存在哪些其他連線。此功能獨立於收集器程序。

27.2.1. Statistics Collection Configuration

由於收集統計資訊會增加查詢執行的成本，因此可以將系統配置為收集或不收集資訊。這由通常由 postgresql.conf 中的配置參數所控制。 （有關設定配置參數的詳細資訊，請參閱第 19 章。）

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 27.2.3.

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.

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

Table 27.1. Dynamic Statistics Views

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

Table 27.2. Collected Statistics Views

View Name	Description
pg_stat_archiver	One row only, showing statistics about the WAL archiver process's activity. See pg_stat_archiver for details.
pg_stat_bgwriter	One row only, showing statistics about the background writer process's activity. See pg_stat_bgwriter for details.
pg_stat_database	One row per database, showing database-wide statistics. See pg_stat_database for details.
pg_stat_database_conflicts	One row per database, showing database-wide statistics about query cancels due to conflict with recovery on standby servers. See pg_stat_database_conflicts for details.
pg_stat_all_tables	One row for each table in the current database, showing statistics about accesses to that specific table. See pg_stat_all_tables for details.
pg_stat_sys_tables	Same as pg_stat_all_tables, except that only system tables are shown.
pg_stat_user_tables	Same as pg_stat_all_tables, except that only user tables are shown.
pg_stat_xact_all_tables	Similar to pg_stat_all_tables, but counts actions taken so far within the current transaction (which are not yet included in pg_stat_all_tables and related views). The columns for numbers of live and dead rows and vacuum and analyze actions are not present in this view.
pg_stat_xact_sys_tables	Same as pg_stat_xact_all_tables, except that only system tables are shown.
pg_stat_xact_user_tables	Same as pg_stat_xact_all_tables, except that only user tables are shown.
pg_stat_all_indexes	One row for each index in the current database, showing statistics about accesses to that specific index. See pg_stat_all_indexes for details.
pg_stat_sys_indexes	Same as pg_stat_all_indexes, except that only indexes on system tables are shown.
pg_stat_user_indexes	Same as pg_stat_all_indexes, except that only indexes on user tables are shown.
pg_statio_all_tables	One row for each table in the current database, showing statistics about I/O on that specific table. See pg_statio_all_tables for details.
pg_statio_sys_tables	Same as pg_statio_all_tables, except that only system tables are shown.
pg_statio_user_tables	Same as pg_statio_all_tables, except that only user tables are shown.
pg_statio_all_indexes	One row for each index in the current database, showing statistics about I/O on that specific index. See pg_statio_all_indexes for details.
pg_statio_sys_indexes	Same as pg_statio_all_indexes, except that only indexes on system tables are shown.
pg_statio_user_indexes	Same as pg_statio_all_indexes, except that only indexes on user tables are shown.
pg_statio_all_sequences	One row for each sequence in the current database, showing statistics about I/O on that specific sequence. See pg_statio_all_sequences for details.
pg_statio_sys_sequences	Same as pg_statio_all_sequences, except that only system sequences are shown. (Presently, no system sequences are defined, so this view is always empty.)
pg_statio_user_sequences	Same as pg_statio_all_sequences, except that only user sequences are shown.
pg_stat_user_functions	One row for each tracked function, showing statistics about executions of that function. See pg_stat_user_functions for details.
pg_stat_xact_user_functions	Similar to pg_stat_user_functions, but counts only calls during the current transaction (which are not yet included in pg_stat_user_functions).

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 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
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 27.4for 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, 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.

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 27.4. 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.
LogicalRepWorkerLock	Waiting for action on logical replication worker to finish.
CLogTruncationLock	Waiting to execute txid_status or update the oldest transaction id available to it.
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 for 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.
serializable_xact	Waiting to perform an operation on a serializable transaction in a parallel query.
parallel_query_dsa	Waiting for parallel query dynamic shared memory allocation lock.
tbm	Waiting for TBM shared iterator lock.
parallel_append	Waiting to choose the next subplan during Parallel Append plan execution.
parallel_hash_join	Waiting to allocate or exchange a chunk of memory or update counters during Parallel Hash plan execution.
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.
LogicalApplyMain	Waiting in main loop of logical apply process.
LogicalLauncherMain	Waiting in main loop of logical launcher process.
PgStatMain	Waiting in main loop of the statistics collector process.
RecoveryWalAll	Waiting for WAL from a stream at recovery.
RecoveryWalStream	Waiting when WAL data is not available from any kind of sources (local, archive or stream) before trying again to retrieve WAL data, 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 to the client.
GSSOpenServer	Waiting to read data from the client while establishing the 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.
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.
CheckpointDone	Waiting for a checkpoint to complete.
CheckpointStart	Waiting for a checkpoint to start.
ClogGroupUpdate	Waiting for group leader to update transaction status at transaction end.
ExecuteGather	Waiting for activity from child process when executing Gather node.
Hash/Batch/Allocating	Waiting for an elected Parallel Hash participant to allocate a hash table.
Hash/Batch/Electing	Electing a Parallel Hash participant to allocate a hash table.
Hash/Batch/Loading	Waiting for other Parallel Hash participants to finish loading a hash table.
Hash/Build/Allocating	Waiting for an elected Parallel Hash participant to allocate the initial hash table.
Hash/Build/Electing	Electing a Parallel Hash participant to allocate the initial hash table.
Hash/Build/HashingInner	Waiting for other Parallel Hash participants to finish hashing the inner relation.
Hash/Build/HashingOuter	Waiting for other Parallel Hash participants to finish partitioning the outer relation.
Hash/GrowBatches/Allocating	Waiting for an elected Parallel Hash participant to allocate more batches.
Hash/GrowBatches/Deciding	Electing a Parallel Hash participant to decide on future batch growth.
Hash/GrowBatches/Electing	Electing a Parallel Hash participant to allocate more batches.
Hash/GrowBatches/Finishing	Waiting for an elected Parallel Hash participant to decide on future batch growth.
Hash/GrowBatches/Repartitioning	Waiting for other Parallel Hash participants to finishing repartitioning.
Hash/GrowBuckets/Allocating	Waiting for an elected Parallel Hash participant to finish allocating more buckets.
Hash/GrowBuckets/Electing	Electing a Parallel Hash participant to allocate more buckets.
Hash/GrowBuckets/Reinserting	Waiting for other Parallel Hash participants to finish inserting tuples into new buckets.
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.
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 group leader to clear transaction id at transaction end.
Promote	Waiting for standby promotion.
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.
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 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.
WALSync	Waiting for a WAL file to reach stable storage.
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 27.5. 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 on incurred while committing if this server was configured as a synchronous standby.
replay_lag	interval	Time elapsed between flushing recent WAL locally and receiving notification that this standby server has written, flushed and applied it. This can be used to gauge the delay that synchronous_commit level remote_apply incurred while committing if this server was configured as a synchronous standby.
sync_priority	integer	Priority of this standby server for being chosen as the synchronous standby in a priority-based synchronous replication. This has no effect in a quorum-based synchronous replication.
sync_state	text	Synchronous state of this standby server. Possible values are: async: This standby server is asynchronous. potential: This standby server is now asynchronous, but can potentially become synchronous if one of current synchronous ones fails. sync: This standby server is synchronous. quorum: This standby server is considered as a candidate for quorum standbys.
reply_time	timestamp with time zone	Send time of last reply message received from standby server

The 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 27.6. 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
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.

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

Table 27.7. pg_stat_subscription View

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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