Wait Events Overview

At any point in time, each database server process is busy processing a request or waiting for a specific event to occur. We say that it is busy, which means that the process needs to use the CPU at that time. For example, when a session process executes a SQL procedure, it may be doing an arithmetic calculation that we say is busy, not waiting. At the same time, another session process may be committing a transaction, and it is waiting for a response from the operating system telling it that the fsync system call it used to flush the log has been completed, and we say it is in a waiting state.

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;

Execute the following query to view collected and summarized statistics of wait events occured in the system:

SELECT * FROM pg_stat_wait_event
   ORDER BY total_time DESC;

Execute the following query to view statistics of wait events occurred in a session:

SELECT pg_stat_reset_wait_event(pg_backend_pid());
-- In current session, execute a series of SQL commands
SELECT * FROM pg_stat_get_wait_event(pg_backend_pid()) AS we
   ORDER BY total_time DESC;

Wait Event Types

Table 1. Wait Event Types

Wait Event Type	Description
`Activity`	The server process is idle. This event type indicates a process waiting for activity in its main processing loop. `wait_event` will identify the specific wait point; see Table 2.
`BufferPin`	The server process is waiting for exclusive access to a data buffer. Buffer pin waits can be protracted if another process holds an open cursor that last read data from the buffer in question. See Table 3.
`Client`	The server process is waiting for activity on a socket connected to a user application. Thus, the server expects something to happen that is independent of its internal processes. `wait_event` will identify the specific wait point; see Table 4.
`CPU`	The server process is active in CPU or is waiting for CPU. See Table 5.
`Extension`	The server process is waiting for some condition defined by an extension module. See Table 6.
`IO`	The server process is waiting for an I/O operation to complete. `wait_event` will identify the specific wait point; see Table 7.
`IPC`	The server process is waiting for some interaction with another server process. `wait_event` will identify the specific wait point; see Table 8.
`Lock`	The server process is waiting for a heavyweight lock. Heavyweight locks, also known as lock manager locks or simply locks, primarily protect SQL-visible objects such as tables. However, they are also used to ensure mutual exclusion for certain internal operations such as relation extension. `wait_event` will identify the type of lock awaited; see Table 9.
`LWLock`	The server process is waiting for a lightweight lock. Most such locks protect a particular data structure in shared memory. `wait_event` will contain a name identifying the purpose of the lightweight lock. (Some locks have specific names; others are part of a group of locks each with a similar purpose.) See Table 10.
`SpinLock`	The server process is waiting for a spin lock. Most such locks protect a particular data structure in shared memory. `wait_event` will contain a name identifying the purpose of the spin lock. (Some locks have specific names; others are part of a group of locks each with a similar purpose.) See Table 11.
`Timeout`	The server process is waiting for a timeout to expire. `wait_event` will identify the specific wait point; see Table 12.

Wait Events of Type Activity

Table 2. Wait Events of Type Activity

`Activity` Wait Event	Description
`ArchiverMain`	Waiting in main loop of archiver process.
`AnalyzeLauncherMain`	Waiting in main loop of autoanalyze 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.
`UndoLauncherMain`	Waiting in main loop of undo maintenance launcher 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.

Wait Events of Type BufferPin

Table 3. Wait Events of Type BufferPin

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

Wait Events of Type Client

Table 4. 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.
`LibPQStorageConnect`	Waiting in compute server to establish connection to remote storage server.
`LibPQStorageReceive`	Waiting in compute server to receive data from remote storage 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.

Wait Events of Type CPU

Table 5. Wait Events of Type CPU

`CPU` Wait Event	Description
`CPU`	Active in CPU or Waiting for CPU.

Wait Events of Type Extension

Table 6. Wait Events of Type Extension

`Extension` Wait Event	Description
`Extension`	Waiting in an extension.

Wait Events of Type IO

Table 7. Wait Events of Type IO

`IO` Wait Event	Description
`BaseBackupRead`	Waiting for base backup to read from a file.
`BufFileRead`	Waiting for a read from a buffered file.
`BufFileWrite`	Waiting for a write to a buffered file.
`BufFileTruncate`	Waiting for a buffered file to be truncated.
`ControlFileRead`	Waiting for a read from the `pg_control` file.
`ControlFileSync`	Waiting for the `pg_control` file to reach durable storage.
`ControlFileSyncUpdate`	Waiting for an update to the `pg_control` file to reach durable storage.
`ControlFileWrite`	Waiting for a write to the `pg_control` file.
`ControlFileWriteUpdate`	Waiting for a write to update the `pg_control` file.
`CopyFileRead`	Waiting for a read during a file copy operation.
`CopyFileWrite`	Waiting for a write during a file copy operation.
`DSMFillZeroWrite`	Waiting to fill a dynamic shared memory backing file with zeroes.
`DataFileExtend`	Waiting for a relation data file to be extended.
`DataFileFlush`	Waiting for a relation data file to reach durable storage.
`DataFileImmediateSync`	Waiting for an immediate synchronization of a relation data file to durable storage.
`DataFilePrefetch`	Waiting for an asynchronous prefetch from a relation data file.
`DataFileRead`	Waiting for a read from a relation data file.
`DataFileSync`	Waiting for changes to a relation data file to reach durable storage.
`DataFileTruncate`	Waiting for a relation data file to be truncated.
`DataFileWrite`	Waiting for a write to a relation data file.
`LockFileAddToDataDirRead`	Waiting for a read while adding a line to the data directory lock file.
`LockFileAddToDataDirSync`	Waiting for data to reach durable storage while adding a line to the data directory lock file.
`LockFileAddToDataDirWrite`	Waiting for a write while adding a line to the data directory lock file.
`LockFileCreateRead`	Waiting to read while creating the data directory lock file.
`LockFileCreateSync`	Waiting for data to reach durable storage while creating the data directory lock file.
`LockFileCreateWrite`	Waiting for a write while creating the data directory lock file.
`LockFileReCheckDataDirRead`	Waiting for a read during recheck of the data directory lock file.
`LogicalRewriteCheckpointSync`	Waiting for logical rewrite mappings to reach durable storage during a checkpoint.
`LogicalRewriteMappingSync`	Waiting for mapping data to reach durable storage during a logical rewrite.
`LogicalRewriteMappingWrite`	Waiting for a write of mapping data during a logical rewrite.
`LogicalRewriteSync`	Waiting for logical rewrite mappings to reach durable storage.
`LogicalRewriteTruncate`	Waiting for truncate of mapping data during a logical rewrite.
`LogicalRewriteWrite`	Waiting for a write of logical rewrite mappings.
`RelationMapRead`	Waiting for a read of the relation map file.
`RelationMapSync`	Waiting for the relation map file to reach durable storage.
`RelationMapWrite`	Waiting for a write to the relation map file.
`ReorderBufferRead`	Waiting for a read during reorder buffer management.
`ReorderBufferWrite`	Waiting for a write during reorder buffer management.
`ReorderLogicalMappingRead`	Waiting for a read of a logical mapping during reorder buffer management.
`ReplicationSlotRead`	Waiting for a read from a replication slot control file.
`ReplicationSlotRestoreSync`	Waiting for a replication slot control file to reach durable storage while restoring it to memory.
`ReplicationSlotSync`	Waiting for a replication slot control file to reach durable storage.
`ReplicationSlotWrite`	Waiting for a write to a replication slot control file.
`SLRUFlushSync`	Waiting for SLRU data to reach durable storage during a checkpoint or database shutdown.
`SLRURead`	Waiting for a read of an SLRU page.
`SLRUSync`	Waiting for SLRU data to reach durable storage following a page write.
`SLRUWrite`	Waiting for a write of an SLRU page.
`SnapbuildRead`	Waiting for a read of a serialized historical catalog snapshot.
`SnapbuildSync`	Waiting for a serialized historical catalog snapshot to reach durable storage.
`SnapbuildWrite`	Waiting for a write of a serialized historical catalog snapshot.
`TimelineHistoryFileSync`	Waiting for a timeline history file received via streaming replication to reach durable storage.
`TimelineHistoryFileWrite`	Waiting for a write of a timeline history file received via streaming replication.
`TimelineHistoryRead`	Waiting for a read of a timeline history file.
`TimelineHistorySync`	Waiting for a newly created timeline history file to reach durable storage.
`TimelineHistoryWrite`	Waiting for a write of a newly created timeline history file.
`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.

Wait Events of Type IPC

Table 8. 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.
`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.
`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.
`WalReceiverExit`	Waiting for the WAL receiver to exit.
`WalReceiverWaitStart`	Waiting for startup process to send initial data for streaming replication.
`XactGroupUpdate`	Waiting for the group leader to update transaction status at end of a parallel operation.

Wait Events of Type Lock

Table 9. Wait Events of Type Lock

`Lock` Wait Event	Description
`advisory`	Waiting to acquire an advisory user lock.
`extend`	Waiting to extend a relation.
`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.
`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.

Wait Events of Type LWLock

Table 10. Wait Events of Type LWLock

`LWLock` Wait Event	Description
`AddinShmemInitLock`	Waiting to manage an extension’s space allocation in shared memory.
`AutoFileLock`	Waiting to update the `postgresql.auto.conf` file.
`AutoanalyzeLock`	Waiting to read or update the current state of autoanalyze workers.
`AutoanalyzeScheduleLock`	Waiting to ensure that a table selected for autoanalyze still needs analyzing.
`BackgroundWorkerLock`	Waiting to read or update background worker state.
`buffer_content`	Waiting to access a data page in memory.
`buffer_io`	Waiting to read or write a data page in disk.
`buffer_mapping`	Waiting to associate a data block with a buffer in the buffer pool.
`CheckpointLock`	Waiting to perform checkpoint.
`CheckpointerCommLock`	Waiting to manage fsync requests.
`ControlFileLock`	Waiting to read or update the `pg_control` file or create a new WAL file.
`DatabaseStateLock`	Waiting to update state of databases.
`DynamicSharedMemoryControlLock`	Waiting to read or update dynamic shared memory allocation information.
`lock_manager`	Waiting to read or update information about “heavyweight” locks.
`LogicalRepWorkerLock`	Waiting to read or update the state of logical replication workers.
`OidGenLock`	Waiting to allocate a new OID.
`OldSerXidLock`	Waiting to read or record conflicting serializable transactions.
`PLogSpaceLock`	Waiting to update state of network tablespaces.
`PLogWriteLock`	Waiting for PLOG buffers to be written to disk.
`predicate_lock_manager`	Waiting to access predicate lock information used by serializable transactions.
`ProcArrayLock`	Waiting to access the shared per-process data structures. Usually occurs when a connection is established and released, or when reporting a session’s transaction ID.
`RelCacheInitLock`	Waiting to read or update a `pg_internal.init` relation cache initialization file.
`ReplicationOriginLock`	Waiting to create, drop or use a replication origin.
`ReplicationSlotAllocationLock`	Waiting to allocate or free a replication slot.
`ReplicationSlotControlLock`	Waiting to read or update replication slot state.
`replication_origin`	Waiting to read or update the replication progress.
`replication_slot_io`	Waiting for I/O on a replication slot.
`SerializableFinishedListLock`	Waiting to access the list of finished serializable transactions.
`SerializablePredicateListLock`	Waiting to access the list of predicate locks held by serializable transactions.
`SerializableXactHashLock`	Waiting to read or update information about serializable transactions.
`ShmemIndexLock`	Waiting to find or allocate space in shared memory.
`SInvalReadLock`	Waiting to retrieve messages from the shared catalog invalidation queue.
`SInvalWriteLock`	Waiting to add a message to the shared catalog invalidation queue.
`SyncRepLock`	Waiting to read or update information about the state of synchronous replication.
`SyncScanLock`	Waiting to select the starting location of a synchronized table scan.
`TablespaceMapLock`	Waiting to read or update the storage location configuration files of tablespaces.
`TwoPhaseStateLock`	Waiting to read or update the state of prepared transactions.
`WALBufMappingLock`	Waiting to replace a page in WAL buffers.
`wal_insert`	Waiting to insert WAL data into a memory buffer.
`WALWriteLock`	Waiting for WAL buffers to be written to disk.

Extensions can add LWLock types to the list shown in Table 9. 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.

Wait Events of Type SpinLock

Table 11. Wait Events of Type SpinLock

`SpinLock` Wait Event	Description
`BufferHeader`	Waiting for access buffer header information.
`BufferStrategyControl`	Waiting for access information about buffer allocation and elimination, and maintain a free buffer list and buffer elimination based on a clock sweep algorithm.
`CheckpointerShmem`	Waiting for access information about checkpoint.
`ConditionVariable`	Waiting to enter or exit the queue used by condition variable.
`FastPathStrongLock`	Waiting to read or update a process’ fast-path lock information.
`HashFreelist`	Waiting to access the free element list in the shared partitioned hash table.
`InvalMessageNumber`	Waiting to retrieve invalidation message counter.
`LogicalTableSync`	Waiting for access information about the initial table synchronization in logical replication.
`LWLockWaitList`	Waiting to enter or exit the queue to used by `LWLock`.
`ParallelBitmapHeapScan`	Waiting for access information used by parallel bitmap scan.
`ParallelBtreeScan`	Waiting for access information used by parallel btree scan.
`ParallelFixedState`	Waiting for access some basic information about parallel query, mainly used to update the latest log sequence number of the parallel worker process.
`ParallelHeapScan`	Waiting for access information used by parallel heap scan.
`PGProcFreeList`	Waiting for allocate or release shared per-process data structure. Usually occurs during the establishment and release of a database connection.
`PLogControl`	Waiting for access information about the PLOG buffer state.
`ReplicationSlot`	Waiting for access information about replication slot.
`ShmemAlloc`	Waiting for allocate shared memory.
`ShmemMessageQueue`	Waiting for access shared memory message queue.
`ShmemSegmentTable`	Waiting for access shared memory segment table of contents.
`StorageServerControl`	Waiting for access information about the storage server process.
`UnloggedLSN`	Wait to retrieve a instance level log sequence number similar to the LSN.
`VFDHeader`	Waiting for read or update information about the virtual file descriptor.
`VSDHeader`	Waiting for read or update information about the virtual storage descriptor.
`WALReceiverData`	Waiting for access information about the WAL receiver process.
`WALSenderControl`	Waiting for access information about the WAL sender process.
`XLogControl`	Waiting for access information about the WAL buffer state.

Wait Events of Type Timeout

Table 12. 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.