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.
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.
Several predefined views, listed in Table 28.1, are available to show the current state of the system. There are also several other views, listed in Table 28.2, available to show the results of statistics collection. Alternatively, one can build custom views using the underlying statistics functions, as discussed in Section 28.2.3.
When using the statistics to monitor collected data, it is important
to realize that the information does not update instantaneously.
Each individual server process transmits new statistical counts to
the collector just before going idle; so a query or transaction still in
progress does not affect the displayed totals. Also, the collector itself
emits a new report at most once per PGSTAT_STAT_INTERVAL
milliseconds (500 ms unless altered while building the server). So the
displayed information lags behind actual activity. However, current-query
information collected by track_activities
is
always up-to-date.
Another important point is that when a server process is asked to display
any of these statistics, it first fetches the most recent report emitted by
the collector process and then continues to use this snapshot for all
statistical views and functions until the end of its current transaction.
So the statistics will show static information as long as you continue the
current transaction. Similarly, information about the current queries of
all sessions is collected when any such information is first requested
within a transaction, and the same information will be displayed throughout
the transaction.
This is a feature, not a bug, because it allows you to perform several
queries on the statistics and correlate the results without worrying that
the numbers are changing underneath you. But if you want to see new
results with each query, be sure to do the queries outside any transaction
block. Alternatively, you can invoke
pg_stat_clear_snapshot
(), which will discard the
current transaction's statistics snapshot (if any). The next use of
statistical information will cause a new snapshot to be fetched.
A transaction can also see its own statistics (as yet untransmitted to the
collector) in the views pg_stat_xact_all_tables
,
pg_stat_xact_sys_tables
,
pg_stat_xact_user_tables
, and
pg_stat_xact_user_functions
. These numbers do not act as
stated above; instead they update continuously throughout the transaction.
Table 28.1. Dynamic Statistics Views
View Name | Description |
---|---|
pg_stat_activity
| One row per server process, showing information related to the current activity of that process, such as state and current query. See pg_stat_activity for details. |
pg_stat_replication | One row per WAL sender process, showing statistics about replication to that sender's connected standby server. See pg_stat_replication for details. |
pg_stat_wal_receiver | Only one row, showing statistics about the WAL receiver from that receiver's connected server. See pg_stat_wal_receiver for details. |
pg_stat_subscription | At least one row per subscription, showing information about the subscription workers. See pg_stat_subscription for details. |
pg_stat_ssl | One row per connection (regular and replication), showing information about SSL used on this connection. See pg_stat_ssl for details. |
pg_stat_progress_vacuum | One row for each backend (including autovacuum worker processes) running
VACUUM , showing current progress.
See Section 28.4.1.
|
Table 28.2. Collected Statistics Views
View Name | Description |
---|---|
pg_stat_archiver | One row only, showing statistics about the WAL archiver process's activity. See pg_stat_archiver for details. |
pg_stat_bgwriter | One row only, showing statistics about the background writer process's activity. See pg_stat_bgwriter for details. |
pg_stat_database | One row per database, showing database-wide statistics. See pg_stat_database for details. |
pg_stat_database_conflicts | One row per database, showing database-wide statistics about query cancels due to conflict with recovery on standby servers. See pg_stat_database_conflicts for details. |
pg_stat_all_tables | One row for each table in the current database, showing statistics about accesses to that specific table. See pg_stat_all_tables for details. |
pg_stat_sys_tables | Same as pg_stat_all_tables , except that only
system tables are shown. |
pg_stat_user_tables | Same as pg_stat_all_tables , except that only user
tables are shown. |
pg_stat_xact_all_tables | Similar to pg_stat_all_tables , but counts actions
taken so far within the current transaction (which are not
yet included in pg_stat_all_tables and related views).
The columns for numbers of live and dead rows and vacuum and
analyze actions are not present in this view. |
pg_stat_xact_sys_tables | Same as pg_stat_xact_all_tables , except that only
system tables are shown. |
pg_stat_xact_user_tables | Same as pg_stat_xact_all_tables , except that only
user tables are shown. |
pg_stat_all_indexes | One row for each index in the current database, showing statistics about accesses to that specific index. See pg_stat_all_indexes for details. |
pg_stat_sys_indexes | Same as pg_stat_all_indexes , except that only
indexes on system tables are shown. |
pg_stat_user_indexes | Same as pg_stat_all_indexes , except that only
indexes on user tables are shown. |
pg_statio_all_tables | One row for each table in the current database, showing statistics about I/O on that specific table. See pg_statio_all_tables for details. |
pg_statio_sys_tables | Same as pg_statio_all_tables , except that only
system tables are shown. |
pg_statio_user_tables | Same as pg_statio_all_tables , except that only
user tables are shown. |
pg_statio_all_indexes | One row for each index in the current database, showing statistics about I/O on that specific index. See pg_statio_all_indexes for details. |
pg_statio_sys_indexes | Same as pg_statio_all_indexes , except that only
indexes on system tables are shown. |
pg_statio_user_indexes | Same as pg_statio_all_indexes , except that only
indexes on user tables are shown. |
pg_statio_all_sequences | One row for each sequence in the current database, showing statistics about I/O on that specific sequence. See pg_statio_all_sequences for details. |
pg_statio_sys_sequences | Same as pg_statio_all_sequences , except that only
system sequences are shown. (Presently, no system sequences are defined,
so this view is always empty.) |
pg_statio_user_sequences | Same as pg_statio_all_sequences , except that only
user sequences are shown. |
pg_stat_user_functions | One row for each tracked function, showing statistics about executions of that function. See pg_stat_user_functions for details. |
pg_stat_xact_user_functions | Similar to pg_stat_user_functions , but counts only
calls during the current transaction (which are not
yet included in pg_stat_user_functions ). |
The per-index statistics are particularly useful to determine which indexes are being used and how effective they are.
The pg_statio_
views are primarily useful to
determine the effectiveness of the buffer cache. When the number
of actual disk reads is much smaller than the number of buffer
hits, then the cache is satisfying most read requests without
invoking a kernel call. However, these statistics do not give the
entire story: due to the way in which PostgreSQL
handles disk I/O, data that is not in the
PostgreSQL buffer cache might still reside in the
kernel's I/O cache, and might therefore still be fetched without
requiring a physical read. Users interested in obtaining more
detailed information on PostgreSQL I/O behavior are
advised to use the PostgreSQL statistics collector
in combination with operating system utilities that allow insight
into the kernel's handling of I/O.
Table 28.3. pg_stat_activity
View
Column | Type | Description |
---|---|---|
datid | oid | OID of the database this backend is connected to |
datname | name | Name of the database this backend is connected to |
pid | integer | Process ID of this backend |
usesysid | oid | OID of the user logged into this backend |
usename | name | Name of the user logged into this backend |
application_name | text | Name of the application that is connected to this backend |
client_addr | inet | IP address of the client connected to this backend. If this field is null, it indicates either that the client is connected via a Unix socket on the server machine or that this is an internal process such as autovacuum. |
client_hostname | text | Host name of the connected client, as reported by a
reverse DNS lookup of client_addr . This field will
only be non-null for IP connections, and only when log_hostname is enabled.
|
client_port | integer | TCP port number that the client is using for communication
with this backend, or -1 if a Unix socket is used
|
backend_start | timestamp with time zone | Time when this process was started. For client backends, this is the time the client connected to the server. |
xact_start | timestamp with time zone | Time when this process' current transaction was started, or null
if no transaction is active. If the current
query is the first of its transaction, this column is equal to the
query_start column.
|
query_start | timestamp with time zone | Time when the currently active query was started, or if
state is not active , when the last query
was started
|
state_change | timestamp with time zone | Time when the state was last changed |
wait_event_type | text | The type of event for which the backend is waiting, if any;
otherwise NULL. Possible values are:
|
wait_event | text | Wait event name if backend is currently waiting, otherwise NULL. See Table 28.4 for details. |
state | text | Current overall state of this backend.
Possible values are:
|
backend_xid | xid | Top-level transaction identifier of this backend, if any. |
backend_xmin | xid | The current backend's xmin horizon. |
query | text | Text of this backend's most recent query. If
state is active this field shows the
currently executing query. In all other states, it shows the last query
that was executed. By default the query text is truncated at 1024
characters; this value can be changed via the parameter
track_activity_query_size.
|
backend_type | text | Type of current backend. Possible types are
autovacuum launcher , autovacuum worker ,
background worker , background writer ,
client backend , checkpointer ,
startup , walreceiver ,
walsender and walwriter .
|
The pg_stat_activity
view will have one row
per server process, showing information related to
the current activity of that process.
The wait_event
and state
columns are
independent. If a backend is in the active
state,
it may or may not be waiting
on some event. If the state
is active
and wait_event
is non-null, it
means that a query is being executed, but is being blocked somewhere
in the system.
Table 28.4. wait_event
Description
Wait Event Type | Wait Event Name | Description |
---|---|---|
LWLock | ShmemIndexLock | Waiting to find or allocate space in shared memory. |
OidGenLock | Waiting to allocate or assign an OID. | |
XidGenLock | Waiting to allocate or assign a transaction id. | |
ProcArrayLock | Waiting to get a snapshot or clearing a transaction id at transaction end. | |
SInvalReadLock | Waiting to retrieve or remove messages from shared invalidation queue. | |
SInvalWriteLock | Waiting to add a message in shared invalidation queue. | |
WALBufMappingLock | Waiting to replace a page in WAL buffers. | |
WALWriteLock | Waiting for WAL buffers to be written to disk. | |
ControlFileLock | Waiting to read or update the control file or creation of a new WAL file. | |
CheckpointLock | Waiting to perform checkpoint. | |
CLogControlLock | Waiting to read or update transaction status. | |
SubtransControlLock | Waiting to read or update sub-transaction information. | |
MultiXactGenLock | Waiting to read or update shared multixact state. | |
MultiXactOffsetControlLock | Waiting to read or update multixact offset mappings. | |
MultiXactMemberControlLock | Waiting to read or update multixact member mappings. | |
RelCacheInitLock | Waiting to read or write relation cache initialization file. | |
CheckpointerCommLock | Waiting to manage fsync requests. | |
TwoPhaseStateLock | Waiting to read or update the state of prepared transactions. | |
TablespaceCreateLock | Waiting to create or drop the tablespace. | |
BtreeVacuumLock | Waiting to read or update vacuum-related information for a B-tree index. | |
AddinShmemInitLock | Waiting to manage space allocation in shared memory. | |
AutovacuumLock | Autovacuum worker or launcher waiting to update or read the current state of autovacuum workers. | |
AutovacuumScheduleLock | Waiting to ensure that the table it has selected for a vacuum still needs vacuuming. | |
SyncScanLock | Waiting to get the start location of a scan on a table for synchronized scans. | |
RelationMappingLock | Waiting to update the relation map file used to store catalog to filenode mapping. | |
AsyncCtlLock | Waiting to read or update shared notification state. | |
AsyncQueueLock | Waiting to read or update notification messages. | |
SerializableXactHashLock | Waiting to retrieve or store information about serializable transactions. | |
SerializableFinishedListLock | Waiting to access the list of finished serializable transactions. | |
SerializablePredicateLockListLock | Waiting to perform an operation on a list of locks held by serializable transactions. | |
OldSerXidLock | Waiting to read or record conflicting serializable transactions. | |
SyncRepLock | Waiting to read or update information about synchronous replicas. | |
BackgroundWorkerLock | Waiting to read or update background worker state. | |
DynamicSharedMemoryControlLock | Waiting to read or update dynamic shared memory state. | |
AutoFileLock | Waiting to update the postgresql.auto.conf file. | |
ReplicationSlotAllocationLock | Waiting to allocate or free a replication slot. | |
ReplicationSlotControlLock | Waiting to read or update replication slot state. | |
CommitTsControlLock | Waiting to read or update transaction commit timestamps. | |
CommitTsLock | Waiting to read or update the last value set for the transaction timestamp. | |
ReplicationOriginLock | Waiting to setup, drop or use replication origin. | |
MultiXactTruncationLock | Waiting to read or truncate multixact information. | |
OldSnapshotTimeMapLock | Waiting to read or update old snapshot control information. | |
BackendRandomLock | Waiting to generate a random number. | |
LogicalRepWorkerLock | Waiting for action on logical replication worker to finish. | |
CLogTruncationLock | Waiting to truncate the write-ahead log or waiting for write-ahead log truncation to finish. | |
clog | Waiting for I/O on a clog (transaction status) buffer. | |
commit_timestamp | Waiting for I/O on commit timestamp buffer. | |
subtrans | Waiting for I/O a subtransaction buffer. | |
multixact_offset | Waiting for I/O on a multixact offset buffer. | |
multixact_member | Waiting for I/O on a multixact_member buffer. | |
async | Waiting for I/O on an async (notify) buffer. | |
oldserxid | Waiting to I/O on an oldserxid buffer. | |
wal_insert | Waiting to insert WAL into a memory buffer. | |
buffer_content | Waiting to read or write a data page in memory. | |
buffer_io | Waiting for I/O on a data page. | |
replication_origin | Waiting to read or update the replication progress. | |
replication_slot_io | Waiting for I/O on a replication slot. | |
proc | Waiting to read or update the fast-path lock information. | |
buffer_mapping | Waiting to associate a data block with a buffer in the buffer pool. | |
lock_manager | Waiting to add or examine locks for backends, or waiting to join or exit a locking group (used by parallel query). | |
predicate_lock_manager | Waiting to add or examine predicate lock information. | |
parallel_query_dsa | Waiting for parallel query dynamic shared memory allocation lock. | |
tbm | Waiting for TBM shared iterator lock. | |
Lock | relation | Waiting to acquire a lock on a relation. |
extend | Waiting to extend a relation. | |
page | Waiting to acquire a lock on page of a relation. | |
tuple | Waiting to acquire a lock on a tuple. | |
transactionid | Waiting for a transaction to finish. | |
virtualxid | Waiting to acquire a virtual xid lock. | |
speculative token | Waiting to acquire a speculative insertion lock. | |
object | Waiting to acquire a lock on a non-relation database object. | |
userlock | Waiting to acquire a user lock. | |
advisory | Waiting to acquire an advisory user lock. | |
BufferPin | BufferPin | Waiting to acquire a pin on a buffer. |
Activity | ArchiverMain | Waiting in main loop of the archiver process. |
AutoVacuumMain | Waiting in main loop of autovacuum launcher process. | |
BgWriterHibernate | Waiting in background writer process, hibernating. | |
BgWriterMain | Waiting in main loop of background writer process background worker. | |
CheckpointerMain | Waiting in main loop of checkpointer process. | |
LogicalLauncherMain | Waiting in main loop of logical launcher process. | |
LogicalApplyMain | Waiting in main loop of logical apply process. | |
PgStatMain | Waiting in main loop of the statistics collector process. | |
RecoveryWalAll | Waiting for WAL from any kind of source (local, archive or stream) at recovery. | |
RecoveryWalStream | Waiting for WAL from a stream at recovery. | |
SysLoggerMain | Waiting in main loop of syslogger process. | |
WalReceiverMain | Waiting in main loop of WAL receiver process. | |
WalSenderMain | Waiting in main loop of WAL sender process. | |
WalWriterMain | Waiting in main loop of WAL writer process. | |
Client | ClientRead | Waiting to read data from the client. |
ClientWrite | Waiting to write data from the client. | |
LibPQWalReceiverConnect | Waiting in WAL receiver to establish connection to remote server. | |
LibPQWalReceiverReceive | Waiting in WAL receiver to receive data from remote server. | |
SSLOpenServer | Waiting for SSL while attempting connection. | |
WalReceiverWaitStart | Waiting for startup process to send initial data for streaming replication. | |
WalSenderWaitForWAL | Waiting for WAL to be flushed in WAL sender process. | |
WalSenderWriteData | Waiting for any activity when processing replies from WAL receiver in WAL sender process. | |
Extension | Extension | Waiting in an extension. |
IPC | BgWorkerShutdown | Waiting for background worker to shut down. |
BgWorkerStartup | Waiting for background worker to start up. | |
BtreePage | Waiting for the page number needed to continue a parallel B-tree scan to become available. | |
ExecuteGather | Waiting for activity from child process when executing Gather node. | |
LogicalSyncData | Waiting for logical replication remote server to send data for initial table synchronization. | |
LogicalSyncStateChange | Waiting for logical replication remote server to change state. | |
MessageQueueInternal | Waiting for other process to be attached in shared message queue. | |
MessageQueuePutMessage | Waiting to write a protocol message to a shared message queue. | |
MessageQueueReceive | Waiting to receive bytes from a shared message queue. | |
MessageQueueSend | Waiting to send bytes to a shared message queue. | |
ParallelFinish | Waiting for parallel workers to finish computing. | |
ParallelBitmapScan | Waiting for parallel bitmap scan to become initialized. | |
ProcArrayGroupUpdate | Waiting for group leader to clear transaction id at transaction end. | |
ReplicationOriginDrop | Waiting for a replication origin to become inactive to be dropped. | |
ReplicationSlotDrop | Waiting for a replication slot to become inactive to be dropped. | |
SafeSnapshot | Waiting for a snapshot for a READ ONLY DEFERRABLE transaction. | |
SyncRep | Waiting for confirmation from remote server during synchronous replication. | |
Timeout | BaseBackupThrottle | Waiting during base backup when throttling activity. |
PgSleep | Waiting in process that called pg_sleep . | |
RecoveryApplyDelay | Waiting to apply WAL at recovery because it is delayed. | |
IO | BufFileRead | Waiting for a read from a buffered file. |
BufFileWrite | Waiting for a write to a buffered file. | |
ControlFileRead | Waiting for a read from the control file. | |
ControlFileSync | Waiting for the control file to reach stable storage. | |
ControlFileSyncUpdate | Waiting for an update to the control file to reach stable storage. | |
ControlFileWrite | Waiting for a write to the control file. | |
ControlFileWriteUpdate | Waiting for a write to update the control file. | |
CopyFileRead | Waiting for a read during a file copy operation. | |
CopyFileWrite | Waiting for a write during a file copy operation. | |
DataFileExtend | Waiting for a relation data file to be extended. | |
DataFileFlush | Waiting for a relation data file to reach stable storage. | |
DataFileImmediateSync | Waiting for an immediate synchronization of a relation data file to stable storage. | |
DataFilePrefetch | Waiting for an asynchronous prefetch from a relation data file. | |
DataFileRead | Waiting for a read from a relation data file. | |
DataFileSync | Waiting for changes to a relation data file to reach stable storage. | |
DataFileTruncate | Waiting for a relation data file to be truncated. | |
DataFileWrite | Waiting for a write to a relation data file. | |
DSMFillZeroWrite | Waiting to write zero bytes to a dynamic shared memory backing file. | |
LockFileAddToDataDirRead | Waiting for a read while adding a line to the data directory lock file. | |
LockFileAddToDataDirSync | Waiting for data to reach stable storage while adding a line to the data directory lock file. | |
LockFileAddToDataDirWrite | Waiting for a write while adding a line to the data directory lock file. | |
LockFileCreateRead | Waiting to read while creating the data directory lock file. | |
LockFileCreateSync | Waiting for data to reach stable storage while creating the data directory lock file. | |
LockFileCreateWrite | Waiting for a write while creating the data directory lock file. | |
LockFileReCheckDataDirRead | Waiting for a read during recheck of the data directory lock file. | |
LogicalRewriteCheckpointSync | Waiting for logical rewrite mappings to reach stable storage during a checkpoint. | |
LogicalRewriteMappingSync | Waiting for mapping data to reach stable storage during a logical rewrite. | |
LogicalRewriteMappingWrite | Waiting for a write of mapping data during a logical rewrite. | |
LogicalRewriteSync | Waiting for logical rewrite mappings to reach stable storage. | |
LogicalRewriteWrite | Waiting for a write of logical rewrite mappings. | |
RelationMapRead | Waiting for a read of the relation map file. | |
RelationMapSync | Waiting for the relation map file to reach stable storage. | |
RelationMapWrite | Waiting for a write to the relation map file. | |
ReorderBufferRead | Waiting for a read during reorder buffer management. | |
ReorderBufferWrite | Waiting for a write during reorder buffer management. | |
ReorderLogicalMappingRead | Waiting for a read of a logical mapping during reorder buffer management. | |
ReplicationSlotRead | Waiting for a read from a replication slot control file. | |
ReplicationSlotRestoreSync | Waiting for a replication slot control file to reach stable storage while restoring it to memory. | |
ReplicationSlotSync | Waiting for a replication slot control file to reach stable storage. | |
ReplicationSlotWrite | Waiting for a write to a replication slot control file. | |
SLRUFlushSync | Waiting for SLRU data to reach stable storage during a checkpoint or database shutdown. | |
SLRURead | Waiting for a read of an SLRU page. | |
SLRUSync | Waiting for SLRU data to reach stable storage following a page write. | |
SLRUWrite | Waiting for a write of an SLRU page. | |
SnapbuildRead | Waiting for a read of a serialized historical catalog snapshot. | |
SnapbuildSync | Waiting for a serialized historical catalog snapshot to reach stable storage. | |
SnapbuildWrite | Waiting for a write of a serialized historical catalog snapshot. | |
TimelineHistoryFileSync | Waiting for a timeline history file received via streaming replication to reach stable storage. | |
TimelineHistoryFileWrite | Waiting for a write of a timeline history file received via streaming replication. | |
TimelineHistoryRead | Waiting for a read of a timeline history file. | |
TimelineHistorySync | Waiting for a newly created timeline history file to reach stable storage. | |
TimelineHistoryWrite | Waiting for a write of a newly created timeline history file. | |
TwophaseFileRead | Waiting for a read of a two phase state file. | |
TwophaseFileSync | Waiting for a two phase state file to reach stable storage. | |
TwophaseFileWrite | Waiting for a write of a two phase state file. | |
WALBootstrapSync | Waiting for WAL to reach stable storage during bootstrapping. | |
WALBootstrapWrite | Waiting for a write of a WAL page during bootstrapping. | |
WALCopyRead | Waiting for a read when creating a new WAL segment by copying an existing one. | |
WALCopySync | Waiting a new WAL segment created by copying an existing one to reach stable storage. | |
WALCopyWrite | Waiting for a write when creating a new WAL segment by copying an existing one. | |
WALInitSync | Waiting for a newly initialized WAL file to reach stable storage. | |
WALInitWrite | Waiting for a write while initializing a new WAL file. | |
WALRead | Waiting for a read from a WAL file. | |
WALSenderTimelineHistoryRead | Waiting for a read from a timeline history file during walsender timeline command. | |
WALSyncMethodAssign | Waiting for data to reach stable storage while assigning WAL sync method. | |
WALWrite | Waiting for a write to a WAL file. |
For tranches registered by extensions, the name is specified by extension
and this will be displayed as wait_event
. It is quite
possible that user has registered the tranche in one of the backends (by
having allocation in dynamic shared memory) in which case other backends
won't have that information, so we display extension
for such
cases.
Here is an example of how wait events can be viewed
SELECT pid, wait_event_type, wait_event FROM pg_stat_activity WHERE wait_event is NOT NULL; pid | wait_event_type | wait_event ------+-----------------+--------------- 2540 | Lock | relation 6644 | LWLock | ProcArrayLock (2 rows)
Table 28.5. pg_stat_replication
View
Column | Type | Description |
---|---|---|
pid | integer | Process ID of a WAL sender process |
usesysid | oid | OID of the user logged into this WAL sender process |
usename | name | Name of the user logged into this WAL sender process |
application_name | text | Name of the application that is connected to this WAL sender |
client_addr | inet | IP address of the client connected to this WAL sender. If this field is null, it indicates that the client is connected via a Unix socket on the server machine. |
client_hostname | text | Host name of the connected client, as reported by a
reverse DNS lookup of client_addr . This field will
only be non-null for IP connections, and only when log_hostname is enabled.
|
client_port | integer | TCP port number that the client is using for communication
with this WAL sender, or -1 if a Unix socket is used
|
backend_start | timestamp with time zone | Time when this process was started, i.e., when the client connected to this WAL sender |
backend_xmin | xid | This standby's xmin horizon reported
by hot_standby_feedback. |
state | text | Current WAL sender state.
Possible values are:
|
sent_lsn | pg_lsn | Last write-ahead log location sent on this connection |
write_lsn | pg_lsn | Last write-ahead log location written to disk by this standby server |
flush_lsn | pg_lsn | Last write-ahead log location flushed to disk by this standby server |
replay_lsn | pg_lsn | Last write-ahead log location replayed into the database on this standby server |
write_lag | interval | Time elapsed between flushing recent WAL locally and receiving
notification that this standby server has written it (but not yet
flushed it or applied it). This can be used to gauge the delay that
synchronous_commit level
remote_write incurred while committing if this
server was configured as a synchronous standby. |
flush_lag | interval | Time elapsed between flushing recent WAL locally and receiving
notification that this standby server has written and flushed it
(but not yet applied it). This can be used to gauge the delay that
synchronous_commit level
remote_flush incurred while committing if this
server was configured as a synchronous standby. |
replay_lag | interval | Time elapsed between flushing recent WAL locally and receiving
notification that this standby server has written, flushed and
applied it. This can be used to gauge the delay that
synchronous_commit level
remote_apply incurred while committing if this
server was configured as a synchronous standby. |
sync_priority | integer | Priority of this standby server for being chosen as the synchronous standby in a priority-based synchronous replication. This has no effect in a quorum-based synchronous replication. |
sync_state | text | Synchronous state of this standby server.
Possible values are:
|
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.
The reported lag times are not predictions of how long it will take for
the standby to catch up with the sending server assuming the current
rate of replay. Such a system would show similar times while new WAL is
being generated, but would differ when the sender becomes idle. In
particular, when the standby has caught up completely,
pg_stat_replication
shows the time taken to
write, flush and replay the most recent reported WAL location rather than
zero as some users might expect. This is consistent with the goal of
measuring synchronous commit and transaction visibility delays for
recent write transactions.
To reduce confusion for users expecting a different model of lag, the
lag columns revert to NULL after a short time on a fully replayed idle
system. Monitoring systems should choose whether to represent this
as missing data, zero or continue to display the last known value.
Table 28.6. pg_stat_wal_receiver
View
Column | Type | Description |
---|---|---|
pid | integer | Process ID of the WAL receiver process |
status | text | Activity status of the WAL receiver process |
receive_start_lsn | pg_lsn | First write-ahead log location used when WAL receiver is started |
receive_start_tli | integer | First timeline number used when WAL receiver is started |
received_lsn | pg_lsn | Last write-ahead log location already received and flushed to disk, the initial value of this field being the first log location used when WAL receiver is started |
received_tli | integer | Timeline number of last write-ahead log location received and flushed to disk, the initial value of this field being the timeline number of the first log location used when WAL receiver is started |
last_msg_send_time | timestamp with time zone | Send time of last message received from origin WAL sender |
last_msg_receipt_time | timestamp with time zone | Receipt time of last message received from origin WAL sender |
latest_end_lsn | pg_lsn | Last write-ahead log location reported to origin WAL sender |
latest_end_time | timestamp with time zone | Time of last write-ahead log location reported to origin WAL sender |
slot_name | text | Replication slot name used by this WAL receiver |
conninfo | text | Connection string used by this WAL receiver, with security-sensitive fields obfuscated. |
The pg_stat_wal_receiver
view will contain only
one row, showing statistics about the WAL receiver from that receiver's
connected server.
Table 28.7. pg_stat_subscription
View
Column | Type | Description |
---|---|---|
subid | oid | OID of the subscription |
subname | text | Name of the subscription |
pid | integer | Process ID of the subscription worker process |
relid | Oid | OID of the relation that the worker is synchronizing; null for the main apply worker |
received_lsn | pg_lsn | Last write-ahead log location received, the initial value of this field being 0 |
last_msg_send_time | timestamp with time zone | Send time of last message received from origin WAL sender |
last_msg_receipt_time | timestamp with time zone | Receipt time of last message received from origin WAL sender |
latest_end_lsn | pg_lsn | Last write-ahead log location reported to origin WAL sender |
latest_end_time | timestamp with time zone | Time of last write-ahead log location reported to origin WAL sender |
The pg_stat_subscription
view will contain one
row per subscription for main worker (with null PID if the worker is
not running), and additional rows for workers handling the initial data
copy of the subscribed tables.
Table 28.8. pg_stat_ssl
View
Column | Type | Description |
---|---|---|
pid | integer | Process ID of a backend or WAL sender process |
ssl | boolean | True if SSL is used on this connection |
version | text | Version of SSL in use, or NULL if SSL is not in use on this connection |
cipher | text | Name of SSL cipher in use, or NULL if SSL is not in use on this connection |
bits | integer | Number of bits in the encryption algorithm used, or NULL if SSL is not used on this connection |
compression | boolean | True if SSL compression is in use, false if not, or NULL if SSL is not in use on this connection |
clientdn | text | Distinguished Name (DN) field from the client certificate
used, or NULL if no client certificate was supplied or if SSL
is not in use on this connection. This field is truncated if the
DN field is longer than NAMEDATALEN (64 characters
in a standard build)
|
The pg_stat_ssl
view will contain one row per
backend or WAL sender process, showing statistics about SSL usage on
this connection. It can be joined to pg_stat_activity
or pg_stat_replication
on the
pid
column to get more details about the
connection.
Table 28.9. pg_stat_archiver
View
Column | Type | Description |
---|---|---|
archived_count | bigint | Number of WAL files that have been successfully archived |
last_archived_wal | text | Name of the last WAL file successfully archived |
last_archived_time | timestamp with time zone | Time of the last successful archive operation |
failed_count | bigint | Number of failed attempts for archiving WAL files |
last_failed_wal | text | Name of the WAL file of the last failed archival operation |
last_failed_time | timestamp with time zone | Time of the last failed archival operation |
stats_reset | timestamp with time zone | Time at which these statistics were last reset |
The pg_stat_archiver
view will always have a
single row, containing data about the archiver process of the cluster.
Table 28.10. pg_stat_bgwriter
View
Column | Type | Description |
---|---|---|
checkpoints_timed | bigint | Number of scheduled checkpoints that have been performed |
checkpoints_req | bigint | Number of requested checkpoints that have been performed |
checkpoint_write_time | double precision | Total amount of time that has been spent in the portion of checkpoint processing where files are written to disk, in milliseconds |
checkpoint_sync_time | double precision | Total amount of time that has been spent in the portion of checkpoint processing where files are synchronized to disk, in milliseconds |
buffers_checkpoint | bigint | Number of buffers written during checkpoints |
buffers_clean | bigint | Number of buffers written by the background writer |
maxwritten_clean | bigint | Number of times the background writer stopped a cleaning scan because it had written too many buffers |
buffers_backend | bigint | Number of buffers written directly by a backend |
buffers_backend_fsync | bigint | Number of times a backend had to execute its own
fsync call (normally the background writer handles those
even when the backend does its own write) |
buffers_alloc | bigint | Number of buffers allocated |
stats_reset | timestamp with time zone | Time at which these statistics were last reset |
The pg_stat_bgwriter
view will always have a
single row, containing global data for the cluster.
Table 28.11. pg_stat_database
View
Column | Type | Description |
---|---|---|
datid | oid | OID of a database |
datname | name | Name of this database |
numbackends | integer | Number of backends currently connected to this database. This is the only column in this view that returns a value reflecting current state; all other columns return the accumulated values since the last reset. |
xact_commit | bigint | Number of transactions in this database that have been committed |
xact_rollback | bigint | Number of transactions in this database that have been rolled back |
blks_read | bigint | Number of disk blocks read in this database |
blks_hit | bigint | Number of times disk blocks were found already in the buffer cache, so that a read was not necessary (this only includes hits in the PostgreSQL buffer cache, not the operating system's file system cache) |
tup_returned | bigint | Number of rows returned by queries in this database |
tup_fetched | bigint | Number of rows fetched by queries in this database |
tup_inserted | bigint | Number of rows inserted by queries in this database |
tup_updated | bigint | Number of rows updated by queries in this database |
tup_deleted | bigint | Number of rows deleted by queries in this database |
conflicts | bigint | Number of queries canceled due to conflicts with recovery in this database. (Conflicts occur only on standby servers; see pg_stat_database_conflicts for details.) |
temp_files | bigint | Number of temporary files created by queries in this database. All temporary files are counted, regardless of why the temporary file was created (e.g., sorting or hashing), and regardless of the log_temp_files setting. |
temp_bytes | bigint | Total amount of data written to temporary files by queries in this database. All temporary files are counted, regardless of why the temporary file was created, and regardless of the log_temp_files setting. |
deadlocks | bigint | Number of deadlocks detected in this database |
blk_read_time | double precision | Time spent reading data file blocks by backends in this database, in milliseconds |
blk_write_time | double precision | Time spent writing data file blocks by backends in this database, in milliseconds |
stats_reset | timestamp with time zone | Time at which these statistics were last reset |
The pg_stat_database
view will contain one row
for each database in the cluster, showing database-wide statistics.
Table 28.12. pg_stat_database_conflicts
View
Column | Type | Description |
---|---|---|
datid | oid | OID of a database |
datname | name | Name of this database |
confl_tablespace | bigint | Number of queries in this database that have been canceled due to dropped tablespaces |
confl_lock | bigint | Number of queries in this database that have been canceled due to lock timeouts |
confl_snapshot | bigint | Number of queries in this database that have been canceled due to old snapshots |
confl_bufferpin | bigint | Number of queries in this database that have been canceled due to pinned buffers |
confl_deadlock | bigint | Number of queries in this database that have been canceled due to deadlocks |
The pg_stat_database_conflicts
view will contain
one row per database, showing database-wide statistics about
query cancels occurring due to conflicts with recovery on standby servers.
This view will only contain information on standby servers, since
conflicts do not occur on master servers.
Table 28.13. pg_stat_all_tables
View
Column | Type | Description |
---|---|---|
relid | oid | OID of a table |
schemaname | name | Name of the schema that this table is in |
relname | name | Name of this table |
seq_scan | bigint | Number of sequential scans initiated on this table |
seq_tup_read | bigint | Number of live rows fetched by sequential scans |
idx_scan | bigint | Number of index scans initiated on this table |
idx_tup_fetch | bigint | Number of live rows fetched by index scans |
n_tup_ins | bigint | Number of rows inserted |
n_tup_upd | bigint | Number of rows updated (includes HOT updated rows) |
n_tup_del | bigint | Number of rows deleted |
n_tup_hot_upd | bigint | Number of rows HOT updated (i.e., with no separate index update required) |
n_live_tup | bigint | Estimated number of live rows |
n_dead_tup | bigint | Estimated number of dead rows |
n_mod_since_analyze | bigint | Estimated number of rows modified since this table was last analyzed |
last_vacuum | timestamp with time zone | Last time at which this table was manually vacuumed
(not counting VACUUM FULL ) |
last_autovacuum | timestamp with time zone | Last time at which this table was vacuumed by the autovacuum daemon |
last_analyze | timestamp with time zone | Last time at which this table was manually analyzed |
last_autoanalyze | timestamp with time zone | Last time at which this table was analyzed by the autovacuum daemon |
vacuum_count | bigint | Number of times this table has been manually vacuumed
(not counting VACUUM FULL ) |
autovacuum_count | bigint | Number of times this table has been vacuumed by the autovacuum daemon |
analyze_count | bigint | Number of times this table has been manually analyzed |
autoanalyze_count | bigint | Number of times this table has been analyzed by the autovacuum daemon |
The pg_stat_all_tables
view will contain
one row for each table in the current database (including TOAST
tables), showing statistics about accesses to that specific table. The
pg_stat_user_tables
and
pg_stat_sys_tables
views
contain the same information,
but filtered to only show user and system tables respectively.
Table 28.14. pg_stat_all_indexes
View
Column | Type | Description |
---|---|---|
relid | oid | OID of the table for this index |
indexrelid | oid | OID of this index |
schemaname | name | Name of the schema this index is in |
relname | name | Name of the table for this index |
indexrelname | name | Name of this index |
idx_scan | bigint | Number of index scans initiated on this index |
idx_tup_read | bigint | Number of index entries returned by scans on this index |
idx_tup_fetch | bigint | Number of live table rows fetched by simple index scans using this index |
The pg_stat_all_indexes
view will contain
one row for each index in the current database,
showing statistics about accesses to that specific index. The
pg_stat_user_indexes
and
pg_stat_sys_indexes
views
contain the same information,
but filtered to only show user and system indexes respectively.
Indexes can be used by simple index scans, “bitmap” index scans,
and the optimizer. In a bitmap scan
the output of several indexes can be combined via AND or OR rules,
so it is difficult to associate individual heap row fetches
with specific indexes when a bitmap scan is used. Therefore, a bitmap
scan increments the
pg_stat_all_indexes
.idx_tup_read
count(s) for the index(es) it uses, and it increments the
pg_stat_all_tables
.idx_tup_fetch
count for the table, but it does not affect
pg_stat_all_indexes
.idx_tup_fetch
.
The optimizer also accesses indexes to check for supplied constants
whose values are outside the recorded range of the optimizer statistics
because the optimizer statistics might be stale.
The idx_tup_read
and idx_tup_fetch
counts
can be different even without any use of bitmap scans,
because idx_tup_read
counts
index entries retrieved from the index while idx_tup_fetch
counts live rows fetched from the table. The latter will be less if any
dead or not-yet-committed rows are fetched using the index, or if any
heap fetches are avoided by means of an index-only scan.
Table 28.15. pg_statio_all_tables
View
Column | Type | Description |
---|---|---|
relid | oid | OID of a table |
schemaname | name | Name of the schema that this table is in |
relname | name | Name of this table |
heap_blks_read | bigint | Number of disk blocks read from this table |
heap_blks_hit | bigint | Number of buffer hits in this table |
idx_blks_read | bigint | Number of disk blocks read from all indexes on this table |
idx_blks_hit | bigint | Number of buffer hits in all indexes on this table |
toast_blks_read | bigint | Number of disk blocks read from this table's TOAST table (if any) |
toast_blks_hit | bigint | Number of buffer hits in this table's TOAST table (if any) |
tidx_blks_read | bigint | Number of disk blocks read from this table's TOAST table indexes (if any) |
tidx_blks_hit | bigint | Number of buffer hits in this table's TOAST table indexes (if any) |
The pg_statio_all_tables
view will contain
one row for each table in the current database (including TOAST
tables), showing statistics about I/O on that specific table. The
pg_statio_user_tables
and
pg_statio_sys_tables
views
contain the same information,
but filtered to only show user and system tables respectively.
Table 28.16. pg_statio_all_indexes
View
Column | Type | Description |
---|---|---|
relid | oid | OID of the table for this index |
indexrelid | oid | OID of this index |
schemaname | name | Name of the schema this index is in |
relname | name | Name of the table for this index |
indexrelname | name | Name of this index |
idx_blks_read | bigint | Number of disk blocks read from this index |
idx_blks_hit | bigint | Number of buffer hits in this index |
The pg_statio_all_indexes
view will contain
one row for each index in the current database,
showing statistics about I/O on that specific index. The
pg_statio_user_indexes
and
pg_statio_sys_indexes
views
contain the same information,
but filtered to only show user and system indexes respectively.
Table 28.17. pg_statio_all_sequences
View
Column | Type | Description |
---|---|---|
relid | oid | OID of a sequence |
schemaname | name | Name of the schema this sequence is in |
relname | name | Name of this sequence |
blks_read | bigint | Number of disk blocks read from this sequence |
blks_hit | bigint | Number of buffer hits in this sequence |
The pg_statio_all_sequences
view will contain
one row for each sequence in the current database,
showing statistics about I/O on that specific sequence.
Table 28.18. pg_stat_user_functions
View
Column | Type | Description |
---|---|---|
funcid | oid | OID of a function |
schemaname | name | Name of the schema this function is in |
funcname | name | Name of this function |
calls | bigint | Number of times this function has been called |
total_time | double precision | Total time spent in this function and all other functions called by it, in milliseconds |
self_time | double precision | Total time spent in this function itself, not including other functions called by it, in milliseconds |
The pg_stat_user_functions
view will contain
one row for each tracked function, showing statistics about executions of
that function. The track_functions parameter
controls exactly which functions are tracked.
Other ways of looking at the statistics can be set up by writing
queries that use the same underlying statistics access functions used by
the standard views shown above. For details such as the functions' names,
consult the definitions of the standard views. (For example, in
psql you could issue \d+ pg_stat_activity
.)
The access functions for per-database statistics take a database OID as an
argument to identify which database to report on.
The per-table and per-index functions take a table or index OID.
The functions for per-function statistics take a function OID.
Note that only tables, indexes, and functions in the current database
can be seen with these functions.
Additional functions related to statistics collection are listed in Table 28.19.
Table 28.19. Additional Statistics Functions
pg_stat_get_activity
, the underlying function of
the pg_stat_activity
view, returns a set of records
containing all the available information about each backend process.
Sometimes it may be more convenient to obtain just a subset of this
information. In such cases, an older set of per-backend statistics
access functions can be used; these are shown in Table 28.20.
These access functions use a backend ID number, which ranges from one
to the number of currently active backends.
The function pg_stat_get_backend_idset
provides a
convenient way to generate one row for each active backend for
invoking these functions. For example, to show the PIDs and
current queries of all backends:
SELECT pg_stat_get_backend_pid(s.backendid) AS pid, pg_stat_get_backend_activity(s.backendid) AS query FROM (SELECT pg_stat_get_backend_idset() AS backendid) AS s;
Table 28.20. Per-Backend Statistics Functions
Function | Return Type | Description |
---|---|---|
| setof integer | Set of currently active backend ID numbers (from 1 to the number of active backends) |
| text | Text of this backend's most recent query |
| timestamp with time zone | Time when the most recent query was started |
| inet | IP address of the client connected to this backend |
| integer | TCP port number that the client is using for communication |
| oid | OID of the database this backend is connected to |
| integer | Process ID of this backend |
| timestamp with time zone | Time when this process was started |
| oid | OID of the user logged into this backend |
| text | Wait event type name if backend is currently waiting, otherwise NULL. See Table 28.4 for details. |
| text | Wait event name if backend is currently waiting, otherwise NULL. See Table 28.4 for details. |
| timestamp with time zone | Time when the current transaction was started |