PostgreSQL
27.2. The Statistics Collector
PostgreSQL’s statistics collector is a subsystem that supports collection and reporting of information about server activity. Presently, the collector can count accesses to tables and indexes in both disk-block and individual-row terms. It also tracks the total number of rows in each table, and information about vacuum and analyze actions for each table. It can also count calls to user-defined functions and the total time spent in each one.
PostgreSQL also supports reporting dynamic information about exactly what is going on in the system right now, such as the exact command currently being executed by other server processes, and which other connections exist in the system. This facility is independent of the collector process.
27.2.1. Statistics Collection Configuration
Since collection of statistics adds some overhead to query execution, the system can be configured to collect or not collect information. This is controlled by configuration parameters that are normally set in postgresql.conf. (See Chapter 19 for details about setting configuration parameters.)
The parameter track_activities enables monitoring of the current command being executed by any server process.
The parameter track_counts controls whether statistics are collected about table and index accesses.
The parameter track_functions enables tracking of usage of user-defined functions.
The parameter track_io_timing enables monitoring of block read and write times.
Normally these parameters are set in postgresql.conf so that they apply to all server processes, but it is possible to turn them on or off in individual sessions using the SET command. (To prevent ordinary users from hiding their activity from the administrator, only superusers are allowed to change these parameters with SET.)
The statistics collector transmits the collected information to other PostgreSQL processes through temporary files. These files are stored in the directory named by the stats_temp_directory parameter, pg_stat_tmp by default. For better performance, stats_temp_directory can be pointed at a RAM-based file system, decreasing physical I/O requirements. When the server shuts down cleanly, a permanent copy of the statistics data is stored in the pg_stat subdirectory, so that statistics can be retained across server restarts. When recovery is performed at server start (e.g., after immediate shutdown, server crash, and point-in-time recovery), all statistics counters are reset.
27.2.2. Viewing Statistics
Several predefined views, listed in Table 27.1, are available to show the current state of the system. There are also several other views, listed in Table 27.2, available to show the results of statistics collection. Alternatively, one can build custom views using the underlying statistics functions, as discussed in Section 27.2.20.
When using the statistics to monitor collected data, it is important to realize that the information does not update instantaneously. Each individual server process transmits new statistical counts to the collector just before going idle; so a query or transaction still in progress does not affect the displayed totals. Also, the collector itself emits a new report at most once per PGSTAT_STAT_INTERVAL milliseconds (500 ms unless altered while building the server). So the displayed information lags behind actual activity. However, current-query information collected by track_activities is always up-to-date.
Another important point is that when a server process is asked to display any of these statistics, it first fetches the most recent report emitted by the collector process and then continues to use this snapshot for all statistical views and functions until the end of its current transaction. So the statistics will show static information as long as you continue the current transaction. Similarly, information about the current queries of all sessions is collected when any such information is first requested within a transaction, and the same information will be displayed throughout the transaction. This is a feature, not a bug, because it allows you to perform several queries on the statistics and correlate the results without worrying that the numbers are changing underneath you. But if you want to see new results with each query, be sure to do the queries outside any transaction block. Alternatively, you can invoke pg_stat_clear_snapshot(), which will discard the current transaction’s statistics snapshot (if any). The next use of statistical information will cause a new snapshot to be fetched.
A transaction can also see its own statistics (as yet untransmitted to the collector) in the views pg_stat_xact_all_tables, pg_stat_xact_sys_tables, pg_stat_xact_user_tables, and pg_stat_xact_user_functions. These numbers do not act as stated above; instead they update continuously throughout the transaction.
Some of the information in the dynamic statistics views shown in Table 27.1 is security restricted. Ordinary users can only see all the information about their own sessions (sessions belonging to a role that they are a member of). In rows about other sessions, many columns will be null. Note, however, that the existence of a session and its general properties such as its sessions user and database are visible to all users. Superusers and members of the built-in role pg_read_all_stats (see also Section 21.5) can see all the information about all sessions.
Table 27.1. Dynamic Statistics Views
| View Name | Description |
|---|---|
|
One row per server process, showing information related to the current activity of that process, such as state and current query. See |
|
One row per WAL sender process, showing statistics about replication to that sender’s connected standby server. See |
|
Only one row, showing statistics about the WAL receiver from that receiver’s connected server. See |
|
At least one row per subscription, showing information about the subscription workers. See |
|
One row per connection (regular and replication), showing information about SSL used on this connection. See |
|
One row per connection (regular and replication), showing information about GSSAPI authentication and encryption used on this connection. See |
|
One row for each backend (including autovacuum worker processes) running |
|
One row for each backend running |
|
One row for each backend (including autovacuum worker processes) running |
|
One row for each backend running |
|
One row for each WAL sender process streaming a base backup, showing current progress. See Section 27.4.5. |
+
Table 27.2. Collected Statistics Views
| View Name | Description |
|---|---|
|
One row only, showing statistics about the WAL archiver process’s activity. See |
|
One row only, showing statistics about the background writer process’s activity. See |
|
One row per database, showing database-wide statistics. See |
|
One row per database, showing database-wide statistics about query cancels due to conflict with recovery on standby servers. See |
|
One row for each table in the current database, showing statistics about accesses to that specific table. See |
|
Same as |
|
Same as |
|
Similar to |
|
Same as |
|
Same as |
|
One row for each index in the current database, showing statistics about accesses to that specific index. See |
|
Same as |
|
Same as |
|
One row for each table in the current database, showing statistics about I/O on that specific table. See |
|
Same as |
|
Same as |
|
One row for each index in the current database, showing statistics about I/O on that specific index. See |
|
Same as |
|
Same as |
|
One row for each sequence in the current database, showing statistics about I/O on that specific sequence. See |
|
Same as |
|
Same as |
|
One row for each tracked function, showing statistics about executions of that function. See |
|
Similar to |
|
One row per SLRU, showing statistics of operations. See |
+
The per-index statistics are particularly useful to determine which indexes are being used and how effective they are.
The pg_statio_ views are primarily useful to determine the effectiveness of the buffer cache. When the number of actual disk reads is much smaller than the number of buffer hits, then the cache is satisfying most read requests without invoking a kernel call. However, these statistics do not give the entire story: due to the way in which PostgreSQL handles disk I/O, data that is not in the PostgreSQL buffer cache might still reside in the kernel’s I/O cache, and might therefore still be fetched without requiring a physical read. Users interested in obtaining more detailed information on PostgreSQL I/O behavior are advised to use the PostgreSQL statistics collector in combination with operating system utilities that allow insight into the kernel’s handling of I/O.
27.2.3. pg_stat_activity
The pg_stat_activity view will have one row per server process, showing information related to the current activity of that process.
Table 27.3. pg_stat_activity View
| Column Type Description |
|---|
OID of the database this backend is connected to |
Name of the database this backend is connected to |
Process ID of this backend |
Process ID of the parallel group leader, if this process is a parallel query worker. |
OID of the user logged into this backend |
Name of the user logged into this backend |
Name of the application that is connected to this backend |
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. |
Host name of the connected client, as reported by a reverse DNS lookup of |
TCP port number that the client is using for communication with this backend, or |
Time when this process was started. For client backends, this is the time the client connected to the server. |
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 |
Time when the currently active query was started, or if |
Time when the |
The type of event for which the backend is waiting, if any; otherwise NULL. See Table 27.4. |
Wait event name if backend is currently waiting, otherwise NULL. See Table 27.5 through Table 27.13. |
Current overall state of this backend. Possible values are:
|
Top-level transaction identifier of this backend, if any. |
The current backend’s |
Text of this backend’s most recent query. If |
Type of current backend. Possible types are |
+
Note
The wait_event and state columns are independent. If a backend is in the active state, it may or may not be waiting on some event. If the state is active and wait_event is non-null, it means that a query is being executed, but is being blocked somewhere in the system.
Table 27.4. Wait Event Types
| Wait Event Type | Description |
|---|---|
|
The server process is idle. This event type indicates a process waiting for activity in its main processing loop. |
|
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 27.6. |
|
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. |
|
The server process is waiting for some condition defined by an extension module. See Table 27.8. |
|
The server process is waiting for an I/O operation to complete. |
|
The server process is waiting for some interaction with another server process. |
|
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. |
|
The server process is waiting for a lightweight lock. Most such locks protect a particular data structure in shared memory. |
|
The server process is waiting for a timeout to expire. |
+
Table 27.5. Wait Events of Type Activity
Activity Wait Event |
Description |
|---|---|
|
Waiting in main loop of archiver process. |
|
Waiting in main loop of autovacuum launcher process. |
|
Waiting in background writer process, hibernating. |
|
Waiting in main loop of background writer process. |
|
Waiting in main loop of checkpointer process. |
|
Waiting in main loop of logical replication apply process. |
|
Waiting in main loop of logical replication launcher process. |
|
Waiting in main loop of statistics collector process. |
|
Waiting in main loop of startup process for WAL to arrive, during streaming recovery. |
|
Waiting in main loop of syslogger process. |
|
Waiting in main loop of WAL receiver process. |
|
Waiting in main loop of WAL sender process. |
|
Waiting in main loop of WAL writer process. |
+
Table 27.6. Wait Events of Type BufferPin
BufferPin Wait Event |
Description |
|---|---|
|
Waiting to acquire an exclusive pin on a buffer. |
+
Table 27.7. Wait Events of Type Client
Client Wait Event |
Description |
|---|---|
|
Waiting to read data from the client. |
|
Waiting to write data to the client. |
|
Waiting to read data from the client while establishing a GSSAPI session. |
|
Waiting in WAL receiver to establish connection to remote server. |
|
Waiting in WAL receiver to receive data from remote server. |
|
Waiting for SSL while attempting connection. |
|
Waiting for startup process to send initial data for streaming replication. |
|
Waiting for WAL to be flushed in WAL sender process. |
|
Waiting for any activity when processing replies from WAL receiver in WAL sender process. |
+
Table 27.8. Wait Events of Type Extension
Extension Wait Event |
Description |
|---|---|
|
Waiting in an extension. |
+
Table 27.9. Wait Events of Type IO
IO Wait Event |
Description |
|---|---|
|
Waiting for a read from a buffered file. |
|
Waiting for a write to a buffered file. |
|
Waiting for a read from the |
|
Waiting for the |
|
Waiting for an update to the |
|
Waiting for a write to the |
|
Waiting for a write to update the |
|
Waiting for a read during a file copy operation. |
|
Waiting for a write during a file copy operation. |
|
Waiting to fill a dynamic shared memory backing file with zeroes. |
|
Waiting for a relation data file to be extended. |
|
Waiting for a relation data file to reach durable storage. |
|
Waiting for an immediate synchronization of a relation data file to durable storage. |
|
Waiting for an asynchronous prefetch from a relation data file. |
|
Waiting for a read from a relation data file. |
|
Waiting for changes to a relation data file to reach durable storage. |
|
Waiting for a relation data file to be truncated. |
|
Waiting for a write to a relation data file. |
|
Waiting for a read while adding a line to the data directory lock file. |
|
Waiting for data to reach durable storage while adding a line to the data directory lock file. |
|
Waiting for a write while adding a line to the data directory lock file. |
|
Waiting to read while creating the data directory lock file. |
|
Waiting for data to reach durable storage while creating the data directory lock file. |
|
Waiting for a write while creating the data directory lock file. |
|
Waiting for a read during recheck of the data directory lock file. |
|
Waiting for logical rewrite mappings to reach durable storage during a checkpoint. |
|
Waiting for mapping data to reach durable storage during a logical rewrite. |
|
Waiting for a write of mapping data during a logical rewrite. |
|
Waiting for logical rewrite mappings to reach durable storage. |
|
Waiting for truncate of mapping data during a logical rewrite. |
|
Waiting for a write of logical rewrite mappings. |
|
Waiting for a read of the relation map file. |
|
Waiting for the relation map file to reach durable storage. |
|
Waiting for a write to the relation map file. |
|
Waiting for a read during reorder buffer management. |
|
Waiting for a write during reorder buffer management. |
|
Waiting for a read of a logical mapping during reorder buffer management. |
|
Waiting for a read from a replication slot control file. |
|
Waiting for a replication slot control file to reach durable storage while restoring it to memory. |
|
Waiting for a replication slot control file to reach durable storage. |
|
Waiting for a write to a replication slot control file. |
|
Waiting for SLRU data to reach durable storage during a checkpoint or database shutdown. |
|
Waiting for a read of an SLRU page. |
|
Waiting for SLRU data to reach durable storage following a page write. |
|
Waiting for a write of an SLRU page. |
|
Waiting for a read of a serialized historical catalog snapshot. |
|
Waiting for a serialized historical catalog snapshot to reach durable storage. |
|
Waiting for a write of a serialized historical catalog snapshot. |
|
Waiting for a timeline history file received via streaming replication to reach durable storage. |
|
Waiting for a write of a timeline history file received via streaming replication. |
|
Waiting for a read of a timeline history file. |
|
Waiting for a newly created timeline history file to reach durable storage. |
|
Waiting for a write of a newly created timeline history file. |
|
Waiting for a read of a two phase state file. |
|
Waiting for a two phase state file to reach durable storage. |
|
Waiting for a write of a two phase state file. |
|
Waiting for WAL to reach durable storage during bootstrapping. |
|
Waiting for a write of a WAL page during bootstrapping. |
|
Waiting for a read when creating a new WAL segment by copying an existing one. |
|
Waiting for a new WAL segment created by copying an existing one to reach durable storage. |
|
Waiting for a write when creating a new WAL segment by copying an existing one. |
|
Waiting for a newly initialized WAL file to reach durable storage. |
|
Waiting for a write while initializing a new WAL file. |
|
Waiting for a read from a WAL file. |
|
Waiting for a read from a timeline history file during a walsender timeline command. |
|
Waiting for a WAL file to reach durable storage. |
|
Waiting for data to reach durable storage while assigning a new WAL sync method. |
|
Waiting for a write to a WAL file. |
+
Table 27.10. Wait Events of Type IPC
IPC Wait Event |
Description |
|---|---|
|
Waiting for WAL files required for a backup to be successfully archived. |
|
Waiting for background worker to shut down. |
|
Waiting for background worker to start up. |
|
Waiting for the page number needed to continue a parallel B-tree scan to become available. |
|
Waiting for a checkpoint to complete. |
|
Waiting for a checkpoint to start. |
|
Waiting for activity from a child process while executing a |
|
Waiting for an elected Parallel Hash participant to allocate a hash table. |
|
Waiting to elect a Parallel Hash participant to allocate a hash table. |
|
Waiting for other Parallel Hash participants to finish loading a hash table. |
|
Waiting for an elected Parallel Hash participant to allocate the initial hash table. |
|
Waiting to elect a Parallel Hash participant to allocate the initial hash table. |
|
Waiting for other Parallel Hash participants to finish hashing the inner relation. |
|
Waiting for other Parallel Hash participants to finish partitioning the outer relation. |
|
Waiting for an elected Parallel Hash participant to allocate more batches. |
|
Waiting to elect a Parallel Hash participant to decide on future batch growth. |
|
Waiting to elect a Parallel Hash participant to allocate more batches. |
|
Waiting for an elected Parallel Hash participant to decide on future batch growth. |
|
Waiting for other Parallel Hash participants to finish repartitioning. |
|
Waiting for an elected Parallel Hash participant to finish allocating more buckets. |
|
Waiting to elect a Parallel Hash participant to allocate more buckets. |
|
Waiting for other Parallel Hash participants to finish inserting tuples into new buckets. |
|
Waiting for a logical replication remote server to send data for initial table synchronization. |
|
Waiting for a logical replication remote server to change state. |
|
Waiting for another process to be attached to a shared message queue. |
|
Waiting to write a protocol message to a shared message queue. |
|
Waiting to receive bytes from a shared message queue. |
|
Waiting to send bytes to a shared message queue. |
|
Waiting for parallel bitmap scan to become initialized. |
|
Waiting for parallel |
|
Waiting for parallel workers to finish computing. |
|
Waiting for the group leader to clear the transaction ID at end of a parallel operation. |
|
Waiting for a barrier event to be processed by all backends. |
|
Waiting for standby promotion. |
|
Waiting for recovery conflict resolution for a vacuum cleanup. |
|
Waiting for recovery conflict resolution for dropping a tablespace. |
|
Waiting for recovery to be resumed. |
|
Waiting for a replication origin to become inactive so it can be dropped. |
|
Waiting for a replication slot to become inactive so it can be dropped. |
|
Waiting to obtain a valid snapshot for a |
|
Waiting for confirmation from a remote server during synchronous replication. |
|
Waiting for the group leader to update transaction status at end of a parallel operation. |
+
Table 27.11. Wait Events of Type Lock
Lock Wait Event |
Description |
|---|---|
|
Waiting to acquire an advisory user lock. |
|
Waiting to extend a relation. |
|
Waiting to update |
|
Waiting to acquire a lock on a non-relation database object. |
|
Waiting to acquire a lock on a page of a relation. |
|
Waiting to acquire a lock on a relation. |
|
Waiting to acquire a speculative insertion lock. |
|
Waiting for a transaction to finish. |
|
Waiting to acquire a lock on a tuple. |
|
Waiting to acquire a user lock. |
|
Waiting to acquire a virtual transaction ID lock. |
+
Table 27.12. Wait Events of Type LWLock
LWLock Wait Event |
Description |
|---|---|
|
Waiting to manage an extension’s space allocation in shared memory. |
|
Waiting to update the |
|
Waiting to read or update the current state of autovacuum workers. |
|
Waiting to ensure that a table selected for autovacuum still needs vacuuming. |
|
Waiting to read or update background worker state. |
|
Waiting to read or update vacuum-related information for a B-tree index. |
|
Waiting to access a data page in memory. |
|
Waiting for I/O on a data page. |
|
Waiting to associate a data block with a buffer in the buffer pool. |
|
Waiting to begin a checkpoint. |
|
Waiting to manage fsync requests. |
|
Waiting to read or update the last value set for a transaction commit timestamp. |
|
Waiting for I/O on a commit timestamp SLRU buffer. |
|
Waiting to access the commit timestamp SLRU cache. |
|
Waiting to read or update the |
|
Waiting to read or update dynamic shared memory allocation information. |
|
Waiting to read or update a process' fast-path lock information. |
|
Waiting to read or update information about “[.quote]#heavyweight”# locks. |
|
Waiting to read or update the state of logical replication workers. |
|
Waiting to read or update shared multixact state. |
|
Waiting for I/O on a multixact member SLRU buffer. |
|
Waiting to access the multixact member SLRU cache. |
|
Waiting for I/O on a multixact offset SLRU buffer. |
|
Waiting to access the multixact offset SLRU cache. |
|
Waiting to read or truncate multixact information. |
|
Waiting for I/O on a |
|
Waiting to read or update |
|
Waiting to update limit on |
|
Waiting to access the |
|
Waiting to allocate a new OID. |
|
Waiting to read or update old snapshot control information. |
|
Waiting to choose the next subplan during Parallel Append plan execution. |
|
Waiting to synchronize workers during Parallel Hash Join plan execution. |
|
Waiting for parallel query dynamic shared memory allocation. |
|
Waiting for parallel query dynamic shared memory allocation. |
|
Waiting to access a parallel query’s information about composite types. |
|
Waiting to access a parallel query’s information about type modifiers that identify anonymous record types. |
|
Waiting to access the list of predicate locks held by the current serializable transaction during a parallel query. |
|
Waiting to access predicate lock information used by serializable transactions. |
|
Waiting to access the shared per-process data structures (typically, to get a snapshot or report a session’s transaction ID). |
|
Waiting to read or update a |
|
Waiting to read or update a |
|
Waiting to create, drop or use a replication origin. |
|
Waiting to read or update the progress of one replication origin. |
|
Waiting to allocate or free a replication slot. |
|
Waiting to read or update replication slot state. |
|
Waiting for I/O on a replication slot. |
|
Waiting for I/O on a serializable transaction conflict SLRU buffer. |
|
Waiting to access the list of finished serializable transactions. |
|
Waiting to access the list of predicate locks held by serializable transactions. |
|
Waiting to read or update information about serializable transactions. |
|
Waiting to access the serializable transaction conflict SLRU cache. |
|
Waiting to access a shared TID bitmap during a parallel bitmap index scan. |
|
Waiting to access a shared tuple store during parallel query. |
|
Waiting to find or allocate space in shared memory. |
|
Waiting to retrieve messages from the shared catalog invalidation queue. |
|
Waiting to add a message to the shared catalog invalidation queue. |
|
Waiting for I/O on a sub-transaction SLRU buffer. |
|
Waiting to access the sub-transaction SLRU cache. |
|
Waiting to read or update information about the state of synchronous replication. |
|
Waiting to select the starting location of a synchronized table scan. |
|
Waiting to create or drop a tablespace. |
|
Waiting to read or update the state of prepared transactions. |
|
Waiting to replace a page in WAL buffers. |
|
Waiting to insert WAL data into a memory buffer. |
|
Waiting for WAL buffers to be written to disk. |
|
Waiting to update limits on transaction id and multixact consumption. |
|
Waiting for I/O on a transaction status SLRU buffer. |
|
Waiting to access the transaction status SLRU cache. |
|
Waiting to execute |
|
Waiting to allocate a new transaction ID. |
+
Note
Extensions can add LWLock types to the list shown in Table 27.12. In some cases, the name assigned by an extension will not be available in all server processes; so an LWLock wait event might be reported as just “[.quote]extension” rather than the extension-assigned name.
Table 27.13. Wait Events of Type Timeout
Timeout Wait Event |
Description |
|---|---|
|
Waiting during base backup when throttling activity. |
|
Waiting due to a call to |
|
Waiting to apply WAL during recovery because of a delay setting. |
|
Waiting during recovery when WAL data is not available from any source ( |
|
Waiting while sending synchronization requests to the checkpointer, because the request queue is full. |
|
Waiting in a cost-based vacuum delay point. |
+
Here is an example of how wait events can be viewed:
SELECT pid, wait_event_type, wait_event FROM pg_stat_activity WHERE wait_event is NOT NULL;
pid | wait_event_type | wait_event
------+-----------------+------------
2540 | Lock | relation
6644 | LWLock | ProcArray
(2 rows)27.2.4. pg_stat_replication
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.
Table 27.14. pg_stat_replication View
| Column Type Description |
|---|
Process ID of a WAL sender process |
OID of the user logged into this WAL sender process |
Name of the user logged into this WAL sender process |
Name of the application that is connected to this WAL sender |
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. |
Host name of the connected client, as reported by a reverse DNS lookup of |
TCP port number that the client is using for communication with this WAL sender, or |
Time when this process was started, i.e., when the client connected to this WAL sender |
This standby’s |
Current WAL sender state. Possible values are:
|
Last write-ahead log location sent on this connection |
Last write-ahead log location written to disk by this standby server |
Last write-ahead log location flushed to disk by this standby server |
Last write-ahead log location replayed into the database on this standby server |
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 |
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 |
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 |
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. |
Synchronous state of this standby server. Possible values are:
|
Send time of last reply message received from standby server |
+
The lag times reported in the pg_stat_replication view are measurements of the time taken for recent WAL to be written, flushed and replayed and for the sender to know about it. These times represent the commit delay that was (or would have been) introduced by each synchronous commit level, if the remote server was configured as a synchronous standby. For an asynchronous standby, the replay_lag column approximates the delay before recent transactions became visible to queries. If the standby server has entirely caught up with the sending server and there is no more WAL activity, the most recently measured lag times will continue to be displayed for a short time and then show NULL.
Lag times work automatically for physical replication. Logical decoding plugins may optionally emit tracking messages; if they do not, the tracking mechanism will simply display NULL lag.
Note
The reported lag times are not predictions of how long it will take for the standby to catch up with the sending server assuming the current rate of replay. Such a system would show similar times while new WAL is being generated, but would differ when the sender becomes idle. In particular, when the standby has caught up completely, pg_stat_replication shows the time taken to write, flush and replay the most recent reported WAL location rather than zero as some users might expect. This is consistent with the goal of measuring synchronous commit and transaction visibility delays for recent write transactions. To reduce confusion for users expecting a different model of lag, the lag columns revert to NULL after a short time on a fully replayed idle system. Monitoring systems should choose whether to represent this as missing data, zero or continue to display the last known value.
27.2.5. pg_stat_wal_receiver
The pg_stat_wal_receiver view will contain only one row, showing statistics about the WAL receiver from that receiver’s connected server.
Table 27.15. pg_stat_wal_receiver View
| Column Type Description |
|---|
Process ID of the WAL receiver process |
Activity status of the WAL receiver process |
First write-ahead log location used when WAL receiver is started |
First timeline number used when WAL receiver is started |
Last write-ahead log location already received and written to disk, but not flushed. This should not be used for data integrity checks. |
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 |
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 |
Send time of last message received from origin WAL sender |
Receipt time of last message received from origin WAL sender |
Last write-ahead log location reported to origin WAL sender |
Time of last write-ahead log location reported to origin WAL sender |
Replication slot name used by this WAL receiver |
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 |
Port number of the PostgreSQL instance this WAL receiver is connected to. |
Connection string used by this WAL receiver, with security-sensitive fields obfuscated. |
+
27.2.6. pg_stat_subscription
The pg_stat_subscription view will contain one row per subscription for main worker (with null PID if the worker is not running), and additional rows for workers handling the initial data copy of the subscribed tables.
Table 27.16. pg_stat_subscription View
| Column Type Description |
|---|
OID of the subscription |
Name of the subscription |
Process ID of the subscription worker process |
OID of the relation that the worker is synchronizing; null for the main apply worker |
Last write-ahead log location received, the initial value of this field being 0 |
Send time of last message received from origin WAL sender |
Receipt time of last message received from origin WAL sender |
Last write-ahead log location reported to origin WAL sender |
Time of last write-ahead log location reported to origin WAL sender |
+
27.2.7. pg_stat_ssl
The pg_stat_ssl view will contain one row per backend or WAL sender process, showing statistics about SSL usage on this connection. It can be joined to pg_stat_activity or pg_stat_replication on the pid column to get more details about the connection.
Table 27.17. pg_stat_ssl View
| Column Type Description |
|---|
Process ID of a backend or WAL sender process |
True if SSL is used on this connection |
Version of SSL in use, or NULL if SSL is not in use on this connection |
Name of SSL cipher in use, or NULL if SSL is not in use on this connection |
Number of bits in the encryption algorithm used, or NULL if SSL is not used on this connection |
True if SSL compression is in use, false if not, or NULL if SSL is not in use on this connection |
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 |
Serial number of the client certificate, or NULL if no client certificate was supplied or if SSL is not in use on this connection. The combination of certificate serial number and certificate issuer uniquely identifies a certificate (unless the issuer erroneously reuses serial numbers). |
DN of the issuer of the client certificate, or NULL if no client certificate was supplied or if SSL is not in use on this connection. This field is truncated like |
+
27.2.8. pg_stat_gssapi
The pg_stat_gssapi view will contain one row per backend, showing information about GSSAPI usage on this connection. It can be joined to pg_stat_activity or pg_stat_replication on the pid column to get more details about the connection.
Table 27.18. pg_stat_gssapi View
| Column Type Description |
|---|
Process ID of a backend |
True if GSSAPI authentication was used for this connection |
Principal used to authenticate this connection, or NULL if GSSAPI was not used to authenticate this connection. This field is truncated if the principal is longer than |
True if GSSAPI encryption is in use on this connection |
+
27.2.9. pg_stat_archiver
The pg_stat_archiver view will always have a single row, containing data about the archiver process of the cluster.
Table 27.19. pg_stat_archiver View
| Column Type Description |
|---|
Number of WAL files that have been successfully archived |
Name of the last WAL file successfully archived |
Time of the last successful archive operation |
Number of failed attempts for archiving WAL files |
Name of the WAL file of the last failed archival operation |
Time of the last failed archival operation |
Time at which these statistics were last reset |
+
27.2.10. pg_stat_bgwriter
The pg_stat_bgwriter view will always have a single row, containing global data for the cluster.
Table 27.20. pg_stat_bgwriter View
| Column Type Description |
|---|
Number of scheduled checkpoints that have been performed |
Number of requested checkpoints that have been performed |
Total amount of time that has been spent in the portion of checkpoint processing where files are written to disk, in milliseconds |
Total amount of time that has been spent in the portion of checkpoint processing where files are synchronized to disk, in milliseconds |
Number of buffers written during checkpoints |
Number of buffers written by the background writer |
Number of times the background writer stopped a cleaning scan because it had written too many buffers |
Number of buffers written directly by a backend |
Number of times a backend had to execute its own |
Number of buffers allocated |
Time at which these statistics were last reset |
+
27.2.11. pg_stat_database
The pg_stat_database view will contain one row for each database in the cluster, plus one for shared objects, showing database-wide statistics.
Table 27.21. pg_stat_database View
| Column Type Description |
|---|
OID of this database, or 0 for objects belonging to a shared relation |
Name of this database, or |
Number of backends currently connected to this database, or |
Number of transactions in this database that have been committed |
Number of transactions in this database that have been rolled back |
Number of disk blocks read in this database |
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) |
Number of rows returned by queries in this database |
Number of rows fetched by queries in this database |
Number of rows inserted by queries in this database |
Number of rows updated by queries in this database |
Number of rows deleted by queries in this database |
Number of queries canceled due to conflicts with recovery in this database. (Conflicts occur only on standby servers; see |
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. |
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. |
Number of deadlocks detected in this database |
Number of data page checksum failures detected in this database (or on a shared object), or NULL if data checksums are not enabled. |
Time at which the last data page checksum failure was detected in this database (or on a shared object), or NULL if data checksums are not enabled. |
Time spent reading data file blocks by backends in this database, in milliseconds (if track_io_timing is enabled, otherwise zero) |
Time spent writing data file blocks by backends in this database, in milliseconds (if track_io_timing is enabled, otherwise zero) |
Time at which these statistics were last reset |
+
27.2.12. pg_stat_database_conflicts
The pg_stat_database_conflicts view will contain one row per database, showing database-wide statistics about query cancels occurring due to conflicts with recovery on standby servers. This view will only contain information on standby servers, since conflicts do not occur on master servers.
Table 27.22. pg_stat_database_conflicts View
| Column Type Description |
|---|
OID of a database |
Name of this database |
Number of queries in this database that have been canceled due to dropped tablespaces |
Number of queries in this database that have been canceled due to lock timeouts |
Number of queries in this database that have been canceled due to old snapshots |
Number of queries in this database that have been canceled due to pinned buffers |
Number of queries in this database that have been canceled due to deadlocks |
+
27.2.13. pg_stat_all_tables
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 27.23. pg_stat_all_tables View
| Column Type Description |
|---|
OID of a table |
Name of the schema that this table is in |
Name of this table |
Number of sequential scans initiated on this table |
Number of live rows fetched by sequential scans |
Number of index scans initiated on this table |
Number of live rows fetched by index scans |
Number of rows inserted |
Number of rows updated (includes HOT updated rows) |
Number of rows deleted |
Number of rows HOT updated (i.e., with no separate index update required) |
Estimated number of live rows |
Estimated number of dead rows |
Estimated number of rows modified since this table was last analyzed |
Estimated number of rows inserted since this table was last vacuumed |
Last time at which this table was manually vacuumed (not counting |
Last time at which this table was vacuumed by the autovacuum daemon |
Last time at which this table was manually analyzed |
Last time at which this table was analyzed by the autovacuum daemon |
Number of times this table has been manually vacuumed (not counting |
Number of times this table has been vacuumed by the autovacuum daemon |
Number of times this table has been manually analyzed |
Number of times this table has been analyzed by the autovacuum daemon |
+
27.2.14. pg_stat_all_indexes
The pg_stat_all_indexes view will contain one row for each index in the current database, showing statistics about accesses to that specific index. The pg_stat_user_indexes and pg_stat_sys_indexes views contain the same information, but filtered to only show user and system indexes respectively.
Table 27.24. pg_stat_all_indexes View
| Column Type Description |
|---|
OID of the table for this index |
OID of this index |
Name of the schema this index is in |
Name of the table for this index |
Name of this index |
Number of index scans initiated on this index |
Number of index entries returned by scans on this index |
Number of live table rows fetched by simple index scans using this index |
+
Indexes can be used by simple index scans, “[.quote]#bitmap”# index scans, and the optimizer. In a bitmap scan the output of several indexes can be combined via AND or OR rules, so it is difficult to associate individual heap row fetches with specific indexes when a bitmap scan is used. Therefore, a bitmap scan increments the pg_stat_all_indexes.idx_tup_read count(s) for the index(es) it uses, and it increments the pg_stat_all_tables.idx_tup_fetch count for the table, but it does not affect pg_stat_all_indexes.idx_tup_fetch. The optimizer also accesses indexes to check for supplied constants whose values are outside the recorded range of the optimizer statistics because the optimizer statistics might be stale.
Note
The idx_tup_read and idx_tup_fetch counts can be different even without any use of bitmap scans, because idx_tup_read counts index entries retrieved from the index while idx_tup_fetch counts live rows fetched from the table. The latter will be less if any dead or not-yet-committed rows are fetched using the index, or if any heap fetches are avoided by means of an index-only scan.
27.2.15. pg_statio_all_tables
The pg_statio_all_tables view will contain one row for each table in the current database (including TOAST tables), showing statistics about I/O on that specific table. The pg_statio_user_tables and pg_statio_sys_tables views contain the same information, but filtered to only show user and system tables respectively.
Table 27.25. pg_statio_all_tables View
| Column Type Description |
|---|
OID of a table |
Name of the schema that this table is in |
Name of this table |
Number of disk blocks read from this table |
Number of buffer hits in this table |
Number of disk blocks read from all indexes on this table |
Number of buffer hits in all indexes on this table |
Number of disk blocks read from this table’s TOAST table (if any) |
Number of buffer hits in this table’s TOAST table (if any) |
Number of disk blocks read from this table’s TOAST table indexes (if any) |
Number of buffer hits in this table’s TOAST table indexes (if any) |
+
27.2.16. pg_statio_all_indexes
The pg_statio_all_indexes view will contain one row for each index in the current database, showing statistics about I/O on that specific index. The pg_statio_user_indexes and pg_statio_sys_indexes views contain the same information, but filtered to only show user and system indexes respectively.
Table 27.26. pg_statio_all_indexes View
| Column Type Description |
|---|
OID of the table for this index |
OID of this index |
Name of the schema this index is in |
Name of the table for this index |
Name of this index |
Number of disk blocks read from this index |
Number of buffer hits in this index |
+
27.2.17. pg_statio_all_sequences
The pg_statio_all_sequences view will contain one row for each sequence in the current database, showing statistics about I/O on that specific sequence.
Table 27.27. pg_statio_all_sequences View
| Column Type Description |
|---|
OID of a sequence |
Name of the schema this sequence is in |
Name of this sequence |
Number of disk blocks read from this sequence |
Number of buffer hits in this sequence |
+
27.2.18. pg_stat_user_functions
The pg_stat_user_functions view will contain one row for each tracked function, showing statistics about executions of that function. The track_functions parameter controls exactly which functions are tracked.
Table 27.28. pg_stat_user_functions View
| Column Type Description |
|---|
OID of a function |
Name of the schema this function is in |
Name of this function |
Number of times this function has been called |
Total time spent in this function and all other functions called by it, in milliseconds |
Total time spent in this function itself, not including other functions called by it, in milliseconds |
+
27.2.19. pg_stat_slru
PostgreSQL accesses certain on-disk information via SLRU (simple least-recently-used) caches. The pg_stat_slru view will contain one row for each tracked SLRU cache, showing statistics about access to cached pages.
Table 27.29. pg_stat_slru View
| Column Type Description |
|---|
Name of the SLRU |
Number of blocks zeroed during initializations |
Number of times disk blocks were found already in the SLRU, so that a read was not necessary (this only includes hits in the SLRU, not the operating system’s file system cache) |
Number of disk blocks read for this SLRU |
Number of disk blocks written for this SLRU |
Number of blocks checked for existence for this SLRU |
Number of flushes of dirty data for this SLRU |
Number of truncates for this SLRU |
Time at which these statistics were last reset |
+
27.2.20. Statistics Functions
Other ways of looking at the statistics can be set up by writing queries that use the same underlying statistics access functions used by the standard views shown above. For details such as the functions' names, consult the definitions of the standard views. (For example, in psql you could issue \d pg_stat_activity+.) The access functions for per-database statistics take a database OID as an argument to identify which database to report on. The per-table and per-index functions take a table or index OID. The functions for per-function statistics take a function OID. Note that only tables, indexes, and functions in the current database can be seen with these functions.
Additional functions related to statistics collection are listed in Table 27.30.
Table 27.30. Additional Statistics Functions
| Function Description |
|---|
Returns the process ID of the server process attached to the current session. |
`+pg_stat_get_activity+` ( `+integer+` ) → `+setof record+` Returns a record of information about the backend with the specified process ID, or one record for each active backend in the system if |
`+pg_stat_get_snapshot_timestamp+` () → `+timestamp with time zone+` Returns the timestamp of the current statistics snapshot. |
`+pg_stat_get_xact_blocks_fetched+` ( `+oid+` ) → `+bigint+` Returns the number of block read requests for table or index, in the current transaction. This number minus |
`+pg_stat_get_xact_blocks_hit+` ( `+oid+` ) → `+bigint+` Returns the number of block read requests for table or index, in the current transaction, found in cache (not triggering kernel |
`+pg_stat_clear_snapshot+` () → `+void+` Discards the current statistics snapshot. |
`+pg_stat_reset+` () → `+void+` Resets all statistics counters for the current database to zero. This function is restricted to superusers by default, but other users can be granted EXECUTE to run the function. |
`+pg_stat_reset_shared+` ( `+text+` ) → `+void+` Resets some cluster-wide statistics counters to zero, depending on the argument. The argument can be This function is restricted to superusers by default, but other users can be granted EXECUTE to run the function. |
`+pg_stat_reset_single_table_counters+` ( `+oid+` ) → `+void+` Resets statistics for a single table or index in the current database to zero. This function is restricted to superusers by default, but other users can be granted EXECUTE to run the function. |
`+pg_stat_reset_single_function_counters+` ( `+oid+` ) → `+void+` Resets statistics for a single function in the current database to zero. This function is restricted to superusers by default, but other users can be granted EXECUTE to run the function. |
`+pg_stat_reset_slru+` ( `+text+` ) → `+void+` Resets statistics to zero for a single SLRU cache, or for all SLRUs in the cluster. If the argument is NULL, all counters shown in the This function is restricted to superusers by default, but other users can be granted EXECUTE to run the function. |
+
Warning
Using pg_stat_reset() also resets counters that autovacuum uses to determine when to trigger a vacuum or an analyze. Resetting these counters can cause autovacuum to not perform necessary work, which can cause problems such as table bloat or out-dated table statistics. A database-wide ANALYZE is recommended after the statistics have been reset.
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 27.31. 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 27.31. Per-Backend Statistics Functions
| Function Description |
|---|
`+pg_stat_get_backend_idset+` () → `+setof integer+` Returns the set of currently active backend ID numbers (from 1 to the number of active backends). |
`+pg_stat_get_backend_activity+` ( `+integer+` ) → `+text+` Returns the text of this backend’s most recent query. |
`+pg_stat_get_backend_activity_start+` ( `+integer+` ) → `+timestamp with time zone+` Returns the time when the backend’s most recent query was started. |
`+pg_stat_get_backend_client_addr+` ( `+integer+` ) → `+inet+` Returns the IP address of the client connected to this backend. |
`+pg_stat_get_backend_client_port+` ( `+integer+` ) → `+integer+` Returns the TCP port number that the client is using for communication. |
`+pg_stat_get_backend_dbid+` ( `+integer+` ) → `+oid+` Returns the OID of the database this backend is connected to. |
`+pg_stat_get_backend_pid+` ( `+integer+` ) → `+integer+` Returns the process ID of this backend. |
`+pg_stat_get_backend_start+` ( `+integer+` ) → `+timestamp with time zone+` Returns the time when this process was started. |
`+pg_stat_get_backend_userid+` ( `+integer+` ) → `+oid+` Returns the OID of the user logged into this backend. |
`+pg_stat_get_backend_wait_event_type+` ( `+integer+` ) → `+text+` Returns the wait event type name if this backend is currently waiting, otherwise NULL. See Table 27.4 for details. |
`+pg_stat_get_backend_wait_event+` ( `+integer+` ) → `+text+` Returns the wait event name if this backend is currently waiting, otherwise NULL. See Table 27.5 through Table 27.13. |
`+pg_stat_get_backend_xact_start+` ( `+integer+` ) → `+timestamp with time zone+` Returns the time when the backend’s current transaction was started. |
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27.3. Viewing Locks |
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