28.2. 統計資訊收集器
PostgreSQL's cumulative statistics system supports collection and reporting of information about server activity. Presently, accesses to tables and indexes in both disk-block and individual-row terms are counted. The total number of rows in each table, and information about vacuum and analyze actions for each table are also counted. If enabled, calls to user-defined functions and the total time spent in each one are counted as well.
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 cumulative statistics system.
28.2.1. 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 20 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 cumulative 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.
The parameter track_wal_io_timing enables monitoring of WAL 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
.)
Cumulative statistics are collected in shared memory. Every PostgreSQL process collects statistics locally, then updates the shared data at appropriate intervals. When a server, including a physical replica, 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. In contrast, when starting from an unclean shutdown (e.g., after an immediate shutdown, a server crash, starting from a base backup, and point-in-time recovery), all statistics counters are reset.
28.2.2. Viewing Statistics
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 accumulated statistics. Alternatively, one can build custom views using the underlying cumulative statistics functions, as discussed in Section 28.2.24.
When using the cumulative statistics views and functions to monitor collected data, it is important to realize that the information does not update instantaneously. Each individual server process flushes out accumulated statistics to shared memory just before going idle, but not more frequently than once per PGSTAT_MIN_INTERVAL
milliseconds (1 second unless altered while building the server); so a query or transaction still in progress does not affect the displayed totals and 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 the accumulated statistics, accessed values are cached until the end of its current transaction in the default configuration. 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. When analyzing statistics interactively, or with expensive queries, the time delta between accesses to individual statistics can lead to significant skew in the cached statistics. To minimize skew, stats_fetch_consistency
can be set to snapshot
, at the price of increased memory usage for caching not-needed statistics data. Conversely, if it's known that statistics are only accessed once, caching accessed statistics is unnecessary and can be avoided by setting stats_fetch_consistency
to none
. You can invoke pg_stat_clear_snapshot
() to discard the current transaction's statistics snapshot or cached values (if any). The next use of statistical information will (when in snapshot mode) cause a new snapshot to be built or (when in cache mode) accessed statistics to be cached.
A transaction can also see its own statistics (not yet flushed out to the shared memory statistics) 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 28.1 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 roles with privileges of built-in role pg_read_all_stats
(see also Section 22.5) can see all the information about all sessions.
Table 28.1. 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 |
| Only one row, showing statistics about blocks prefetched during recovery. 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 28.4.5. |
| One row for each backend running |
Table 28.2. 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 only, showing statistics about WAL 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 |
| One row per replication slot, showing statistics about the replication slot's usage. See |
| One row per subscription, showing statistics about errors. 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 views in combination with operating system utilities that allow insight into the kernel's handling of I/O.
28.2.3. pg_stat_activity
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 28.3. pg_stat_activity
View
pg_stat_activity
ViewColumn 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 28.4. |
Wait event name if backend is currently waiting, otherwise NULL. See Table 28.5 through Table 28.13. |
Current overall state of this backend. Possible values are:
|
Top-level transaction identifier of this backend, if any. |
The current backend's |
Identifier of this backend's most recent query. If |
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 28.4. 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 28.6. |
| 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 28.8. |
| 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 28.5. Wait Events of Type Activity
Activity
| 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 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 28.6. Wait Events of Type BufferPin
BufferPin
| Description |
| Waiting to acquire an exclusive pin on a buffer. |
Table 28.7. Wait Events of Type Client
Client
| 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 WAL to be flushed in WAL sender process. |
| Waiting for any activity when processing replies from WAL receiver in WAL sender process. |
Table 28.8. Wait Events of Type Extension
Extension
| Description |
| Waiting in an extension. |
Table 28.9. Wait Events of Type IO
IO
| Description |
| Waiting for base backup to read from a file. |
| Waiting for a read from a buffered file. |
| Waiting for a write to a buffered file. |
| Waiting for a buffered file to be truncated. |
| 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 the version file to be written while creating a database. |
| 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 28.10. Wait Events of Type IPC
IPC
| Description |
| Waiting for subplan nodes of an |
| Waiting for archive_cleanup_command to complete. |
| Waiting for archive_command to complete. |
| Waiting for the termination of another backend. |
| 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 buffer I/O to complete. |
| 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_end_command to complete. |
| 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 for restore_command to complete. |
| Waiting to obtain a valid snapshot for a |
| Waiting for confirmation from a remote server during synchronous replication. |
| Waiting for the WAL receiver to exit. |
| Waiting for startup process to send initial data for streaming replication. |
| Waiting for the group leader to update transaction status at end of a parallel operation. |
Table 28.11. Wait Events of Type Lock
Lock
| 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 28.12. Wait Events of Type LWLock
LWLock
| 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 to associate a data block with a buffer in the buffer pool. |
| 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 “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 for stats dynamic shared memory allocator access |
| Waiting for stats shared memory hash table access |
| Waiting for shared memory stats data access |
| 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 28.12. In some cases, the name assigned by an extension will not be available in all server processes; so an LWLock
wait event might be reported as just “extension
” rather than the extension-assigned name.
Table 28.13. Wait Events of Type Timeout
Timeout
| Description |
| Waiting during base backup when throttling activity. |
| Waiting between writes while performing a checkpoint. |
| 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. |
| Waiting to acquire an exclusive lock to truncate off any empty pages at the end of a table vacuumed. |
Here is an example of how wait events can be viewed:
28.2.4. pg_stat_replication
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 28.14. pg_stat_replication
View
pg_stat_replication
ViewColumn 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. |
|