Prevent sudden database connection spikes by using a connection pool. 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. Waiting for a write of a WAL page during bootstrapping. Amount of transaction data decoded for streaming in-progress transactions to the decoding output plugin while decoding changes from WAL for this slot. Waiting in main loop of WAL receiver process. For client backends, this is the time the client connected to the server. Waiting for a replication slot control file to reach durable storage. Waiting for a write of a serialized historical catalog snapshot. Per-Backend Statistics Functions, pg_stat_get_backend_idset () setof integer. Simple test for lock_waits log messages. Time when this process' current transaction was started, or null if no transaction is active. Waiting to read or update information about the state of synchronous replication. 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. The pg_stat_subscription_stats view will contain one row per subscription. 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. Waiting to read or update old snapshot control information. Waiting for the group leader to update transaction status at end of a parallel operation. See Section30.5 for more information about the internal WAL function issue_xlog_fsync. It can also count calls to user-defined functions and the total time spent in each one. Waiting to allocate or free a replication slot. Waiting to access a parallel query's information about type modifiers that identify anonymous record types. LWLock:BufferIO - Amazon Relational Database Service AWSDocumentationAmazon RDS and Aurora DocumentationUser Guide Relevant engine versionsContextCausesActions LWLock:BufferIO 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. If this field is null, it indicates that the client is connected via a Unix socket on the server machine. potential: This standby server is now asynchronous, but can potentially become synchronous if one of current synchronous ones fails. Write-Ahead Logging (WAL) is a standard method for ensuring data integrity. Waiting to receive bytes from a shared message queue. Waiting for a write during reorder buffer management. Name of this database, or NULL for shared objects. Waiting to read or update information about synchronous replicas. 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. 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. Returns the OID of the database this backend is connected to. Sometimes it may be more convenient to obtain just a subset of this information. Waiting for a relation data file to be extended. Amount of transaction data decoded for sending transactions to the decoding output plugin while decoding changes from WAL for this slot. Locks in PostgreSQL: 4. Locks in memory - Habr The server process is waiting for activity on a socket connected to a user application. The pg_statio_ views are primarily useful to determine the effectiveness of the buffer cache. Last write-ahead log location already received and written to disk, but not flushed. 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. The argument can be one of CommitTs, MultiXactMember, MultiXactOffset, Notify, Serial, Subtrans, or Xact to reset the counters for only that entry. So the displayed information lags behind actual activity. Waiting for SSL while attempting connection. If you've got a moment, please tell us how we can make the documentation better. The server process is waiting for an I/O operation to complete. Choose the appropriate target Region. Returns the wait event type name if this backend is currently waiting, otherwise NULL. The server process is waiting for some interaction with another server process. PostgreSQL's cumulative statistics system supports collection and reporting of information about server activity. Table28.19.pg_stat_subscription_stats View, Number of times an error occurred while applying changes, Number of times an error occurred during the initial table synchronization. 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. Re: [HACKERS] Sequence Access Method WIP postgres 51 LWLock--2_Serendipity_Shy-CSDN Waiting for a write of logical rewrite mappings. 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. Waiting for an asynchronous prefetch from a relation data file. Did this page help you? The pg_stat_recovery_prefetch view will contain only one row. 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. Wait Events of Type BufferPin, Table28.8. See, One row per database, showing database-wide statistics. 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. Its Time when the currently active query was started, or if state is not active, when the last query was started. Waiting for a read of the relation map file. Resetting these counters can cause autovacuum to not perform necessary work, which can cause problems such as table bloat or out-dated table statistics. LWLock:BufferIO. However, current-query information collected by track_activities is always up-to-date. See. See, One row for each sequence in the current database, showing statistics about I/O on that specific sequence. pg_blocking_pids function. Waiting for group leader to clear transaction id at transaction end. Waiting for a relation data file to be extended. to report a documentation issue. OID of this database, or 0 for objects belonging to a shared relation. Waiting to access the sub-transaction SLRU cache. This field will only be non-null for IP connections, and only when log_hostname is enabled. 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. Waiting for a replication origin to become inactive to be dropped. Waiting for a write to a replication slot control file. This facility is independent of the cumulative statistics system. See, One row for each table in the current database, showing statistics about I/O on that specific table. These numbers do not act as stated above; instead they update continuously throughout the transaction. Waiting to acquire an advisory user lock. Aurora PostgreSQL wait events PDF RSS The following table lists the wait events for Aurora PostgreSQL that most commonly indicate performance problems, and summarizes the most common causes and corrective actions. See, One row for each index in the current database, showing statistics about accesses to that specific index. Waiting for I/O on a serializable transaction conflict SLRU buffer. Waiting for SLRU data to reach durable storage during a checkpoint or database shutdown. Waiting to retrieve messages from the shared catalog invalidation queue. See, One row for each sequence in the current database, showing statistics about I/O on that specific sequence. Waiting for truncate of mapping data during a logical rewrite. The pg_stat_slru view will contain one row for each tracked SLRU cache, showing statistics about access to cached pages. idle: The backend is waiting for a new client command. Waiting for a write of a newly created timeline history file. Waiting for a read when creating a new WAL segment by copying an existing one. Restrict the maximum number of connections to the database as a best practice. Table28.17.pg_statio_all_sequences View. 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. 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. 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. Waiting in main loop of WAL sender process. Total amount of time spent writing WAL buffers to disk via XLogWrite request, in milliseconds (if track_wal_io_timing is enabled, otherwise zero). Waiting to read or update replication slot state. 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. PostgreSQL utilizes lightweight locks (LWLocks) to synchronize and control access to the buffer content. a page) has to be retrieved outside the shared buffer pool. Locks in PostgreSQL: 4. Locks in memory : Postgres Professional Waiting to read or update vacuum-related information for a B-tree index. 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. The per-table and per-index functions take a table or index OID. The parameter track_io_timing enables monitoring of block read and write times. 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. Waiting for a read from a timeline history file during a walsender timeline command. Waiting in main loop of autovacuum launcher process. For details such as the functions' names, consult the definitions of the standard views. See Table28.4 for details. wait_event will identify the specific wait point. 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. Waiting for startup process to send initial data for streaming replication. 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. Number of WAL files that have been successfully archived, Name of the WAL file most recently successfully archived, last_archived_time timestamp with time zone, Time of the most recent successful archive operation, Number of failed attempts for archiving WAL files, Name of the WAL file of the most recent failed archival operation, last_failed_time timestamp with time zone, Time of the most recent failed archival operation. This counter is incremented each time a transaction is spilled, and the same transaction may be spilled multiple times. Waiting for a newly created timeline history file to reach durable storage. Monitoring systems should choose whether to represent this as missing data, zero or continue to display the last known value. 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. 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. The parameter track_io_timing enables monitoring of block read and write times. Waiting for data to reach durable storage while creating the data directory lock file. The pg_stat_user_functions view will contain one row for each tracked function, showing statistics about executions of that function. But if you want to see new results with each query, be sure to do the queries outside any transaction block. The access functions for per-database statistics take a database OID as an argument to identify which database to report on. This counter is incremented each time a transaction is streamed, and the same transaction may be streamed multiple times. 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 live rows fetched by sequential scans and index entries returned by index scans in this database, Number of live rows fetched by index scans 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. 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). Waiting for a buffered file to be truncated. Buffer pin waits can be protracted if another process holds an open cursor that last read data from the buffer in question. Waiting for a replication slot control file to reach durable storage. BufferCacheHitRatio metric dips. Waiting to elect a Parallel Hash participant to allocate more buckets. Each buffer header also contains an LWLock, the "buffer content lock", that *does* represent the right to access the data: in the buffer. Connection string used by this WAL receiver, with security-sensitive fields obfuscated. wait_event will contain a name identifying the purpose of the lightweight lock. 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 has no effect in a quorum-based synchronous replication. Table28.17.pg_stat_recovery_prefetch View, Number of blocks prefetched because they were not in the buffer pool, Number of blocks not prefetched because they were already in the buffer pool, Number of blocks not prefetched because they would be zero-initialized, Number of blocks not prefetched because they didn't exist yet, Number of blocks not prefetched because a full page image was included in the WAL, Number of blocks not prefetched because they were already recently prefetched, How many bytes ahead the prefetcher is looking, How many blocks ahead the prefetcher is looking, How many prefetches have been initiated but are not yet known to have completed, 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. , LWTRANCHE_MXACTMEMBER_BUFFERS, LWTRANCHE_ASYNC_BUFFERS, LWTRANCHE_OLDSERXID_BUFFERS, LWTRANCHE_WAL_INSERT, LWTRANCHE_BUFFER_CONTENT, LWTRANCHE_BUFFER_IO_IN_PROGRESS, LWTRANCHE . Waiting to acquire an exclusive lock to truncate off any empty pages at the end of a table vacuumed. The pg_stat_bgwriter view will always have a single row, containing global data for the cluster. Waiting for a read from a replication slot control file. purpose is for the same page to be read into the shared buffer. Waiting for a timeline history file received via streaming replication to reach durable storage. The total number of rows in each table, and information about vacuum and analyze actions for each table are also counted. Waiting for a read during recheck of the data directory lock file. Waiting for logical rewrite mappings to reach durable storage. Waiting for SLRU data to reach durable storage following a page write. See, One row for each index in the current database, showing statistics about accesses to that specific index. Superusers and roles with privileges of built-in role pg_read_all_stats (see also Section22.5) can see all the information about all sessions. Statistics Functions. Alternatively, one can build custom views using the underlying cumulative statistics functions, as discussed in Section28.2.24. The pg_stat_ssl view will contain one row per backend or WAL sender process, showing statistics about SSL usage on this connection. Waiting for other Parallel Hash participants to finish loading a hash table. LWLock:buffer_mapping. Waiting to insert WAL into a memory buffer. Note that this includes data that is streamed and/or spilled. Waiting for a read during reorder buffer management. Re: Improve WALRead() to suck data directly from WAL buffers when Resetting these counters can cause autovacuum to not perform necessary work, which can cause problems such as table bloat or out-dated table statistics. Using pg_stat_reset() also resets counters that autovacuum uses to determine when to trigger a vacuum or an analyze. Waiting for the version file to be written while creating a database. Waiting for confirmation from remote server during synchronous replication. Topics Relevant engine versions Context Causes Actions Relevant engine versions Identifier of this backend's most recent query. quorum: This standby server is considered as a candidate for quorum standbys. For better performance, stats_temp_directory can be pointed at a RAM-based file system, decreasing physical I/O requirements. The pg_stat_wal view will always have a single row, containing data about WAL activity of the cluster. Waiting in WAL receiver to receive data from remote server. 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. Statistics Collection Configuration, One row per server process, showing information related to the current activity of that process, such as state and current query. Waiting for the page number needed to continue a parallel B-tree scan to become available. If you've got a moment, please tell us what we did right so we can do more of it. In all other states, it shows the last query that was executed. Waiting for a write to the relation map file. See, One row per connection (regular and replication), showing information about SSL used on this connection. Send time of last reply message received from standby server. Returns the wait event name if this backend is currently waiting, otherwise NULL. Waiting for a relation data file to be truncated. 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. also pending an I/O operation, The ratio between the size of the shared buffer pool (defined by the shared_buffers parameter) and the Waiting to access the multixact member SLRU cache. As soon as the page is read inside the shared buffer pool, the LWLock:BufferIO lock is released. 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. Possible values are: 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 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.