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. The following wait events are a subset of the list in Amazon Aurora PostgreSQL wait events. Copyright 1996-2023 The PostgreSQL Global Development Group, PostgreSQL 15.2, 14.7, 13.10, 12.14, and 11.19 Released, 28.2.1. Possible values are: Top-level transaction identifier of this backend, if any. Since collection of statistics adds some overhead to query execution, the system can be configured to collect or not collect information. Waiting for background worker to shut down. Waiting to choose the next subplan during Parallel Append plan execution. replication_origin: Waiting to read or update the replication progress. Waiting in WAL receiver to receive data from remote server. 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. Lock: The backend is waiting for a heavyweight lock. Waiting for parallel workers to finish computing. Waiting to find or allocate space in shared memory. This counts top-level transactions only, and is not incremented for subtransactions. 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. Resets statistics of the replication slot defined by the argument. might need to increase it or scale up your DB instance class. Waiting for background worker to start up. Waiting to read or update shared multixact state. Simple test for lock_waits log messages. Waiting for a replication slot to become inactive to be dropped. Waiting for a relation data file to be extended. The pg_stat_ssl view will contain one row per backend or WAL sender process, showing statistics about SSL usage on this connection. There are also several other views, listed in Table28.2, available to show the accumulated statistics. fastpath function call: The backend is executing a fast-path function. The pg_stat_wal_receiver view will contain only one row, showing statistics about the WAL receiver from that receiver's connected server. Waiting to acquire an exclusive lock to truncate off any empty pages at the end of a table vacuumed. It also tracks the total number of rows in each table, and information about vacuum and analyze actions for each table. The parameter track_counts controls whether cumulative statistics are collected about table and index accesses. Waiting for activity from a child process while executing a. Waiting a new WAL segment created by copying an existing one to reach durable storage. NULL if this process is a parallel group leader or does not participate in parallel query. For more information, see LWLock:buffer_mapping. Waiting for a write of a timeline history file received via streaming replication. 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. Streaming only works with top-level transactions (subtransactions can't be streamed independently), so the counter is not incremented for subtransactions. See. Buffer pin waits can be protracted if another process holds an open cursor which last read data from the buffer in question. The LWLock:BufferIO event occurs when RDS for PostgreSQL or Aurora PostgreSQL is waiting for other processes to finish their I/O operations. Waiting to access the list of finished serializable transactions. (Conflicts occur only on standby servers; see, Number of temporary files created by queries in this database. Waiting for action on logical replication worker to finish. Waiting for a read of a serialized historical catalog snapshot. Number of times WAL buffers were written out to disk via XLogWrite request. Waiting for group leader to clear transaction id at transaction end. Indexes can be used by simple index scans, bitmap index scans, and the optimizer. The pg_stat_replication_slots view will contain one row per logical replication slot, showing statistics about its usage. Autovacuum worker or launcher waiting to update or read the current state of autovacuum workers. Waiting for a write to a replication slot control file. 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). It is quite possible that user has registered the tranche in one of the backends (by having allocation in dynamic shared memory) in which case other backends won't have that information, so we display extension for such cases. Waiting to replace a page in WAL buffers. Waiting to update limit on notification message storage. These access functions use a backend ID number, which ranges from one to the number of currently active backends. Waiting when WAL data is not available from any kind of sources (local, archive or stream) before trying again to retrieve WAL data, at recovery. Timeout: The server process is waiting for a timeout to expire. Waiting to read or update notification messages. By default the query text is truncated at 1024 bytes; this value can be changed via the parameter track_activity_query_size. 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. For example, to show the PIDs and current queries of all backends: Table28.20. Wait Events of Type BufferPin, Table28.8. The functions for per-function statistics take a function OID. The parameter track_counts controls whether statistics are collected about table and index accesses. Thanks for letting us know we're doing a good job! In all other states, it shows the identifier of last query that was executed. Waiting during recovery when WAL data is not available from any source (. Waiting for a write while creating the data directory lock file. Waiting to acquire an advisory user lock. Waiting to access a shared TID bitmap during a parallel bitmap index scan. Most such locks protect a particular data structure in shared memory. Waiting for parallel query dynamic shared memory allocation. Waiting for I/O on a clog (transaction status) buffer. Waiting to elect a Parallel Hash participant to allocate the initial hash table. Table28.19. (The path case can be distinguished because it will always be an absolute path, beginning with /.). Waiting to allocate or free a replication slot. (See Chapter19 for details about setting configuration parameters.). Waiting for a write to update the control file. This effect can mean that you have a small shared buffers setting. PostgreSQL utilizes lightweight locks (LWLocks) to synchronize and control access to the buffer content. 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. For more information on lightweight locks, see Locking Overview. Using pg_stat_reset() also resets counters that autovacuum uses to determine when to trigger a vacuum or an analyze. Waiting in main loop of logical replication apply process. If the argument is NULL, resets statistics for all the replication slots. There is no order to the granting of LWLocks and in a high concurrency system this can cause contention. Waiting for confirmation from remote server during synchronous replication. Number of data page checksum failures detected in this database (or on a shared object), or NULL if data checksums are not enabled. 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. Waiting to write zero bytes to a dynamic shared memory backing file. PostgreSQL 15.2, 14.7, 13.10, 12.14, and 11.19 Released, 28.2.1. Here is an example of how wait events can be viewed: The pg_stat_replication view will contain one row per WAL sender process, showing statistics about replication to that sender's connected standby server. Waiting to associate a data block with a buffer in the buffer pool. 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 elect a Parallel Hash participant to allocate a hash table. If the argument is NULL, reset statistics for all subscriptions. But access to that shared memory requires the protection of light-weight locks, which should last for only nanoseconds or microseconds while the memory access is actually occuring. Presently, the collector can count accesses to tables and indexes in both disk-block and individual-row terms. Waiting for a write during reorder buffer management. Waiting in main loop of autovacuum launcher process. Waiting to read or update the state of logical replication workers. Waiting for an elected Parallel Hash participant to finish allocating more buckets. Waiting for a timeline history file received via streaming replication to reach durable storage. The server process is waiting for a timeout to expire. Waiting for a write of mapping data during a logical rewrite. Extension: The server process is waiting for activity in an extension module. Waiting for a read from a timeline history file during walsender timeline command. Write-Ahead Logging (WAL) is a standard method for ensuring data integrity. PostgreSQL is one of the most popular open-source databases in the world and has successful implementations across several mission-critical environments across various domains, using real-time high-end OLTP applications performing millions and billions of transactions per day. Waiting for a two phase state file to reach durable storage. 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. Waiting for logical rewrite mappings to reach durable storage. Additional functions related to statistics collection are listed in Table28.19. Waiting to get a snapshot or clearing a transaction id at transaction end. When recovery is performed at server start (e.g., after immediate shutdown, server crash, and point-in-time recovery), all statistics counters are reset. Waiting for a relation data file to reach durable storage. Provide feedback LWLock:BufferIO. The pg_statio_ views are primarily useful to determine the effectiveness of the buffer cache. Waiting for a WAL file to reach durable storage. The pg_stat_activity view will have one row per server process, showing information related to the current activity of that process. Resetting these counters can cause autovacuum to not perform necessary work, which can cause problems such as table bloat or out-dated table statistics. See, One row for each tracked function, showing statistics about executions of that function. So the statistics will show static information as long as you continue the current transaction. See Table28.4 for details. Waiting for a write of a WAL page during bootstrapping. Waiting to create, drop or use a replication origin. @ LWTRANCHE_REPLICATION_SLOT_IO. The pg_stat_wal view will always have a single row, containing data about WAL activity of the cluster. Waiting to elect a Parallel Hash participant to allocate more buckets. There have been several occasions when a query is being executed dozens of times simultaneously by one or many users. Waiting for a read during a file copy operation. This documentation is for an unsupported version of PostgreSQL. You can split your Possible values are: active: The backend is executing a query. a page) has to be retrieved outside the shared buffer pool. Waiting for any activity when processing replies from WAL receiver in WAL sender process. Waiting for SLRU data to reach durable storage during a checkpoint or database shutdown. Waiting for I/O on a multixact member SLRU buffer. A backend process wants to read a page into shared memory. IP address of the client connected to this backend. Waiting for I/O on commit timestamp buffer. 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. Waiting for WAL buffers to be written to disk. Waiting to apply WAL at recovery because it is delayed. Waiting to read or write a data page in memory. 39919 LWLock buffer_mapping 5119 Client ClientRead 3116 IO DataFileRead With C-Hash Event Count Event Type Event Name See, One row for each table in the current database, showing statistics about I/O on that specific table. Detailed Description . 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 updated (includes HOT updated rows), Number of rows HOT updated (i.e., with no separate index update required), 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 VACUUM FULL), Last time at which this table was vacuumed by the autovacuum daemon, Last time at which this table was manually analyzed, last_autoanalyze timestamp with time zone, Last time at which this table was analyzed by the autovacuum daemon, Number of times this table has been manually vacuumed (not counting VACUUM FULL), 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. finish their input/output (I/O) operations when concurrently trying to access a page. For details such as the functions' names, consult the definitions of the standard views. Sometimes it may be more convenient to obtain just a subset of this information. See, One row for each table in the current database, showing statistics about I/O on that specific table. Choose the appropriate target Region. After v9.5, the buffer cache is protected by 128 hash tables each protected by a LWLock. The pg_stat_gssapi view will contain one row per backend, showing information about GSSAPI usage on this connection. Number of backends currently connected to this database, or NULL for shared objects. to keep index reordering low and reduces its impact. See, OID of the database this backend is connected to, Name of the database this backend is connected to, Name of the user logged into this backend, Name of the application that is connected to this backend. If this field is null, it indicates that this is an internal server process. Alone the requirement of separate fsyncs and everything is pretty bothersome. Time at which these statistics were last reset. Number of disk blocks read from this index. Indexes can be used by simple index scans, bitmap index scans, and the optimizer. 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. Possible values are: Wait event name if backend is currently waiting, otherwise NULL. Several predefined views, listed in Table28.1, are available to show the current state of the system. 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. Waiting for data to reach durable storage while creating the data directory lock file. LWLock:buffer_mapping. The pg_stat_wal_receiver view will contain only one row, showing statistics about the WAL receiver from that receiver's connected server. Waiting to access the list of predicate locks held by serializable transactions. 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. 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. Waiting between writes while performing a checkpoint. I'd like to know more about what these locks could imply if anything. Waiting to read or update background worker state. Only directly connected standbys are listed; no information is available about downstream standby servers. Increase the number of wal_buffers available to the database. Priority of this standby server for being chosen as the synchronous standby in a priority-based synchronous replication. Waits for a buffer pin ( BufferPin ). This is used by system processes waiting for activity in their main processing loop. 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. Postgres 10.3: SELECT queries hang for hours - Stack Overflow See, Only one row, showing statistics about the WAL receiver from that receiver's connected server. If enabled, calls to user-defined functions and the total time spent in each one are counted as well. When the buffer manager receives a request, PostgreSQL uses the buffer_tag of the desired page. 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. Name of the user logged into this backend, Name of the application that is connected to this backend. If the argument is NULL, all counters shown in the pg_stat_slru view for all SLRU caches are reset. Waiting to read or update the last value set for a transaction commit timestamp. ; Ensure that filesystem journaling is turned off for data files and WAL files. potential: This standby server is now asynchronous, but can potentially become synchronous if one of current synchronous ones fails. Waiting to access predicate lock information used by serializable transactions. Waiting for a logical replication remote server to change state. Waiting for a read of a serialized historical catalog snapshot. Waiting to read or update replication slot state. Waiting for other Parallel Hash participants to finish inserting tuples into new buckets. Resets all statistics counters for the current database to zero. Waiting for I/O on a multixact offset SLRU buffer. 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. Several predefined views, listed in Table28.1, are available to show the current state of the system. Table28.26.pg_stat_database_conflicts View, 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. Note that only tables, indexes, and functions in the current database can be seen with these functions. Waiting for a replication slot control file to reach durable storage. pg_stat_get_backend_dbid ( integer ) oid. The last article introduced SpinLock in PostgreSQL. Waiting to acquire an advisory user lock. See, One row for each backend (including autovacuum worker processes) running, One row for each WAL sender process streaming a base backup, showing current progress. 5mins of Postgres E25: Postgres lock monitoring, LWLocks and the log Postgres Locking: When is it Concerning? OID of the database this backend is connected to, Name of the database this backend is connected to. Heavyweight locks, also known as lock manager locks or simply locks, primarily protect SQL-visible objects such as tables. Waiting for any activity when processing replies from WAL receiver in WAL sender process. 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. Returns the time when the backend's current transaction was started. Waiting to replace a page in WAL buffers. Waiting for activity from child process when executing. The total number of rows in each table, and information about vacuum and analyze actions for each table are also counted. Therefore it is not safe to assume that all files older than last_archived_wal have also been successfully archived. Waiting for truncate of mapping data during a logical rewrite. PostgreSQL: Documentation: 10: 28.2. The Statistics Collector This can be a host name, an IP address, or a directory path if the connection is via Unix socket. gorthx on Twitter Waiting in WAL receiver to establish connection to remote server. Doing this helps Number of in-progress transactions streamed to the decoding output plugin after the memory used by logical decoding to decode changes from WAL for this slot has exceeded logical_decoding_work_mem. Waiting to read or write relation cache initialization file. Per-Backend Statistics Functions, Copyright 1996-2023 The PostgreSQL Global Development Group. 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. TCP port number that the client is using for communication with this backend, or -1 if a Unix socket is used. LWLock:BufferIO - Amazon Relational Database Service Waiting to select the starting location of a synchronized table scan. Waiting in main loop of WAL sender process. The per-index statistics are particularly useful to determine which indexes are being used and how effective they are. Did this page help you? The combination of certificate serial number and certificate issuer uniquely identifies a certificate (unless the issuer erroneously reuses serial numbers). The counter gets incremented for both top-level transactions and subtransactions. BK_1935: "IObuffer_locks,ControlLock()"IOControlLockControlLockIOSlruSharedData The overhead of a file is much more than wasting the remainder of a page. Waiting for recovery conflict resolution for a vacuum cleanup. Waiting in main loop of syslogger process. 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. Waiting for a write during a file copy operation. checksum_last_failure timestamp with time zone. postgres 26 Heap_Insert 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 to acquire a virtual transaction ID lock. Waiting to add or examine locks for backends, or waiting to join or exit a locking group (used by parallel query). Extensions can register their specific waits ( Extension ). Time when this process' current transaction was started, or null if no transaction is active. Waiting for I/O on a multixact_member buffer. This is consistent with the goal of measuring synchronous commit and transaction visibility delays for recent write transactions. being read from storage. The track_functions parameter controls exactly which functions are tracked. Table28.15.pg_stat_replication_slots View, A unique, cluster-wide identifier for the replication slot. Waiting for the termination of another backend. disabled: This state is reported if track_activities is disabled in this backend. Waiting to perform an operation on a list of locks held by serializable transactions. pg_stat_get_backend_wait_event ( integer ) text. See, One row for each index in the current database, showing statistics about I/O on that specific index. active: The backend is executing a query. When using the statistics to monitor collected data, it is important to realize that the information does not update instantaneously. Waiting to read or update the state of prepared transactions. Waiting to allocate or free a replication slot. query performance - PostgreSQL LWLock: lock_manager issue - Database 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. 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. Waiting for a read of the relation map file. 106 . Returns the process ID of the server process attached to the current session. If this field is null, it indicates that the client is connected via a Unix socket on the server machine. The pg_stat_database view will contain one row for each database in the cluster, plus one for shared objects, showing database-wide statistics. (To prevent ordinary users from hiding their activity from the administrator, only superusers are allowed to change these parameters with SET.). PostgreSQL accesses certain on-disk information via SLRU (simple least-recently-used) caches. Waiting for a write when creating a new WAL segment by copying an existing one. Waiting for a write of a WAL page during bootstrapping. A database-wide ANALYZE is recommended after the statistics have been reset. Listen The most possible reason for why you see LWLockTranche/buffer_mapping wait event in PostgreSQL Well, if you are here you probably came across an issue where your database had CPU spikes. Total amount of data written to temporary files by queries in this database. See, One row for each index in the current database, showing statistics about accesses to that specific index. 28.2.3. The fields returned are a subset of those in the pg_stat_activity view. Waiting for a read when creating a new WAL segment by copying an existing one. PostgreSQL 's statistics collector is a subsystem that supports collection and reporting of information about server activity. Per-Backend Statistics Functions, pg_stat_get_backend_idset () setof integer. Wait Events of Type Extension. Waiting for a write to the relation map file. streaming: This WAL sender is streaming changes after its connected standby server has caught up with the primary. Waiting for a read of the relation map file. Waiting for a write to the relation map file. 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.