If this field is null, it indicates that this is an internal server process. 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. Waiting to update limit on notification message storage. Waiting for another process to be attached to a shared message queue. Waiting to access the serializable transaction conflict SLRU cache. See, One row per connection (regular and replication), showing information about GSSAPI authentication and encryption used on this connection. Waiting in main loop of syslogger process. Resets statistics of the replication slot defined by the argument. If you've got a moment, please tell us what we did right so we can do more of it. Doing this helps Common causes for the LWLock:BufferIO event to appear in top waits include the following: Multiple backends or connections trying to access the same page that's Waiting to update limits on transaction id and multixact consumption. Waiting for a newly created timeline history file to reach durable storage. Waiting to elect a Parallel Hash participant to allocate a hash table. 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. pg_stat_get_backend_pid ( integer ) integer, pg_stat_get_backend_start ( integer ) timestamp with time zone. When the buffer manager receives a request, PostgreSQL uses the buffer_tag of the desired page. There are also several other views, listed in Table28.2, available to show the results of statistics collection. Waiting in main loop of WAL writer process. replication_slot_io: Waiting for I/O on a replication slot. Waiting for parallel query dynamic shared memory allocation. Waiting in main loop of the archiver process. Waiting for a serialized historical catalog snapshot to reach durable storage. Waiting in a cost-based vacuum delay point. Returns the TCP port number that the client is using for communication. Monitoring systems should choose whether to represent this as missing data, zero or continue to display the last known value. 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. Wait Events of Type BufferPin, Table28.8. Waiting for a buffered file to be truncated. Waiting to read or update sub-transaction information. If the argument is NULL, all counters shown in the pg_stat_slru view for all SLRU caches are reset. Its Returns the time when the backend's most recent query was started. Waiting to find or allocate space in shared memory. The server process is waiting for a lightweight lock. This category is useful for modules to track custom waiting points. (Conflicts occur only on standby servers; see, Number of temporary files created by queries in this database. Waiting for a replication slot control file to reach durable storage while restoring it to memory. Waiting for a write to a replication slot control file. Number of times WAL files were synced to disk via issue_xlog_fsync request (if fsync is on and wal_sync_method is either fdatasync, fsync or fsync_writethrough, otherwise zero). This field will only be non-null for IP connections, and only when log_hostname is enabled. wait_event will identify the specific wait point. Waiting to read or update the state of logical replication workers. Resets all statistics counters for the current database to zero. Number of temporary files created by queries in this database. fastpath function call: The backend is executing a fast-path function. 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. Waiting to acquire an exclusive pin on a buffer. The LWLock:BufferIO event occurs when RDS for PostgreSQL or Aurora PostgreSQL is waiting for other processes to finish their I/O operations. quorum: This standby server is considered as a candidate for quorum standbys. 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. Statistics Functions. Waiting for WAL to reach durable storage during bootstrapping. Synchronous state of this standby server. Waiting to read or update dynamic shared memory state. The buffer_mapping LWLock wait event will be . 39919 LWLock buffer_mapping 5119 Client ClientRead 3116 IO DataFileRead . Waiting to receive bytes from a shared message queue. If the argument is NULL, resets statistics for all the replication slots. It also tracks the total number of rows in each table, and information about vacuum and analyze actions for each table. Waiting for the relation map file to reach durable storage. Waiting for other process to be attached in shared message queue. Waiting to read or truncate multixact information. Waiting for background worker to shut down. Waiting to access a parallel query's information about composite types. Waiting for a write to the relation map file. 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. Waiting for I/O on a multixact_member buffer. pg_stat_get_backend_dbid ( integer ) oid. The total number of rows in each table, and information about vacuum and analyze actions for each table are also counted. Waiting for I/O on a clog (transaction status) buffer. Waiting for a write of logical rewrite mappings. Waiting for a write to a relation data file. In all other states, it shows the identifier of last query that was executed. NULL if this process is a parallel group leader or does not participate in parallel query. Waiting for a new WAL segment created by copying an existing one to reach durable storage. (Some locks have specific names; others are part of a group of locks each with a similar purpose.). Table28.17.pg_statio_all_sequences View. 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. See. Per-Backend Statistics Functions, pg_stat_get_backend_idset () setof integer. The track_functions parameter controls exactly which functions are tracked. The wait_event and state columns are independent. Tune max_wal_size and checkpoint_timeout based on Waiting for a read of a serialized historical catalog snapshot. Waiting for the termination of another backend. The new locking scheme avoids grabbing system-wide exclusive locks in common code paths. 28.2.3. Waiting for a write of mapping data during a logical rewrite. Simple test for lock_waits log messages. Possible values are: active: The backend is executing a query. LWLock: The backend is waiting for a lightweight lock. Waiting to read or update information about synchronous replicas. 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 written for this SLRU, Number of blocks checked for existence for this SLRU, Number of flushes of dirty data for this SLRU. There is no order to the granting of LWLocks and in a high concurrency system this can cause contention. Waiting for a read from a timeline history file during walsender timeline command. The functions for per-function statistics take a function OID. Waiting for a write while adding a line to the data directory lock file. Waiting to access a shared tuple store during parallel query. Waiting in background writer process, hibernating. Current overall state of this backend. The next use of statistical information will cause a new snapshot to be fetched. Waiting for a read while adding a line to the data directory lock file. Priority of this standby server for being chosen as the synchronous standby in a priority-based synchronous replication. Waiting for a write during reorder buffer management. Waiting for WAL to reach durable storage during bootstrapping. The WALWriteLock wait occurs while PostgreSQL flushes WAL records to disk or during a WAL segment switch.. How to reduce this wait . Waiting to read or update the fast-path lock information. 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. Thanks for letting us know this page needs work. Waiting for a read during a file copy operation. 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. Use partitioned tables (which also have partitioned indexes). Time when this process' current transaction was started, or null if no transaction is active. Waiting for a read from the control file. It can be joined to pg_stat_activity or pg_stat_replication on the pid column to get more details about the connection. However, current-query information collected by track_activities is always up-to-date. Waiting to select the starting location of a synchronized table scan. Such a system would show similar times while new WAL is being generated, but would differ when the sender becomes idle. 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. Waiting for I/O on commit timestamp buffer. The track_functions parameter controls exactly which functions are tracked. The pg_statio_ views are primarily useful to determine the effectiveness of the buffer cache. This lock is used to handle multiple sessions that all require access to the same Waiting for other Parallel Hash participants to finish loading a hash table. Waiting for a read of a timeline history file. By default the query text is truncated at 1024 bytes; this value can be changed via the parameter track_activity_query_size. 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. The server process is waiting for a timeout to expire. A database-wide ANALYZE is recommended after the statistics have been reset. Waiting for WAL to be flushed in WAL sender process. Waiting in main loop of WAL receiver process. checksum_last_failure timestamp with time zone. When a buffer is read from disk (or written to disk), an IO in progress lock is also acquired, which indicates to other processes that the page is being read (or written) they can queue up if they need to do something with this page. your experience with the particular feature or requires further clarification, 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. 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. finish their input/output (I/O) operations when concurrently trying to access a page. Waiting to allocate or free a replication slot. PostgreSQL 15.2, 14.7, 13.10, 12.14, and 11.19 Released, 28.2.1. Waiting for WAL from a stream at recovery. It works like this: 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. A backend process wants to read a page into shared memory. Waiting for a read during recheck of the data directory lock file. See Section30.5 for more information about the internal WAL function XLogWrite. Occasionally i noticed that in random interval of times the dbms become slow and get stuck on a few SELECT queries. Last write-ahead log location already received and written to disk, but not flushed. Waiting to update the relation map file used to store catalog to filenode mapping. However, current-query information collected by track_activities is always up-to-date. From the Actions drop-down menu, choose Create Read Replica. The parameter track_functions enables tracking of usage of user-defined functions. Serial number of the client certificate, or NULL if no client certificate was supplied or if SSL is not in use on this connection. See Table28.5 through Table28.13. pg_stat_get_backend_xact_start ( integer ) timestamp with time zone. Waiting to get a snapshot or clearing a transaction id at transaction end. 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. See Table28.5 through Table28.13. shared_buffers parameter. The pg_statio_ views are primarily useful to determine the effectiveness of the buffer cache. Waiting for a replication slot control file to reach durable storage while restoring it to memory. Topics Relevant engine versions Context Causes Actions Relevant engine versions 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. This documentation is for an unsupported version of PostgreSQL. Waiting to access a shared TID bitmap during a parallel bitmap index scan. See, One row per SLRU, showing statistics of operations. 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. Waiting for other Parallel Hash participants to finish repartitioning. Waiting to apply WAL during recovery because of a delay setting. 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. idle in transaction: The backend is in a transaction, but is not currently executing a query. 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. 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. If this field is null, it indicates that the client is connected via a Unix socket on the server machine. Resets statistics to zero for a single SLRU cache, or for all SLRUs in the cluster. Provide feedback Returns the IP address of the client connected to this backend. Waiting to read or update information about serializable transactions. Waiting to read or update the replication progress. Waiting to retrieve messages from the shared catalog invalidation queue. This counter is incremented each time a transaction is spilled, and the same transaction may be spilled multiple times. being read from storage. 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. Synchronous state of this standby server. 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. Since collection of statistics adds some overhead to query execution, the system can be configured to collect or not collect information. Waiting for a newly initialized WAL file to reach durable storage. Waiting for a timeline history file received via streaming replication to reach durable storage. Javascript is disabled or is unavailable in your browser. Waiting for a write during a file copy operation. 39919 LWLock buffer_mapping 5119 Client ClientRead 3116 IO DataFileRead With C-Hash Event Count Event Type Event Name a page) has to be retrieved outside the shared buffer pool. The pg_stat_subscription_stats view will contain one row per subscription. 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). Sometimes it may be more convenient to obtain just a subset of this information. See Section30.5 for more information about the internal WAL function issue_xlog_fsync. Waiting to retrieve or remove messages from shared invalidation queue. See, At least one row per subscription, showing information about the subscription workers. 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 SLRU data to reach durable storage following a page write. Logical decoding plugins may optionally emit tracking messages; if they do not, the tracking mechanism will simply display NULL lag. Waiting for a two phase state file to reach durable storage. Resets statistics for a single function in the current database to zero. From pg_stat_activity i noticed that the wait_event_type and wait_event of these queries is as follows: Waiting for a write of mapping data during a logical rewrite. The full object locks which last (usually) for the duration of a transaction and which you can see in pg_locks have info about them stored in shared memory. Waiting to read or update transaction status. Waiting for parallel workers to finish computing. Waiting for the page number needed to continue a parallel B-tree scan to become available. Waiting in main loop of logical launcher process. Waiting for a write of a newly created timeline history file. This has no effect in a quorum-based synchronous replication. See, One row per subscription, showing statistics about errors. Waiting for confirmation from a remote server during synchronous replication. Waiting for mapping data to reach durable storage during a logical rewrite. (The path case can be distinguished because it will always be an absolute path, beginning with /.). 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. Indexes can be used by simple index scans, bitmap index scans, and the optimizer. The parameter track_io_timing enables monitoring of block read and write times. A database-wide ANALYZE is recommended after the statistics have been reset. Note that only tables, indexes, and functions in the current database can be seen with these functions. Definition: lwlock.h:190. Therefore, while holding an exclusive lock, a process prevents other processes from acquiring a shared or exclusive lock. Waiting for data to reach durable storage while creating the data directory lock file. pg_stat_reset_replication_slot ( text ) void. 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.