Waiting for a write while adding a line to the data directory lock file. to keep index reordering low and reduces its impact. Waiting to read or update the progress of one replication origin. Waiting to read or update a process' fast-path lock information. See, One row for each tracked function, showing statistics about executions of that function. See, One row for each sequence in the current database, showing statistics about I/O on that specific sequence. The pg_stat_wal_receiver view will contain only one row, showing statistics about the WAL receiver from that receiver's connected server. Waiting in main loop of background writer process. The access functions for per-database statistics take a database OID as an argument to identify which database to report on. PostgreSQL 15.2, 14.7, 13.10, 12.14, and 11.19 Released, 28.2.1. Waiting for a two phase state file to reach durable storage. Waiting to add a message in shared invalidation queue. Waiting for the control file to reach durable storage. Number of data page checksum failures detected in this database (or on a shared object), or NULL if data checksums are not enabled. Current WAL sender state. Waiting to access the commit timestamp SLRU cache. wait_event will identify the type of lock awaited. In rows about other sessions, many columns will be null. Waiting for a write while creating the data directory lock file. 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. See, One row for each index in the current database, showing statistics about accesses to that specific index. Number of transactions in this database that have been committed, Number of transactions in this database that have been rolled back, Number of disk blocks read in this database, Number of times disk blocks were found already in the buffer cache, so that a read was not necessary (this only includes hits in the PostgreSQL buffer cache, not the operating system's file system cache), Number of rows returned by queries in this database, Number of rows fetched by queries in this database, Number of rows inserted by queries in this database, Number of rows updated by queries in this database, Number of rows deleted by queries in this database, Number of queries canceled due to conflicts with recovery in this database. Waiting for confirmation from a remote server during synchronous replication. In addition, background workers registered by extensions may have additional types. Waiting for a write of logical rewrite mappings. Distinguished Name (DN) field from the client certificate used, or NULL if no client certificate was supplied or if SSL is not in use on this connection. Waiting for a serialized historical catalog snapshot to reach durable storage. Waiting for SLRU data to reach durable storage following a page write. Number of times WAL buffers were written out to disk via XLogWrite request. 213 * Prior to PostgreSQL 9.4, we used an enum type called LWLockId to refer. IPC: The server process is waiting for some activity from another process in the server. Text of this backend's most recent query. Waiting to perform an operation on a list of locks held by serializable transactions. 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. Waiting for group leader to clear transaction id at transaction end. Waiting for a write of a newly created timeline history file. The pg_stat_subscription view will contain one row per subscription for main worker (with null PID if the worker is not running), and additional rows for workers handling the initial data copy of the subscribed tables. Waiting in background writer process, hibernating. 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 changes to a relation data file to reach durable storage. Waiting to acquire a lock on page of a relation. Waiting to allocate or free a replication slot. This should not be used for data integrity checks. After v9.5, the buffer cache is protected by 128 hash tables each protected by a LWLock. The LWLock that this article will introduce is a lightweight lock (Lightweight Lock) based on SpinLock. Waiting for WAL buffers to be written to disk. fastpath function call: The backend is executing a fast-path function. Possible values are: Wait event name if backend is currently waiting, otherwise NULL. Query identifiers are not computed by default so this field will be null unless compute_query_id parameter is enabled or a third-party module that computes query identifiers is configured. Priority of this standby server for being chosen as the synchronous standby in a priority-based synchronous replication. Waiting for a read during recheck of the data directory lock file. However, they are also used to ensure mutual exclusion for certain internal operations such as relation extension. This lock is used to handle multiple sessions that all require access to the same Waiting for a write of a timeline history file received via streaming replication. If a backend is in the active state, it may or may not be waiting on some event. Waiting to elect a Parallel Hash participant to decide on future batch growth. 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. Waiting for data to reach durable storage while creating the data directory lock file. Waiting for WAL buffers to be written to disk. Waiting for an elected Parallel Hash participant to allocate the initial hash table. PostgreSQL utilizes lightweight locks (LWLocks) to synchronize and control access to the buffer content. Waiting to acquire a speculative insertion lock. Waiting for a write of a two phase state file. Waiting for startup process to send initial data for streaming replication. Returns the OID of the database this backend is connected to. pg_stat_get_backend_activity ( integer ) text. wait_event will identify the specific wait point. 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. This view will only contain information on standby servers, since conflicts do not occur on master servers. Waiting to access the list of predicate locks held by serializable transactions. Waiting for a timeline history file received via streaming replication to reach durable storage. Definition: lwlock.h:190. Waiting to add or examine predicate lock information. Waiting to get a snapshot or clearing a transaction id at transaction end. Waiting to read or update transaction status. Please refer to your browser's Help pages for instructions. Number of disk blocks read from this index. Time when this process' current transaction was started, or null if no transaction is active. 39919 LWLock buffer_mapping 5119 Client ClientRead 3116 IO DataFileRead With C-Hash Event Count Event Type Event Name Possible types are autovacuum launcher, autovacuum worker, logical replication launcher, logical replication worker, parallel worker, background writer, client backend, checkpointer, archiver, startup, walreceiver, walsender and walwriter. pg_stat_get_activity ( integer ) setof record. Waiting for logical rewrite mappings to reach durable storage. idle: The backend is waiting for a new client command. 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 parallel workers to finish computing. Thanks for letting us know we're doing a good job! Sometimes it may be more convenient to obtain just a subset of this information. 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. The generated IO patterns are also much worse. Occasionally i noticed that in random interval of times the dbms become slow and get stuck on a few SELECT queries. 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. Waiting for a read from a replication slot control file. Waiting in main loop of startup process for WAL to arrive, during streaming recovery. Therefore, while holding an exclusive lock, a process prevents other processes from acquiring a shared or exclusive lock. Note that only tables, indexes, and functions in the current database can be seen with these functions. Waiting to access a parallel query's information about type modifiers that identify anonymous record types. Identifier of this backend's most recent query. 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 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. Waiting to read or update notification messages. Returns the text of this backend's most recent query. Lag times work automatically for physical replication. Statistics Functions. See, One row only, showing statistics about WAL activity. PostgreSQL accesses certain on-disk information via SLRU (simple least-recently-used) caches. Waiting to read or update the state of prepared transactions. Waiting during recovery when WAL data is not available from any source (. Thus, the server expects something to happen that is independent of its internal processes. The pg_stat_bgwriter view will always have a single row, containing global data for the cluster. Waiting in main loop of autovacuum launcher process. Returns the time when the backend's current transaction was started. Discards the current statistics snapshot or cached information. Presently, the collector can count accesses to tables and indexes in both disk-block and individual-row terms. Waiting for a buffered file to be truncated. Waiting in main loop of logical launcher process. 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. Number of temporary files created by queries in this database. replication_origin: Waiting to read or update the replication progress. The LWLock:BufferIO event occurs when Aurora PostgreSQL or RDS for PostgreSQL is waiting for other processes to finish their input/output (I/O) operations when concurrently trying to access a page. 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. 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. Waiting in main loop of logical apply process. This has no effect in a quorum-based synchronous replication. 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. This is used by system processes waiting for activity in their main processing loop. Waiting for stats dynamic shared memory allocator access, Waiting for stats shared memory hash table access, Waiting for shared memory stats data access. Waiting for SSL while attempting connection. > However, someone with deeper knowledge of page pinning and buffer manager > internals could certainly devise a better solution. Thanks for letting us know this page needs work. Waiting to find or allocate space in shared memory. It can be joined to pg_stat_activity or pg_stat_replication on the pid column to get more details about the connection. might need to increase it or scale up your DB instance class. Waiting to send bytes to a shared message queue. From the Actions drop-down menu, choose Create Read Replica. All temporary files are counted, regardless of why the temporary file was created, and regardless of the log_temp_files setting. The parameter track_wal_io_timing enables monitoring of WAL write times. See, One row for each index in the current database, showing statistics about I/O on that specific index. This counter is incremented each time a transaction is spilled, and the same transaction may be spilled multiple times. pg_stat_get_backend_activity_start ( integer ) timestamp with time zone. quorum: This standby server is considered as a candidate for quorum standbys. Waiting a new WAL segment created by copying an existing one to reach durable storage. Table28.12.pg_stat_database_conflicts View. 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. Waiting for a read of a serialized historical catalog snapshot. 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. Waiting for a replication origin to become inactive to be dropped. Amount of transaction data decoded for sending transactions to the decoding output plugin while decoding changes from WAL for this slot. Waiting for other Parallel Hash participants to finish partitioning the outer relation. 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. See Table28.5 through Table28.13. pg_stat_get_backend_xact_start ( integer ) timestamp with time zone. LWLock- buffer_mapping. Waiting for a write during a file copy operation. The pg_stat_subscription_stats view will contain one row per subscription. Waiting for a read from the control file.

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