Waiting to access the list of finished serializable transactions. Waiting for WAL buffers to be written to disk. Waiting for a write of a serialized historical catalog snapshot. The server process is waiting for an I/O operation to complete. This standby's xmin horizon reported by hot_standby_feedback. Waiting to replace a page in WAL buffers. 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. Note that this includes the transactions that are streamed and/or spilled. pg_stat_get_backend_wait_event ( integer ) text. Waiting for other Parallel Hash participants to finish loading a hash table. Waiting for a newly initialized WAL file to reach durable storage. Waiting for a replication slot control file to reach durable storage. For an asynchronous standby, the replay_lag column approximates the delay before recent transactions became visible to queries. Waiting for a write of a WAL page during bootstrapping. Resets some cluster-wide statistics counters to zero, depending on the argument. Waiting for a read from the control file. Time when this process' current transaction was started, or null if no transaction is active. Waiting for other Parallel Hash participants to finish partitioning the outer relation. ), Reset some cluster-wide statistics counters to zero, depending on the argument (requires superuser privileges by default, but EXECUTE for this function can be granted to others). 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. lock_manager Returns the OID of the user logged into this backend. The pg_stat_gssapi view will contain one row per backend, showing information about GSSAPI usage on this connection. Such a system would show similar times while new WAL is being generated, but would differ when the sender becomes idle. wait_event will contain a name identifying the purpose of the lightweight lock. Text of this backend's most recent query. PostgreSQL 's statistics collector is a subsystem that supports collection and reporting of information about server activity. Waiting for parallel workers to finish computing. Number of backends currently connected to this database, or NULL for shared objects. A process acquires an LWLock in a shared mode to read from the buffer and . Every PostgreSQL process collects statistics locally, then updates the shared data at appropriate intervals. streaming: This WAL sender is streaming changes after its connected standby server has caught up with the primary. For example, to show the PIDs and current queries of all backends: Table28.35. Waiting to read or update information about serializable transactions. Waiting for a relation data file to be truncated. pg_stat_get_backend_activity_start ( integer ) timestamp with time zone. The fields returned are a subset of those in the pg_stat_activity view. The per-table and per-index functions take a table or index OID. The function pg_stat_get_backend_idset provides a convenient way to generate one row for each active backend for invoking these functions. 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. 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. It can be joined to pg_stat_activity or pg_stat_replication on the pid column to get more details about the connection. Waiting for confirmation from remote server during synchronous replication. 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. . Waiting for I/O on an async (notify) buffer. Number of times transactions were spilled to disk while decoding changes from WAL for this slot. Waiting to read or write relation cache initialization file. Waiting to choose the next subplan during Parallel Append plan execution. The pg_stat_bgwriter view will always have a single row, containing global data for the cluster. Detailed Description . See, One row per WAL sender process, showing statistics about replication to that sender's connected standby server. Note that only tables, indexes, and functions in the current database can be seen with these functions. The pg_statio_ views are primarily useful to determine the effectiveness of the buffer cache. The track_functions parameter controls exactly which functions are tracked. The parameter track_counts controls whether cumulative statistics are collected about table and index accesses. Waiting for a barrier event to be processed by all backends. This field is truncated if the DN field is longer than, Number of WAL files that have been successfully archived, Name of the last WAL file successfully archived, Time of the last successful archive operation, Number of failed attempts for archiving WAL files, Name of the WAL file of the last failed archival operation, Time of the last failed archival operation, Time at which these statistics were last reset, Number of scheduled checkpoints that have been performed, Number of requested checkpoints that have been performed, Total amount of time that has been spent in the portion of checkpoint processing where files are written to disk, in milliseconds, Total amount of time that has been spent in the portion of checkpoint processing where files are synchronized to disk, in milliseconds, Number of buffers written during checkpoints, Number of buffers written by the background writer, Number of times the background writer stopped a cleaning scan because it had written too many buffers, Number of buffers written directly by a backend, Number of times a backend had to execute its own. Time when the currently active query was started, or if state is not active, when the last query was started. 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. 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. [prev in list] [next in list] [prev in thread] [next in thread] List: postgresql-general Subject: Re: [HACKERS] [PATCH] Refactoring of LWLock tranches From: Ildus Kurbangaliev <i.kurbangaliev postgrespro ! 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. Possible types are. Serial number of the client certificate, or NULL if no client certificate was supplied or if SSL is not in use on this connection. Waiting for a write during a file copy operation. Waiting in main loop of autovacuum launcher process. Time elapsed between flushing recent WAL locally and receiving notification that this standby server has written, flushed and applied it. 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. (See Chapter19 for details about setting configuration parameters.). See, One row for each sequence in the current database, showing statistics about I/O on that specific sequence. pg_blocking_pids function. IP address of the client connected to this backend. 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 other Parallel Hash participants to finish repartitioning. LWLock:BufferIO. Waiting for a read from a timeline history file during walsender timeline command. Waiting to access predicate lock information used by serializable transactions. The pg_stat_subscription_stats view will contain one row per subscription. 28.2.3. Waiting for changes to a relation data file to reach durable storage. Waiting a new WAL segment created by copying an existing one to reach durable storage. Waiting to read or update the state of logical replication workers. Waiting for startup process to send initial data for streaming replication. Waiting for other Parallel Hash participants to finish hashing the inner relation. Number of deadlocks detected in this database. Waiting for an asynchronous prefetch from a relation data file. Waiting to synchronize workers during Parallel Hash Join plan execution. Waiting for a read during reorder buffer management. number of buffers needed by the current workload, The size of the shared buffer pool not being well balanced with the number of pages being evicted by other For client backends, this is the time the client connected to the server. Waiting for a write to the relation map file. events. IP address of the client connected to this backend. 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. Only directly connected standbys are listed; no information is available about downstream standby servers. Waiting to read or update the last value set for a transaction commit timestamp. If the argument is other (or indeed, any unrecognized name), then the counters for all other SLRU caches, such as extension-defined caches, are reset. Waiting to access the multixact offset SLRU cache. Table28.19. Waiting to insert WAL into a memory buffer. 106 . Waiting to read or update shared multixact state. For more information on lightweight locks, see Locking Overview. 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. Note that only tables, indexes, and functions in the current database can be seen with these functions. The per-table and per-index functions take a table or index OID. Waiting for WAL to reach durable storage during bootstrapping. The pg_statio_all_tables view will contain one row for each table in the current database (including TOAST tables), showing statistics about I/O on that specific table. PostgreSQL utilizes lightweight locks (LWLocks) to synchronize and control access to the buffer content. Waiting to access the serializable transaction conflict SLRU cache. Waiting for a write while creating the data directory lock 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 to get a snapshot or clearing a transaction id at transaction end. gorthx on Twitter Waiting for I/O on a multixact offset buffer. Amount of transaction data decoded for streaming in-progress transactions to the decoding output plugin while decoding changes from WAL for this slot. Waiting for a read during a file copy operation. 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. 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 insert WAL data into a memory buffer. Waiting for a newly created timeline history file to reach durable storage. PostgreSQL Entangled in Locks: Attempts to free it - Amit Kapila - Dilip Kumar PGCon 2017 . 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. Extensions can add LWLock types to the list shown in Table28.12. You can invoke pg_stat_clear_snapshot() to discard the current transaction's statistics snapshot or cached values (if any). 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. 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. 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. The LWLock:BufferIO event occurs when Aurora PostgreSQL or RDS for PostgreSQL is waiting for other processes to The counter gets incremented for both top-level transactions and subtransactions. Waiting to read or update shared notification state. Waiting to replace a page in WAL buffers. 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. This lock is used to handle multiple sessions that all require access to the same Number of times this function has been called, Total time spent in this function and all other functions called by it, in milliseconds, Total time spent in this function itself, not including other functions called by it, in milliseconds. This can be used to gauge the delay that. If you see anything in the documentation that is not correct, does not match Each such lock protects a particular data structure in shared memory. Since collection of statistics adds some overhead to query execution, the system can be configured to collect or not collect information. 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. 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. In addition, background workers registered by extensions may have additional types. This can be used to gauge the delay that synchronous_commit level on incurred while committing if this server was configured as a synchronous standby. Waiting for logical rewrite mappings to reach durable storage. Calling, Reset statistics for a single table or index in the current database to zero (requires superuser privileges by default, but EXECUTE for this function can be granted to others), Reset statistics for a single function in the current database to zero (requires superuser privileges by default, but EXECUTE for this function can be granted to others), Set of currently active backend ID numbers (from 1 to the number of active backends), Time when the most recent query was started, IP address of the client connected to this backend, TCP port number that the client is using for communication, Wait event type name if backend is currently waiting, otherwise NULL. Waiting for confirmation from a remote server during synchronous replication. 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. , LWTRANCHE_MXACTMEMBER_BUFFERS, LWTRANCHE_ASYNC_BUFFERS, LWTRANCHE_OLDSERXID_BUFFERS, LWTRANCHE_WAL_INSERT, LWTRANCHE_BUFFER_CONTENT, LWTRANCHE_BUFFER_IO_IN_PROGRESS, LWTRANCHE . 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. (The path case can be distinguished because it will always be an absolute path, beginning with /.). ru> Date: 2015-11-19 14:04:28 Message-ID: 20151119170428.490de41d lp [Download RAW message or body] Thank you for the review. Waiting for parallel query dynamic shared memory allocation. Occasionally i noticed that in random interval of times the dbms become slow and get stuck on a few SELECT queries. 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. See. The parameter track_io_timing enables monitoring of block read and write times. Waiting in main loop of the archiver process. Waiting for activity from child process when executing. Waiting to read or update transaction status. Waiting while sending synchronization requests to the checkpointer, because the request queue is full. Note that this includes data that is streamed and/or spilled. 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 WAL to be flushed in WAL sender process. Waiting for an elected Parallel Hash participant to allocate the initial hash table. But processes can also await other events: Waits for input/output ( IO) occur when a process needs to read or write data. Waiting for a read of a two phase state file. Waiting for logical replication remote server to send data for initial table synchronization. David Christensen on Twitter. Waiting for mapping data to reach durable storage during a logical rewrite. 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. Presently, accesses to tables and indexes in both disk-block and individual-row terms are counted. This has no effect in a quorum-based synchronous replication. 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. Waiting for data to reach durable storage while assigning WAL sync method. streaming: This WAL sender is streaming changes after its connected standby server has caught up with the primary. Alternatively, you can invoke pg_stat_clear_snapshot(), which will discard the current transaction's statistics snapshot (if any). Resets statistics for a single function in the current database to zero. This has no effect in a quorum-based synchronous replication. Alternatively, one can build custom views using the underlying statistics functions, as discussed in Section28.2.3. Number of disk blocks read from this table, Number of disk blocks read from all indexes on this table, Number of buffer hits in all indexes on this table, Number of disk blocks read from this table's TOAST table (if any), Number of buffer hits in this table's TOAST table (if any), Number of disk blocks read from this table's TOAST table indexes (if any), Number of buffer hits in this table's TOAST table indexes (if any). 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. The generated IO patterns are also much worse. 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. This can be a host name, an IP address, or a directory path if the connection is via Unix socket. idle in transaction (aborted): This state is similar to idle in transaction, except one of the statements in the transaction caused an error. Waiting for a timeline history file received via streaming replication to reach durable storage. See, One row for each table in the current database, showing statistics about accesses to that specific table. Indexes can be used by simple index scans, bitmap index scans, and the optimizer.