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. This function is restricted to superusers by default, but other users can be granted EXECUTE to run the function. The track_functions parameter controls exactly which functions are tracked. Waiting for a logical replication remote server to send data for initial table synchronization. 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 for a write to a relation data file. The server process is waiting for some condition defined by an extension module. 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. Waiting for SSL while attempting connection. The generated IO patterns are also much worse. Waiting to acquire a lock on page of a relation. The pg_stat_replication_slots view will contain one row per logical replication slot, showing statistics about its usage. 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. Waiting to replace a page in WAL buffers. Waiting to access the list of finished serializable transactions. Waiting for the group leader to clear the transaction ID at end of a parallel operation. Number of data page checksum failures detected in this database (or on a shared object), or NULL if data checksums are not enabled. 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. [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 ! 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 to associate a data block with a buffer in the buffer pool. For example, to show the PIDs and current queries of all backends: Table28.20. Waiting to retrieve or store information about serializable transactions. For details such as the functions' names, consult the definitions of the standard views. Waiting for I/O on a serializable transaction conflict SLRU buffer. Waiting to read or update transaction commit timestamps. This is used by system processes waiting for activity in their main processing loop. This facility is independent of the cumulative statistics system. This includes the sync time when wal_sync_method is either open_datasync or open_sync. 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. Returns the text of this backend's most recent query. So the statistics will show static information as long as you continue the current transaction. pg_stat_get_activity ( integer ) setof record. As soon as the page is read inside the shared buffer pool, the LWLock:BufferIO lock is released. Number of transactions spilled to disk once the memory used by logical decoding to decode changes from WAL has exceeded logical_decoding_work_mem. Waiting to retrieve messages from the shared catalog invalidation queue. Waiting in main loop of the archiver process. When the buffer manager receives a request, PostgreSQL uses the buffer_tag of the desired page. Its Waiting for I/O on a multixact member SLRU buffer. Waiting in main loop of WAL writer process. See Table28.4. A transaction can also see its own statistics (not yet flushed out to the shared memory statistics) in the views pg_stat_xact_all_tables, pg_stat_xact_sys_tables, pg_stat_xact_user_tables, and pg_stat_xact_user_functions. Amount of transaction data decoded for streaming in-progress transactions to the decoding output plugin while decoding changes from WAL for this slot. If state is active this field shows the currently executing query. Waiting for a replication origin to become inactive to be dropped. Waiting for a write of mapping data during a logical rewrite. For client backends, this is the time the client connected to the server. Waiting to read or update sub-transaction information. idle in transaction: The backend is in a transaction, but is not currently executing a query. fastpath function call: The backend is executing a fast-path function. Streaming only works with top-level transactions (subtransactions can't be streamed independently), so the counter is not incremented for subtransactions. Waiting to manage space allocation in shared memory. 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. Waiting for a relation data file to reach durable storage. your workload peak time if you see LWLock:BufferIO coinciding with Waiting to allocate or free a replication slot. Waiting for parallel query dynamic shared memory allocation lock. Using pg_stat_reset() also resets counters that autovacuum uses to determine when to trigger a vacuum or an analyze. See, One row for each index in the current database, showing statistics about accesses to that specific index. Statistics Collection Configuration, One row per server process, showing information related to the current activity of that process, such as state and current query. ; Ensure that filesystem journaling is turned off for data files and WAL files. Waiting for a write of logical rewrite mappings. Waiting in main loop of autovacuum launcher process. Waiting to read data from the client while establishing a GSSAPI session. Normally, WAL files are archived in order, oldest to newest, but that is not guaranteed, and does not hold under special circumstances like when promoting a standby or after crash recovery. Waiting in background writer process, hibernating. Waiting for a replication slot control file to reach durable storage while restoring it to memory. Number of index scans initiated on this index, Number of index entries returned by scans on this index, Number of live table rows fetched by simple index scans using this index. 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. See. See, One row only, showing statistics about the background writer process's activity. Waiting to elect a Parallel Hash participant to allocate a hash table. Waiting to read or record conflicting serializable transactions. Waiting to select the starting location of a synchronized table scan. wait_event will identify the specific wait point. Host name of the connected client, as reported by a reverse DNS lookup of client_addr. Logical decoding plugins may optionally emit tracking messages; if they do not, the tracking mechanism will simply display NULL lag. Waiting to read or update multixact offset mappings. Waiting in main loop of logical replication apply process. Doing this helps Logical decoding plugins may optionally emit tracking messages; if they do not, the tracking mechanism will simply display NULL lag. See, At least one row per subscription, showing information about the subscription workers. Waiting during base backup when throttling activity. Waiting to read or update information about synchronous replicas. See, Only one row, showing statistics about the WAL receiver from that receiver's connected server. 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. See, One row for each sequence in the current database, showing statistics about I/O on that specific sequence. Waiting for a serialized historical catalog snapshot to reach durable storage. The parameter track_wal_io_timing enables monitoring of WAL write times. 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. Waiting to receive bytes from a shared message queue. checksum_last_failure timestamp with time zone. In all other states, it shows the last query that was executed. Waiting for a write of a two phase state file. Waiting for a write while adding a line to the data directory lock file. Waiting for a read from a timeline history file during walsender timeline command. pg_stat_get_snapshot_timestamp () timestamp with time zone, Returns the timestamp of the current statistics snapshot, or NULL if no statistics snapshot has been taken. 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 a relation data file to be extended. Waiting to read or update information about serializable transactions. Per-Backend Statistics Functions, Copyright 1996-2023 The PostgreSQL Global Development Group. For tranches registered by extensions, the name is specified by extension and this will be displayed as wait_event. After v9.5, the buffer cache is protected by 128 hash tables each protected by a LWLock. You can invoke pg_stat_clear_snapshot() to discard the current transaction's statistics snapshot or cached values (if any). All temporary files are counted, regardless of why the temporary file was created (e.g., sorting or hashing), and regardless of the, Total amount of data written to temporary files by queries in this database. Waiting for a read of the relation map file. potential: This standby server is now asynchronous, but can potentially become synchronous if one of current synchronous ones fails. 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 to read or update shared multixact state. 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. lock_manager IP address of the client connected to this backend. See, At least one row per subscription, showing information about the subscription workers. Time elapsed between flushing recent WAL locally and receiving notification that this standby server has written, flushed and applied it. The IO:DataFileRead wait event occurs while data is Waiting to read or update the last value set for a transaction commit timestamp. Note, however, that the existence of a session and its general properties such as its sessions user and database are visible to all users. Waiting for a read from a timeline history file during a walsender timeline command. Returns the process ID of the server process attached to the current session. 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. But if you want to see new results with each query, be sure to do the queries outside any transaction block. Postgres Locking: When is it Concerning? TCP port number that the client is using for communication with this WAL sender, or -1 if a Unix socket is used, Time when this process was started, i.e., when the client connected to this WAL sender. Re: Improve WALRead() to suck data directly from WAL buffers when possible Waiting for an update to the control file to reach durable storage. IO: The server process is waiting for a IO to complete. Waiting to insert WAL data into a memory buffer. See, One row per database, showing database-wide statistics about query cancels due to conflict with recovery on standby servers. 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. Some of the information in the dynamic statistics views shown in Table28.1 is security restricted. These files are stored in the directory named by the stats_temp_directory parameter, pg_stat_tmp by default. 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 fsync call (normally the background writer handles those even when the backend does its own write). Waiting for a read of a timeline history file. Waiting for I/O on a multixact_member buffer. Waiting for a write of mapping data during a logical rewrite. Waiting to acquire a lock on a page of a relation. See, One row for each tracked function, showing statistics about executions of that function. Waiting to read or write a data page in memory. This category is useful for modules to track custom waiting points. If this field is null, it indicates that the client is connected via a Unix socket on the server machine. Waiting to find or allocate space in shared memory. Each shared buffer has an I/O lock that is associated with the LWLock:BufferIO wait event, each time a block (or fastpath function call: The backend is executing a fast-path function. There are also several other views, listed in Table28.2, available to show the results of statistics collection. BufferCacheHitRatio and LWLock:BufferIO wait 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. Waiting for a read while adding a line to the data directory lock file. For an asynchronous standby, the replay_lag column approximates the delay before recent transactions became visible to queries. Possible values are: Wait event name if backend is currently waiting, otherwise NULL. Waiting in main loop of logical launcher process. The argument can be bgwriter to reset all the counters shown in the pg_stat_bgwriter view, archiver to reset all the counters shown in the pg_stat_archiver view, wal to reset all the counters shown in the pg_stat_wal view or recovery_prefetch to reset all the counters shown in the pg_stat_recovery_prefetch view. This view will only contain information on standby servers, since conflicts do not occur on primary servers. If this field is null, it indicates that the client is connected via a Unix socket on the server machine. The total number of rows in each table, and information about vacuum and analyze actions for each table are also counted. These access functions use a backend ID number, which ranges from one to the number of currently active backends. Waits for a buffer pin ( BufferPin ). Waiting for an elected Parallel Hash participant to finish allocating more buckets. pg_stat_get_backend_pid ( integer ) integer, pg_stat_get_backend_start ( integer ) timestamp with time zone. 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. Waiting to write a protocol message to a shared message queue. Time spent reading data file blocks by backends in this database, in milliseconds (if track_io_timing is enabled, otherwise zero), Time spent writing data file blocks by backends in this database, in milliseconds (if track_io_timing is enabled, otherwise zero), Time spent by database sessions in this database, in milliseconds (note that statistics are only updated when the state of a session changes, so if sessions have been idle for a long time, this idle time won't be included), Time spent executing SQL statements in this database, in milliseconds (this corresponds to the states active and fastpath function call in pg_stat_activity), idle_in_transaction_time double precision, Time spent idling while in a transaction in this database, in milliseconds (this corresponds to the states idle in transaction and idle in transaction (aborted) in pg_stat_activity), Total number of sessions established to this database, Number of database sessions to this database that were terminated because connection to the client was lost, Number of database sessions to this database that were terminated by fatal errors, Number of database sessions to this database that were terminated by operator intervention. Waiting for SLRU data to reach durable storage following a page write. Waiting to read or update the state of prepared transactions. Waiting to read or update the fast-path lock information. Resets some cluster-wide statistics counters to zero, depending on the argument. This standby's xmin horizon reported by hot_standby_feedback. The argument can be one of CommitTs, MultiXactMember, MultiXactOffset, Notify, Serial, Subtrans, or Xact to reset the counters for only that entry. Waiting for another process to be attached to a shared message queue. Waiting for recovery conflict resolution for dropping a tablespace. In order to write the disk block into buffer memory, the buffer cache's hash table entry needs updating. Waiting for the relation map file to reach durable storage. If you see anything in the documentation that is not correct, does not match Waiting for other Parallel Hash participants to finish repartitioning. 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. Waiting for the page number needed to continue a parallel B-tree scan to become available. (See Chapter20 for details about setting configuration parameters.). Waiting in WAL receiver to establish connection to remote server. Choose the appropriate target Region. See, One row per connection (regular and replication), showing information about SSL used on this connection. The pg_stat_database view will contain one row for each database in the cluster, plus one for shared objects, showing database-wide statistics. BufferPin: The server process is waiting to access to a data buffer during a period when no other process can be examining that buffer. Waiting to read or update old snapshot control information. Waiting for a write while adding a line to the data directory lock file. Amount of decoded transaction data spilled to disk while performing decoding of changes from WAL for this slot. Returns the time when the backend's most recent query was started. The pg_stat_database view will contain one row for each database in the cluster, showing database-wide statistics. A database-wide ANALYZE is recommended after the statistics have been reset. The parameter track_activities enables monitoring of the current command being executed by any server process. Send time of last reply message received from standby server. Waiting for SSL while attempting connection. Waiting for a write when creating a new WAL segment by copying an existing one. A backend process wants to read a page into shared memory. See, One row per subscription, showing statistics about errors. If this field is null, it indicates that this is an internal server process. Monitoring systems should choose whether to represent this as missing data, zero or continue to display the last known value. Waiting to acquire an exclusive lock to truncate off any empty pages at the end of a table vacuumed. backup: This WAL sender is sending a backup. 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. Waiting for a read of the relation map file. Waiting to read or update the current state of autovacuum workers. LWLock:buffer_mapping. See Section30.5 for more information about the internal WAL function issue_xlog_fsync. Waiting to read or update the state of logical replication workers. (Some locks have specific names; others are part of a group of locks each with a similar purpose.). Waiting to write zero bytes to a dynamic shared memory backing file. Increase the number of wal_buffers available to the database. 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. Waiting for a read from a relation data file. 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 any activity when processing replies from WAL receiver in WAL sender process. The function pg_stat_get_backend_idset provides a convenient way to generate one row for each active backend for invoking these functions. Waiting to ensure that the table it has selected for a vacuum still needs vacuuming. Waiting for a newly initialized WAL file to reach durable storage. Identifier of this backend's most recent query. Waiting to access the shared per-process data structures (typically, to get a snapshot or report a session's transaction ID). Waiting to acquire an advisory user lock. Waiting to read or update background worker state. 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 data to reach durable storage while assigning WAL sync method. 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 write relation cache initialization file. This can be used to gauge the delay that synchronous_commit level remote_apply incurred while committing if this server was configured as a synchronous standby. - a BufFreeList LWLock was getting acquired to find a free buffer for a page - to change the association of buffer in buffer mapping hash table a LWLock is acquired on a hash partition to which the buffer to be associated belongs and as there were just 16 such partitions, there was huge contention when multiple clients Waiting for startup process to send initial data for streaming replication. Waiting to read or update the state of prepared transactions. 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. The server process is waiting for a lightweight lock. 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). Synchronous state of this standby server. Alternatively, you can invoke pg_stat_clear_snapshot(), which will discard the current transaction's statistics snapshot (if any). The parameter track_activities enables monitoring of the current command being executed by any server process. Waiting to ensure that a table selected for autovacuum still needs vacuuming. 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. quorum: This standby server is considered as a candidate for quorum standbys. The columns wal_distance, block_distance and io_depth show current values, and the other columns show cumulative counters that can be reset with the pg_stat_reset_shared function. In all other states, it shows the identifier of last query that was executed. I've made . If, Type of current backend. Number of deadlocks detected in this database. The buffer_mapping LWLock wait event will be . catchup: This WAL sender's connected standby is catching up with the primary. Waiting for a read of a logical mapping during reorder buffer management. The pg_stat_wal view will always have a single row, containing data about WAL activity of the cluster. In rows about other sessions, many columns will be null. 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. Waiting for a write to the relation map file. 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. Additional Statistics Functions. Waiting for a read when creating a new WAL segment by copying an existing one. wait_event will identify the specific wait point. 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 to read or truncate multixact information. might need to increase it or scale up your DB instance class. This facility is independent of the collector process. 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. (For example, in psql you could issue \d+ pg_stat_activity.) pg_stat_reset_single_function_counters ( oid ) void. disabled: This state is reported if track_activities is disabled in this backend. Connection string used by this WAL receiver, with security-sensitive fields obfuscated. Waiting to update limit on notification message storage. potential: This standby server is now asynchronous, but can potentially become synchronous if one of current synchronous ones fails. Copyright 1996-2023 The PostgreSQL Global Development Group, PostgreSQL 15.2, 14.7, 13.10, 12.14, and 11.19 Released, 28.2.1. workload into more reader nodes. Waiting for a write during reorder buffer management. Per-Backend Statistics Functions, pg_stat_get_backend_idset () setof integer. Time at which these statistics were last reset. LWLock: The backend is waiting for a lightweight lock. 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.
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