Re: Improve WALRead() to suck data directly from WAL buffers when possible Waiting in main loop of background writer process background worker. Waiting for a write during a file copy operation. OID of the database this backend is connected to, Name of the database this backend is connected to. Number of times WAL buffers were written out to disk via XLogWrite request. Waiting in WAL receiver to receive data from remote server. 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, last_msg_send_time timestamp with time zone, Send time of last message received from origin WAL sender, last_msg_receipt_time timestamp with time zone, 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, Host of the PostgreSQL instance this WAL receiver is connected to. PostgreSQL utilizes lightweight locks (LWLocks) to synchronize and control access to the buffer content. 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. Time when the currently active query was started, or if state is not active, when the last query was started. Waiting for truncate of mapping data during a logical rewrite. The parameter track_counts controls whether statistics are collected about table and index accesses. It also tracks the total number of rows in each table, and information about vacuum and analyze actions for each table. events. Waiting for other Parallel Hash participants to finish partitioning the outer relation. Waiting to read or update transaction status. Thanks for letting us know this page needs work. Waiting for I/O on a commit timestamp SLRU buffer. Cumulative statistics are collected in shared memory. If this field is null, it indicates that this is an internal server process. finish their input/output (I/O) operations when concurrently trying to access a page. Table28.31.pg_statio_all_sequences View, Number of disk blocks read from this sequence. Possible values are: async: This standby server is asynchronous. Waiting to read or update background worker state. Amount of decoded transaction data spilled to disk while performing decoding of changes from WAL for this slot. 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. IPC: The server process is waiting for some activity from another process in the server. The pg_stat_wal view will always have a single row, containing data about WAL activity of the cluster. Number of deadlocks detected in this database. This and other streaming counters for this slot can be used to tune logical_decoding_work_mem. streaming: This WAL sender is streaming changes after its connected standby server has caught up with the primary. Waiting for an elected Parallel Hash participant to decide on future batch growth. Waiting for a write during a file copy operation. catchup: This WAL sender's connected standby is catching up with the primary. The per-table and per-index functions take a table or index OID. quorum: This standby server is considered as a candidate for quorum standbys. See, At least one row per subscription, showing information about the subscription workers. Waiting for a WAL file to reach durable storage. 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. 39919 LWLock buffer_mapping 5119 Client ClientRead 3116 IO DataFileRead . 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. A process acquires an LWLock in a shared mode to read from the buffer and an exclusive mode to write to the buffer. 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. A backend process wants to read a page into shared memory. The pg_stat_archiver view will always have a single row, containing data about the archiver process of the cluster. Waiting for a read from a relation data file. Resetting these counters can cause autovacuum to not perform necessary work, which can cause problems such as table bloat or out-dated table statistics. The track_functions parameter controls exactly which functions are tracked. A backend process is waiting to associate a data block with a buffer in the shared buffer pool. Waiting for background worker to start up. Waiting for an immediate synchronization of a relation data file to durable storage. replication_slot_io: Waiting for I/O on a replication slot. Tune max_wal_size and checkpoint_timeout based on The LWLock that this article will introduce is a lightweight lock (Lightweight Lock) based on SpinLock. Per-Backend Statistics Functions, Copyright 1996-2023 The PostgreSQL Global Development Group. If, Type of current backend. 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. Waiting for a two phase state file to reach durable storage. Client: The server process is waiting for some activity on a socket from user applications, and that the server expects something to happen that is independent from its internal processes. Waiting for a read from a replication slot control file. Amount of transaction data decoded for sending transactions to the decoding output plugin while decoding changes from WAL for this slot. Waiting to acquire an exclusive lock to truncate off any empty pages at the end of a table vacuumed. 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. Lock: The backend is waiting for a heavyweight lock. Waiting for the relation map file to reach durable storage. your experience with the particular feature or requires further clarification, Waiting to read or update old snapshot control information. For better performance, stats_temp_directory can be pointed at a RAM-based file system, decreasing physical I/O requirements. Waiting to read or record conflicting serializable transactions. This is consistent with the goal of measuring synchronous commit and transaction visibility delays for recent write transactions. Waiting for a read during reorder buffer management. Copyright 1996-2023 The PostgreSQL Global Development Group, PostgreSQL 15.2, 14.7, 13.10, 12.14, and 11.19 Released, 28.2.1. 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. 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. 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. 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. Some of the information in the dynamic statistics views shown in Table28.1 is security restricted. 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 a read during a file copy operation. postgres7 Slru--1. Waiting to read or write a data page in memory. Returns the time when this process was started. Waiting for a read while adding a line to the data directory lock file. See, One row for each index in the current database, showing statistics about accesses to that specific index. 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 read or update the state of logical replication workers. Waiting for background worker to shut down. Waiting in main loop of the statistics collector process. 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). This field will only be non-null for IP connections, and only when log_hostname is enabled. Waiting to access the list of predicate locks held by the current serializable transaction during a parallel query. IP address of the client connected to this WAL sender. Waiting for a relation data file to be extended. Waiting for the version file to be written while creating a database. Waiting in main loop of WAL sender process. Waiting for a write while initializing a new WAL file. Returns the text of this backend's most recent query. 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. See, Only one row, showing statistics about blocks prefetched during recovery. 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 live rows fetched by sequential scans and index entries returned by index scans in this database, Number of live rows fetched by index scans 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 to retrieve messages from the shared catalog invalidation queue. 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. Waiting to apply WAL during recovery because of a delay setting. Waiting to access the sub-transaction SLRU cache. Waiting for a buffered file to be truncated. Waiting to access the serializable transaction conflict SLRU cache. Waiting for I/O on a clog (transaction status) buffer. Waiting to acquire a speculative insertion lock. The pg_stat_wal_receiver view will contain only one row, showing statistics about the WAL receiver from that receiver's connected server. Type of current backend. 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. 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. The parameter track_activities enables monitoring of the current command being executed by any server process. The server process is waiting for activity on a socket connected to a user application. The functions for per-function statistics take a function OID. Waiting for a newly initialized WAL file to reach durable storage. If state is active this field shows the identifier of the currently executing query. See, One row only, showing statistics about the WAL archiver process's activity. Using pg_stat_reset() also resets counters that autovacuum uses to determine when to trigger a vacuum or an analyze. Waiting for I/O on a serializable transaction conflict SLRU buffer. However, they are also used to ensure mutual exclusion for certain internal operations such as relation extension. Current WAL sender state. Wait event name if backend is currently waiting, otherwise NULL. 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. Identifier of this backend's most recent query. If enabled, calls to user-defined functions and the total time spent in each one are counted as well. 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 to acquire an advisory user lock. In all other states, it shows the identifier of last query that was executed. Each shared buffer has an I/O lock that is associated with the LWLock:BufferIO wait event, each time a block (or PostgreSQL's statistics collector is a subsystem that supports collection and reporting of information about server activity. If the argument is NULL, reset statistics for all subscriptions. The pg_stat_recovery_prefetch view will contain only one row. sync: This standby server is synchronous. 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. Waiting in main loop of syslogger process. IP address of the client connected to this WAL sender. Waiting to write a protocol message to a shared message queue. Amount of transaction data decoded for streaming in-progress transactions to the decoding output plugin while decoding changes from WAL for this slot. pg_stat_reset_subscription_stats ( oid ) void. Waiting for a write while initializing a new WAL file. Waiting to access the list of predicate locks held by serializable transactions. Waiting for data to reach durable storage while creating the data directory lock file. Time when this process' current transaction was started, or null if no transaction is active. Waiting to insert WAL into a memory buffer. The parameter track_activities enables monitoring of the current command being executed by any server process. Waiting for a relation data file to reach durable storage. From the Actions drop-down menu, choose Create Read Replica. Several predefined views, listed in Table28.1, are available to show the current state of the system. Waiting to synchronize workers during Parallel Hash Join plan execution. Wait Events of Type BufferPin, Table28.8. Additional Statistics Functions. A database-wide ANALYZE is recommended after the statistics have been reset. Waiting for a write of a serialized historical catalog snapshot. Returns the wait event name if this backend is currently waiting, otherwise NULL. Number of WAL files that have been successfully archived, Name of the WAL file most recently successfully archived, last_archived_time timestamp with time zone, Time of the most recent successful archive operation, Number of failed attempts for archiving WAL files, Name of the WAL file of the most recent failed archival operation, last_failed_time timestamp with time zone, Time of the most recent failed archival operation. Waiting to ensure that the table it has selected for a vacuum still needs vacuuming. The argument can be one of CommitTs, MultiXactMember, MultiXactOffset, Notify, Serial, Subtrans, or Xact to reset the counters for only that entry. This view will only contain information on standby servers, since conflicts do not occur on master servers. 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. See, One row for each tracked function, showing statistics about executions of that function. Presently, accesses to tables and indexes in both disk-block and individual-row terms are counted. Waiting to read or update shared notification state. 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. For details such as the functions' names, consult the definitions of the standard views. This counter is incremented each time a transaction is streamed, and the same transaction may be streamed multiple times. 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. 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. Waiting to ensure that a table selected for autovacuum still needs vacuuming. You can split your The server process is waiting for an I/O operation to complete. pg_stat_reset_replication_slot ( text ) void. The server process is waiting for a timeout to expire. Waiting in main loop of checkpointer process. Waiting for an asynchronous prefetch from a relation data file. The server process is waiting for a lightweight lock. Waiting to elect a Parallel Hash participant to decide on future batch growth. Waiting for I/O on a multixact member SLRU buffer. Waiting for WAL buffers to be written to disk. Waiting for a write to the relation map file. Waiting in WAL receiver to establish connection to remote server. The management of the buffers in PostgreSQL consists of a buffer descriptor that contains metadata about the buffer and the buffer content that is read from the disk. Increase the number of wal_buffers available to the database. The server process is idle. This facility is independent of the cumulative statistics system. Waiting to acquire a lock on page of a relation. The parameter track_io_timing enables monitoring of block read and write times. LWLock- buffer_mapping. (Some locks have specific names; others are part of a group of locks each with a similar purpose.). See, One row only, showing statistics about the background writer process's activity. See, Only one row, showing statistics about the WAL receiver from that receiver's connected server. wait_event will identify the specific wait point. Waiting for a replication slot control file to reach durable storage while restoring it to memory. Waiting to read or update information about serializable transactions. So the statistics will show static information as long as you continue the current transaction. Waiting for the page number needed to continue a parallel B-tree scan to become available. Ordinary users can only see all the information about their own sessions (sessions belonging to a role that they are a member of). 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. purpose is for the same page to be read into the shared buffer. Waiting to read or update vacuum-related information for a B-tree index. Time when this process was started. 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. Serial number of the client certificate, or NULL if no client certificate was supplied or if SSL is not in use on this connection. 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. For an asynchronous standby, the replay_lag column approximates the delay before recent transactions became visible to queries. Waiting for recovery conflict resolution for a vacuum cleanup. Returns the time when the backend's most recent query was started. Waiting to read or update information about the state of synchronous replication. Waiting to apply WAL at recovery because it is delayed. These access functions use a backend ID number, which ranges from one to the number of currently active backends. Logical decoding plugins may optionally emit tracking messages; if they do not, the tracking mechanism will simply display NULL lag. wait_event will identify the specific wait point. See, One row for each index in the current database, showing statistics about I/O on that specific index. This field is truncated like client_dn. 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. Table28.15.pg_stat_replication_slots View, A unique, cluster-wide identifier for the replication slot. Priority of this standby server for being chosen as the synchronous standby in a priority-based synchronous replication. Waiting to update limits on transaction id and multixact consumption. Port number of the PostgreSQL instance this WAL receiver is connected to. Connection string used by this WAL receiver, with security-sensitive fields obfuscated. (Conflicts occur only on standby servers; see, Number of temporary files created by queries in this database. Waiting to read or truncate multixact information. Waiting to associate a data block with a buffer in the buffer pool. Extension: The server process is waiting for activity in an extension module. Waiting to write zero bytes to a dynamic shared memory backing file. Waiting for a write of a WAL page during bootstrapping. Waiting in WAL receiver to establish connection to remote server. checksum_last_failure timestamp with time zone. Waiting for a read while adding a line to the data directory lock file. 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. Additional functions related to statistics collection are listed in Table28.19. Buffer pin waits can be protracted if another process holds an open cursor which last read data from the buffer in question. Waiting for a read of the relation map file. 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. The pg_stat_user_functions view will contain one row for each tracked function, showing statistics about executions of that function. What we have discussed in this episode of 5mins of Postgres. 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. Waiting to get the start location of a scan on a table for synchronized scans. 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. 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. ), 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). Waiting for activity from a child process while executing a. 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. Several predefined views, listed in Table28.1, are available to show the current state of the system. Waiting for logical rewrite mappings to reach durable storage. potential: This standby server is now asynchronous, but can potentially become synchronous if one of current synchronous ones fails. Waiting to receive bytes from a shared message queue. also pending an I/O operation, The ratio between the size of the shared buffer pool (defined by the shared_buffers parameter) and the This documentation is for an unsupported version of PostgreSQL. pg_stat_get_backend_pid ( integer ) integer, pg_stat_get_backend_start ( integer ) timestamp with time zone. 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 a read of a two phase state file. The parameter track_counts controls whether cumulative statistics are collected about table and index accesses. Waiting to update the relation map file used to store catalog to filenode mapping. LWTRANCHE_BUFFER_CONTENT @ LWTRANCHE_BUFFER_CONTENT. See, One row per WAL sender process, showing statistics about replication to that sender's connected standby server. Waiting for a read when creating a new WAL segment by copying an existing one.
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