Logical decoding plugins may optionally emit tracking messages; if they do not, the tracking mechanism will simply display NULL lag. Waiting in a cost-based vacuum delay point. So the displayed information lags behind actual activity. Statistics Functions. 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. Possible values are: catchup: This WAL sender's connected standby is catching up with the primary. Waiting in WAL receiver to receive data from remote server. Waiting to acquire an advisory user lock. David Christensen on Twitter. Additional functions related to the cumulative statistics system are listed in Table28.34. The pg_stat_replication view will contain one row per WAL sender process, showing statistics about replication to that sender's connected standby server. 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. 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. The parameter track_wal_io_timing enables monitoring of WAL write times. When a buffer is read from disk (or written to disk), an IO in progress lock is also acquired, which indicates to other processes that the page is being read (or written) they can queue up if they need to do something with this page. Thanks for letting us know we're doing a good job! Waiting to read or update the current state of autovacuum workers. fastpath function call: The backend is executing a fast-path function. Table28.31.pg_statio_all_sequences View, Number of disk blocks read from this sequence. wait_event will identify the specific wait point. Waiting for a relation data file to reach durable storage. If state is active this field shows the identifier of the currently executing query. Waiting for a write while initializing a new WAL file. Waiting for a read when creating a new WAL segment by copying an existing one. 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. The next use of statistical information will (when in snapshot mode) cause a new snapshot to be built or (when in cache mode) accessed statistics to be cached. Alone the requirement of separate fsyncs and everything is pretty bothersome. OID of this database, or 0 for objects belonging to a shared relation. Waiting for a read of the relation map file. The pg_statio_ views are primarily useful to determine the effectiveness of the buffer cache. Detailed Description . pg_stat_reset_replication_slot ( text ) void. (Conflicts occur only on standby servers; see, Number of temporary files created by queries in this database. Waiting for a write when creating a new WAL segment by copying an existing one. Waiting while sending synchronization requests to the checkpointer, because the request queue is full. Waiting during recovery when WAL data is not available from any source (. See, One row per connection (regular and replication), showing information about GSSAPI authentication and encryption used on this connection. 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. Each shared buffer has an I/O lock that is associated with the LWLock:BufferIO wait event, each time a block (or Waiting in main loop of background writer process. From pg_stat_activity i noticed that the wait_event_type and wait_event of these queries is as follows: This and other streaming counters for this slot can be used to tune logical_decoding_work_mem. Copyright 1996-2023 The PostgreSQL Global Development Group, PostgreSQL 15.2, 14.7, 13.10, 12.14, and 11.19 Released, 28.2.1. Waiting for parallel bitmap scan to become initialized. 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. OID of the user logged into this WAL sender process, Name of the user logged into this WAL sender process, Name of the application that is connected to this WAL sender. Waiting to access the multixact offset SLRU cache. Waiting in main loop of syslogger process. Last write-ahead log location already received and written to disk, but not flushed. The server process is waiting for some interaction with another server process. Waiting for a read from a timeline history file during a walsender timeline command. The following wait events are a subset of the list in Amazon Aurora PostgreSQL wait events. Waiting for the group leader to clear the transaction ID at end of a parallel operation. Waiting for a read of a serialized historical catalog snapshot. Waiting for a newly initialized WAL file to reach durable storage. Per-Backend Statistics Functions, pg_stat_get_backend_idset () setof integer. The buffer_tag comprises three values: the RelFileNode and the fork number of the relation to which its page belongs, and the block number of its 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. Waiting to read while creating the data directory lock file. Resets statistics of the replication slot defined by the argument. Table28.12.pg_stat_database_conflicts View. Waiting for a write of a serialized historical catalog snapshot. Time when this process' current transaction was started, or null if no transaction is active. Waiting for startup process to send initial data for streaming replication. 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. The server process is waiting for activity on a socket connected to a user application. Port number of the PostgreSQL instance this WAL receiver is connected to. also pending an I/O operation, The ratio between the size of the shared buffer pool (defined by the shared_buffers parameter) and the However, current-query information collected by track_activities is always up-to-date. 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. In all other states, it shows the identifier of last query that was executed. All temporary files are counted, regardless of why the temporary file was created (e.g., sorting or hashing), and regardless of the log_temp_files setting. Waiting for a relation data file to be truncated. ), 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). 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 access the multixact member SLRU cache. Waiting to allocate or free a replication slot. TCP port number that the client is using for communication with this WAL sender, or, Time when this process was started, i.e., when the client connected to this WAL sender. Each buffer header also contains an LWLock, the "buffer content lock", that *does* represent the right to access the data: in the buffer. The pg_stat_archiver view will always have a single row, containing data about the archiver process of the cluster. Wait event name if backend is currently waiting, otherwise NULL. Waiting for background worker to start up. Note that only tables, indexes, and functions in the current database can be seen with these functions. Waiting in main loop of WAL sender process. Waiting to access the shared per-process data structures (typically, to get a snapshot or report a session's transaction ID). In such cases, an older set of per-backend statistics access functions can be used; these are shown in Table28.35. Waiting to acquire an exclusive pin on a buffer. Waiting for the relation map file to reach durable storage. Waiting for SLRU data to reach durable storage during a checkpoint or database shutdown. Waiting to read or update multixact offset mappings. Waiting for an immediate synchronization of a relation data file to durable storage. See Table28.4 for details. Waiting for data to reach durable storage while creating the data directory lock file. Waiting when WAL data is not available from any kind of sources (local, archive or stream) before trying again to retrieve WAL data, at recovery. Waiting to read or record conflicting serializable transactions. pg_stat_get_backend_wait_event_type ( integer ) text. When a server, including a physical replica, shuts down cleanly, a permanent copy of the statistics data is stored in the pg_stat subdirectory, so that statistics can be retained across server restarts. 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. The pg_stat_gssapi view will contain one row per backend, showing information about GSSAPI usage on this connection. For example, to show the PIDs and current queries of all backends: Table28.20. This view will only contain information on standby servers, since conflicts do not occur on master servers. So the statistics will show static information as long as you continue the current transaction. 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. Increase the number of wal_buffers available to the database. Verify whether you have unused indexes, then remove them. Waiting to write zero bytes to a dynamic shared memory backing file. The type of event for which the backend is waiting, if any; otherwise NULL. idle in transaction: The backend is in a transaction, but is not currently executing a query. The server process is idle. However, current-query information collected by track_activities is always up-to-date. Time when the currently active query was started, or if state is not active, when the last query was started. Only directly connected standbys are listed; no information is available about downstream standby servers. See, One row for each tracked function, showing statistics about executions of that function. If the argument is NULL, resets statistics for all the replication slots. For client backends, this is the time the client connected to the server. See, One row per SLRU, showing statistics of operations. streaming: This WAL sender is streaming changes after its connected standby server has caught up with the primary. Waiting for the page number needed to continue a parallel B-tree scan to become available. Simple test for lock_waits log messages. Restrict the maximum number of connections to the database as a best practice. 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. Type of current backend. We recommend different actions depending on the causes of your wait event: Observe Amazon CloudWatch metrics for correlation between sharp decreases in the Waiting to read or update background worker state. The LWLock:BufferIO event occurs when RDS for PostgreSQL or Aurora PostgreSQL is waiting for other processes to finish their I/O operations. Amount of transaction data decoded for sending transactions to the decoding output plugin while decoding changes from WAL for this slot. Waiting for a write while adding a line to the data directory lock file. 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. Alternatively, one can build custom views using the underlying cumulative statistics functions, as discussed in Section28.2.24. It is used per the rules above. The pg_stat_user_functions view will contain one row for each tracked function, showing statistics about executions of that function. Text of this backend's most recent query. This is controlled by configuration parameters that are normally set in postgresql.conf. 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. Waiting for a write during reorder buffer management. This can be a host name, an IP address, or a directory path if the connection is via Unix socket. please use Waiting for a write of a newly created timeline history file. Waiting for a write of logical rewrite mappings. This wait event information is relevant for all Aurora PostgreSQL 13 versions. Waiting for a barrier event to be processed by all backends. It can be joined to pg_stat_activity or pg_stat_replication on the pid column to get more details about the connection. Waiting to allocate or assign a transaction id. Waiting to access predicate lock information used by serializable transactions. Waiting to read or update transaction status. Did this page help you? Waiting to synchronize workers during Parallel Hash Join plan execution. 213 * Prior to PostgreSQL 9.4, we used an enum type called LWLockId to refer. pg_stat_get_backend_activity_start ( integer ) timestamp with time zone. Waiting in WAL receiver to establish connection to remote server. Waiting to access the list of finished serializable transactions. To use the Amazon Web Services Documentation, Javascript must be enabled. Waiting in main loop of autovacuum launcher process. A backend process wants to read a page into shared memory. ; Ensure that filesystem journaling is turned off for data files and WAL files. 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. LWLock:BufferIO. 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. If enabled, calls to user-defined functions and the total time spent in each one are counted as well. Waiting for a write of mapping data during a logical rewrite. idle: The backend is waiting for a new client command. Waiting for a write while initializing a new WAL file. In rows about other sessions, many columns will be null. IP address of the client connected to this backend. Send time of last reply message received from standby server. Waiting for other Parallel Hash participants to finish repartitioning. Then identify which query 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. Such a system would show similar times while new WAL is being generated, but would differ when the sender becomes idle. Waiting for other Parallel Hash participants to finish hashing the inner relation. BK_1935: "IObuffer_locks,ControlLock()"IOControlLockControlLockIOSlruSharedData. 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 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. pg_stat_get_backend_client_addr ( integer ) inet. See, One row per subscription, showing statistics about errors. 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 to acquire a lock on a non-relation database object. Waiting to read or update replication slot state. This field will only be non-null for IP connections, and only when log_hostname is enabled. Waits for a buffer pin ( BufferPin ). See, One row per database, showing database-wide statistics. Waiting for a serialized historical catalog snapshot to reach durable storage. Its purpose is for the same page to be read into the shared buffer. Avoid PostgreSQL LWLock:buffer_content locks in Amazon Aurora: Tips and best practices. See, One row only, showing statistics about the background writer process's activity. The parameter track_activities enables monitoring of the current command being executed by any server process. The pg_stat_slru view will contain one row for each tracked SLRU cache, showing statistics about access to cached pages. Waiting to read or update shared notification state. Tune max_wal_size and checkpoint_timeout based on Resets all statistics counters for the current database to zero. Waiting to retrieve or remove messages from shared invalidation queue. By default the query text is truncated at 1024 bytes; this value can be changed via the parameter track_activity_query_size. Time at which these statistics were last reset. See, One row for each backend (including autovacuum worker processes) running, One row only, showing statistics about the WAL archiver process's activity. Waiting to read or update multixact member mappings. Waiting in main loop of logical apply process. Waiting to read or update information about serializable transactions. The type of event for which the backend is waiting, if any; otherwise 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. Resets statistics for a single subscription shown in the pg_stat_subscription_stats view to zero. Waiting for parallel query dynamic shared memory allocation lock. Such a system would show similar times while new WAL is being generated, but would differ when the sender becomes idle. Waiting for WAL files required for a backup to be successfully archived. Waiting for changes to a relation data file to reach durable storage. Another important point is that when a server process is asked to display any of these statistics, it first fetches the most recent report emitted by the collector process and then continues to use this snapshot for all statistical views and functions until the end of its current transaction. Waiting for I/O on a multixact offset SLRU buffer. 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. Waiting for a read during reorder buffer management. Waiting for startup process to send initial data for streaming replication. The pg_stat_database view will contain one row for each database in the cluster, showing database-wide statistics. Waiting to elect a Parallel Hash participant to decide on future batch growth. idle: The backend is waiting for a new client command. The parameter track_io_timing enables monitoring of block read and write times. Waiting to manage an extension's space allocation in shared memory. finish their input/output (I/O) operations when concurrently trying to access a page. replication_slot_io: Waiting for I/O on a replication slot. The parameter track_functions enables tracking of usage of user-defined functions. 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. 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). 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. 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. Waiting for a newly created timeline history file to reach durable storage. Waiting for the control file to reach durable storage. Statistics Collection Configuration, One row per server process, showing information related to the current activity of that process, such as state and current query. As soon as the page is read inside the shared buffer pool, the LWLock:BufferIO lock is released. 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. Time when this process' current transaction was started, or null if no transaction is active. Resetting these counters can cause autovacuum to not perform necessary work, which can cause problems such as table bloat or out-dated table statistics. Waiting to access the serializable transaction conflict SLRU cache. The per-table and per-index functions take a table or index OID. Waiting for a timeline history file received via streaming replication to reach durable storage. See, One row only, showing statistics about the background writer process's activity. Waiting to allocate a new transaction ID. sync: This standby server is synchronous. The next use of statistical information will cause a new snapshot to be fetched. A process acquires an LWLock in a shared mode to read from the buffer and an exclusive mode to write to the buffer. Waiting to add or examine predicate lock information. Returns the set of currently active backend ID numbers (from 1 to the number of active backends). Waiting for a write to a relation data file. proc: Waiting to read or update the fast-path lock information. Provide feedback LWTRANCHE_BUFFER_CONTENT @ LWTRANCHE_BUFFER_CONTENT. Waiting for an elected Parallel Hash participant to finish allocating more buckets. 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. 39919 LWLock buffer_mapping 5119 Client ClientRead 3116 IO DataFileRead With C-Hash Event Count Event Type Event Name The access functions for per-database statistics take a database OID as an argument to identify which database to report on. The parameter track_counts controls whether statistics are collected about table and index accesses. Alternatively, you can invoke pg_stat_clear_snapshot(), which will discard the current transaction's statistics snapshot (if any). Waiting to update limit on notification message storage. See, Only one row, showing statistics about the WAL receiver from that receiver's connected server. Waiting to apply WAL at recovery because it is delayed. The pg_stat_wal_receiver view will contain only one row, showing statistics about the WAL receiver from that receiver's connected server. 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. Waiting to read or update background worker state. 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 for background worker to shut down. The pg_stat_all_indexes view will contain one row for each index in the current database, showing statistics about accesses to that specific index. The LWLock:BufferIO wait event precedes the IO:DataFileRead wait event. pg_stat_get_backend_wait_event ( integer ) text. Waiting for parallel workers to finish computing. Waiting for a read from a replication slot control file. Table28.19. Waiting for WAL from a stream at recovery. shared_buffers parameter. IP address of the client connected to this WAL sender. pg_stat_get_backend_activity ( integer ) text. pg_stat_reset_single_function_counters ( oid ) void. Waiting for a read of a two phase state file. The per-index statistics are particularly useful to determine which indexes are being used and how effective they are. Time when this process was started. PostgreSQL 15.2, 14.7, 13.10, 12.14, and 11.19 Released, 28.2.1. Table28.34. From the Actions drop-down menu, choose Create Read Replica. Waiting to associate a data block with a buffer in the buffer pool. Number of times transactions were spilled to disk while decoding changes from WAL for this slot. The parameter track_activities enables monitoring of the current command being executed by any server process. Waiting to elect a Parallel Hash participant to allocate a hash table. Waiting to read or update notification messages. Presently, the collector can count accesses to tables and indexes in both disk-block and individual-row terms. The total number of rows in each table, and information about vacuum and analyze actions for each table are also counted. LWLock:buffer_mapping. True if GSSAPI authentication was used for this connection. Waiting for a read while adding a line to the data directory lock file. Waiting to perform an operation on a list of locks held by serializable transactions. For details such as the functions' names, consult the definitions of the standard views. Resetting these counters can cause autovacuum to not perform necessary work, which can cause problems such as table bloat or out-dated table statistics. might need to increase it or scale up your DB instance class. 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 to find or allocate space in shared memory. Each individual server process transmits new statistical counts to the collector just before going idle; so a query or transaction still in progress does not affect the displayed totals.

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