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. This is controlled by configuration parameters that are normally set in postgresql.conf. Presently, the collector can count accesses to tables and indexes in both disk-block and individual-row terms. Possible types are. wait_event will identify the specific wait point. 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 timeline history file received via streaming replication to reach durable storage. buffer_mapping | DBmarlin Docs and Knowledge Base Amount of transaction data decoded for sending transactions to the decoding output plugin while decoding changes from WAL for this slot. Waiting to read or update background worker state. (Conflicts occur only on standby servers; see, Number of temporary files created by queries in this database. Waiting for base backup to read from a file. Waiting in background writer process, hibernating. 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 recovery conflict resolution for dropping a tablespace. 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. Postgres 10.3: SELECT queries hang for hours - Stack Overflow Waiting for a write to a relation data file. Waiting to synchronize workers during Parallel Hash Join plan execution. Waiting for a read of the relation map file. Returns the time when this process was started. Waiting to allocate or free a replication slot. Query identifiers are not computed by default so this field will be null unless compute_query_id parameter is enabled or a third-party module that computes query identifiers is configured. See, One row per database, showing database-wide statistics about query cancels due to conflict with recovery on standby servers. Waiting in main loop of checkpointer process. The pg_stat_user_functions view will contain 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. 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. A process acquires an LWLock in a shared mode to read from the buffer and an exclusive mode to write to the buffer. 213 * Prior to PostgreSQL 9.4, we used an enum type called LWLockId to refer. Possible values are: catchup: This WAL sender's connected standby is catching up with the primary. Returns the TCP port number that the client is using for communication. Waiting to allocate or assign a transaction id. LWLock:BufferIO - Amazon Relational Database Service AWSDocumentationAmazon RDS and Aurora DocumentationUser Guide Relevant engine versionsContextCausesActions LWLock:BufferIO 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. Waiting for WAL to be flushed in WAL sender process. There are also several other views, listed in Table28.2, available to show the accumulated statistics. purpose is for the same page to be read into the shared buffer. For example, to show the PIDs and current queries of all backends: Table28.35. See, One row for each index in the current database, showing statistics about accesses to that specific index. The WALWriteLock wait occurs while PostgreSQL flushes WAL records to disk or during a WAL segment switch.. How to reduce this wait . See, One row only, showing statistics about the background writer process's activity. Waiting in main loop of WAL receiver process. Table28.12.pg_stat_database_conflicts View. Waiting to get the start location of a scan on a table for synchronized scans. Waiting for recovery conflict resolution for a vacuum cleanup. 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. After v9.5, the buffer cache is protected by 128 hash tables each protected by a LWLock. Waiting in main loop of autovacuum launcher process. In all other states, it shows the last query that was executed. Waiting for I/O on commit timestamp buffer. Sometimes it may be more convenient to obtain just a subset of this information. In such cases, an older set of per-backend statistics access functions can be used; these are shown in Table28.20. I am not the DBA, but receive reports occasionally when an application is causing load on the system. PostgreSQL utilizes lightweight locks (LWLocks) to synchronize and control access to the buffer content. Synchronous state of this standby server. PDF Scalability And Performance Improvements In PostgreSQL 9 - PGCon Waiting for other process to be attached in shared message queue. (See Chapter20 for details about setting configuration parameters.). Waiting for I/O on a transaction status SLRU buffer. Waiting in main loop of archiver process. Waiting for a replication slot to become inactive so it can be dropped. The LWLock:BufferIO wait event precedes the IO:DataFileRead wait event. pg_blocking_pids function. PostgreSQL accesses certain on-disk information via SLRU (simple least-recently-used) caches. 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. 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. Alternatively, one can build custom views using the underlying cumulative statistics functions, as discussed in Section28.2.24. Every PostgreSQL process collects statistics locally, then updates the shared data at appropriate intervals. disabled: This state is reported if track_activities is disabled in this backend. Waiting for a read during a file copy operation. Only directly connected standbys are listed; no information is available about downstream standby servers. Waiting in WAL receiver to receive data from remote server. For details such as the functions' names, consult the definitions of the standard views. 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 a read of a serialized historical catalog snapshot. Statistics Collection Configuration, One row per server process, showing information related to the current activity of that process, such as state and current query. Increase the number of wal_buffers available to the database. Waiting for a read during reorder buffer management. The pg_statio_user_tables and pg_statio_sys_tables views contain the same information, but filtered to only show user and system tables respectively. your workload peak time if you see LWLock:BufferIO coinciding with See, At least one row per subscription, showing information about the subscription workers. Number of times in-progress transactions were streamed to the decoding output plugin while decoding changes from WAL for this slot. Waiting for data to reach durable storage while adding a line to the data directory lock file. Waiting to associate a data block with a buffer in the buffer pool. 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 write while creating the data directory lock file. LWLock:BufferIO. Logical decoding plugins may optionally emit tracking messages; if they do not, the tracking mechanism will simply display NULL lag. Waiting to acquire an exclusive pin on a buffer. 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. The parameter track_io_timing enables monitoring of block read and write times. Alternatively, one can build custom views using the underlying statistics functions, as discussed in Section28.2.3. 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. Pointers to free buffers and to the next victim are protected by one buffer strategy lock spinlock. Waiting for a read during recheck of the data directory lock file. A database-wide ANALYZE is recommended after the statistics have been reset. Returns the OID of the database this backend is connected to. Waiting for background worker to start up. Table28.6. It is used per the rules above. 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. (See Chapter19 for details about setting configuration parameters.). block. Text of this backend's most recent query. 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. If enabled, calls to user-defined functions and the total time spent in each one are counted as well. Per-Backend Statistics Functions, Copyright 1996-2023 The PostgreSQL Global Development Group. See, One row per connection (regular and replication), showing information about GSSAPI authentication and encryption used on this connection. shared_buffers parameter. See, One row only, showing statistics about the background writer process's activity. We're sorry we let you down. Waiting for SSL while attempting connection. Waiting for stats dynamic shared memory allocator access, Waiting for stats shared memory hash table access, Waiting for shared memory stats data access. This should not be used for data integrity checks. 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. Waiting to read or update the last value set for a transaction commit timestamp. events. Waiting for a write while creating the data directory lock file. 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 server process is waiting for an I/O operation to complete. Waiting to create, drop or use a replication origin. 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. Waiting for an asynchronous prefetch from a relation data file. If you've got a moment, please tell us how we can make the documentation better. Waiting for a write when creating a new WAL segment by copying an existing one. 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. 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. The parameter track_io_timing enables monitoring of block read and write times. The parameter track_io_timing enables monitoring of block read and write times. 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 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. Waiting to associate a data block with a buffer in the buffer pool. This and other streaming counters for this slot can be used to tune logical_decoding_work_mem. These numbers do not act as stated above; instead they update continuously throughout the transaction. 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. Waiting for a read during recheck of the data directory lock file. Re: Improve WALRead() to suck data directly from WAL buffers when possible The IO:DataFileRead wait event occurs while data is Time elapsed between flushing recent WAL locally and receiving notification that this standby server has written, flushed and applied it. Waiting to read or update replication slot state. Waiting to add a message in shared invalidation queue. My application is using Postgres as DBMS, the version of Postgres that i'm using is 10.3 with the extension Postgis installed. See. 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 combination of certificate serial number and certificate issuer uniquely identifies a certificate (unless the issuer erroneously reuses serial numbers). 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. Waiting for parallel workers to finish computing. 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 write of logical rewrite mappings. Waiting to acquire an advisory user lock. The counter gets incremented for both top-level transactions and subtransactions. Waiting for a logical replication remote server to send data for initial table synchronization. 28.2.3. replication_origin: Waiting to read or update the replication progress. Per-Backend Statistics Functions, pg_stat_get_backend_idset () setof integer. sync: This standby server is synchronous. WALWriteLock | DBmarlin Docs and Knowledge Base Returns the text of this backend's most recent query. The reported lag times are not predictions of how long it will take for the standby to catch up with the sending server assuming the current rate of replay. The pg_stat_user_tables and pg_stat_sys_tables views contain the same information, but filtered to only show user and system tables respectively. See, Time when the current transaction was started. See, One row for each table in the current database, showing statistics about I/O on that specific table. idle in transaction: The backend is in a transaction, but is not currently executing a query. For client backends, this is the time the client connected to the server. True if GSSAPI authentication was used for this connection. Returns the OID of the user logged into this backend. This has no effect in a quorum-based synchronous replication. Waiting in main loop of logical replication apply process. This counter is incremented each time a transaction is spilled, and the same transaction may be spilled multiple times. 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. pg_stat_reset_subscription_stats ( oid ) void. Waiting to access the shared per-process data structures (typically, to get a snapshot or report a session's transaction ID). Waiting to read or update the state of prepared transactions. Waiting to access the list of predicate locks held by serializable transactions. Each buffer header also contains an LWLock, the "buffer content lock", that *does* represent the right to access the data: in the buffer. idle: The backend is waiting for a new client command. The following wait events are a subset of the list in Amazon Aurora PostgreSQL wait events. Waiting to read or write relation cache initialization file. pg_stat_get_activity ( integer ) setof record. Waiting for an elected Parallel Hash participant to allocate the initial hash table. async: This standby server is asynchronous. Waiting for other Parallel Hash participants to finish partitioning the outer relation. Waiting for a read of a two phase state file. Heavyweight locks, also known as lock manager locks or simply locks, primarily protect SQL-visible objects such as tables. 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. The pg_stat_archiver view will always have a single row, containing data about the archiver process of the cluster. The parameter track_wal_io_timing enables monitoring of WAL write times. Waiting to read or update the state of logical replication workers. See, One row per database, showing database-wide statistics. Waiting for a replication origin to become inactive to be dropped. Its purpose is for the same page to be read into the shared buffer. But if you want to see new results with each query, be sure to do the queries outside any transaction block. Waiting for another process to be attached to a shared message queue. Since collection of statistics adds some overhead to query execution, the system can be configured to collect or not collect information. pg_stat_get_backend_activity_start ( integer ) timestamp with time zone. Use partitioned tables (which also have partitioned indexes). catchup: This WAL sender's connected standby is catching up with the primary. If you've got a moment, please tell us what we did right so we can do more of it. Waiting for logical rewrite mappings to reach durable storage. Waiting for a newly created timeline history file to reach durable storage. Waiting to elect a Parallel Hash participant to allocate more buckets. Number of transactions spilled to disk once the memory used by logical decoding to decode changes from WAL has exceeded logical_decoding_work_mem. The per-table and per-index functions take a table or index OID. - 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 See. 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. Waiting in main loop of WAL receiver process. Waiting for a read of a logical mapping during reorder buffer management. 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. to keep index reordering low and reduces its impact. 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. There have been several occasions when a query is being executed dozens of times simultaneously by one or many users. Waiting to replace a page in WAL buffers. Table28.17.pg_statio_all_sequences View. Waiting to acquire a speculative insertion lock. fastpath function call: The backend is executing a fast-path function. 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. 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. Similarly, information about the current queries of all sessions is collected when any such information is first requested within a transaction, and the same information will be displayed throughout the transaction. Copyright 1996-2023 The PostgreSQL Global Development Group, PostgreSQL 15.2, 14.7, 13.10, 12.14, and 11.19 Released, 28.2.1. 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 a two phase state file to reach durable storage. See, One row for each backend (including autovacuum worker processes) running, One row only, showing statistics about the WAL archiver process's activity. Each such lock protects a particular data structure in shared memory. If you see anything in the documentation that is not correct, does not match 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. Topics Relevant engine versions Context Causes Actions Relevant engine versions Waiting for a write during reorder buffer management. Best practices for Amazon RDS for PostgreSQL cross-Region read replicas 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. 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. This function is restricted to superusers by default, but other users can be granted EXECUTE to run the function. idle in transaction (aborted): This state is similar to idle in transaction, except one of the statements in the transaction caused an error. Number of data page checksum failures detected in this database (or on a shared object), or NULL if data checksums are not enabled. Waiting to read or update sub-transaction information. Its Waiting for a two phase state file to reach durable storage. Waiting for I/O on a commit timestamp SLRU buffer. The parameter track_activities enables monitoring of the current command being executed by any server process. Waiting for a read of a timeline history file. Waiting to read or update background worker state. Table28.17.pg_stat_recovery_prefetch View, Number of blocks prefetched because they were not in the buffer pool, Number of blocks not prefetched because they were already in the buffer pool, Number of blocks not prefetched because they would be zero-initialized, Number of blocks not prefetched because they didn't exist yet, Number of blocks not prefetched because a full page image was included in the WAL, Number of blocks not prefetched because they were already recently prefetched, How many bytes ahead the prefetcher is looking, How many blocks ahead the prefetcher is looking, How many prefetches have been initiated but are not yet known to have completed, Process ID of the subscription worker process, OID of the relation that the worker is synchronizing; null for the main apply worker, Last write-ahead log location received, the initial value of this field being 0. Waiting to manage space allocation in shared memory. In all other states, it shows the identifier of last query that was executed. This field will only be non-null for IP connections, and only when log_hostname is 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. potential: This standby server is now asynchronous, but can potentially become synchronous if one of current synchronous ones fails. Lag times work automatically for physical replication. Waiting to access a parallel query's information about type modifiers that identify anonymous record types. Waiting to read while creating the data directory lock file. 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. Waiting to access the list of finished serializable transactions. Waiting to read or update dynamic shared memory allocation information. The server process is waiting for exclusive access to a data buffer. Waiting for parallel bitmap scan to become initialized. Connection string used by this WAL receiver, with security-sensitive fields obfuscated. 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 the group leader to update transaction status at end of a parallel operation. Waiting to read or update notification messages. Waiting for a write of a two phase state file. If the current query is the first of its transaction, this column is equal to the, Time when the currently active query was started, or if. Waiting to read or update multixact offset mappings. IP address of the client connected to this backend. Priority of this standby server for being chosen as the synchronous standby in a priority-based synchronous replication. LWLock- buffer_mapping : r/PostgreSQL - reddit.com 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. The fields returned are a subset of those in the pg_stat_activity view. Most such locks protect a particular data structure in shared memory. 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. streaming: This WAL sender is streaming changes after its connected standby server has caught up with the primary. Waiting for parallel workers to finish computing. ), 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). Waits for lightweight locks ( LWLock ). Waiting for a write of mapping data during a logical rewrite. pg_stat_get_backend_pid ( integer ) integer, pg_stat_get_backend_start ( integer ) timestamp with time zone. The parameter track_counts controls whether cumulative statistics are collected about table and index accesses.