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. See, One row per database, showing database-wide statistics. Before PostgreSQL 8.1, all operations of the shared buffer manager itself were protected by a single system-wide lock, the BufMgrLock, which unsurprisingly proved to be a source of contention. Waiting to access the list of finished serializable transactions. Using pg_stat_reset() also resets counters that autovacuum uses to determine when to trigger a vacuum or an analyze. Waiting to acquire an exclusive pin on a buffer. Waiting to read or update vacuum-related information for a B-tree index. There is no order to the granting of LWLocks and in a high concurrency system this can cause contention. Pointers to free buffers and to the next victim are protected by one buffer strategy lock spinlock. When using the cumulative statistics views and functions to monitor collected data, it is important to realize that the information does not update instantaneously. Waiting to acquire a lock on a non-relation database object. Time at which the last data page checksum failure was detected in this database (or on a shared object), or NULL if data checksums are not enabled. Table28.12.pg_stat_database_conflicts View. 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. Waiting to apply WAL at recovery because it is delayed. If the argument is NULL, all counters shown in the pg_stat_slru view for all SLRU caches are reset. Waiting for a read of a logical mapping during reorder buffer management. Per-Backend Statistics Functions, pg_stat_get_backend_idset () setof integer. Waiting to access the list of predicate locks held by the current serializable transaction during a parallel query. 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. As soon as the page is read inside the shared buffer pool, the LWLock:BufferIO lock is released. 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. Tune max_wal_size and checkpoint_timeout based on Buffer pin waits can be protracted if another process holds an open cursor that last read data from the buffer in question. 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. Only directly connected standbys are listed; no information is available about downstream standby servers. Ordinary users can only see all the information about their own sessions (sessions belonging to a role that they are a member of). See, One row for each backend (including autovacuum worker processes) running, One row only, showing statistics about the WAL archiver process's activity. To minimize skew, stats_fetch_consistency can be set to snapshot, at the price of increased memory usage for caching not-needed statistics data. See, One row only, showing statistics about the background writer process's activity. Use partitioned tables (which also have partitioned indexes). This facility is independent of the collector process. Waiting for I/O on a commit timestamp SLRU buffer. Total amount of data written to temporary files by queries in this database. Waiting in background writer process, hibernating. 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). 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. This block has to be read from outside the shared buffer pool, defined by the The IO:DataFileRead wait event occurs while data is Waiting for a write of a serialized historical catalog snapshot. Waiting to read or update notification messages. Waiting for a buffered file to be truncated. LWTRANCHE_BUFFER_CONTENT @ LWTRANCHE_BUFFER_CONTENT. Waiting for a relation data file to be truncated. Wait Events of Type BufferPin, Table28.8. 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. Number of transactions spilled to disk once the memory used by logical decoding to decode changes from WAL has exceeded logical_decoding_work_mem. pg_stat_get_backend_activity_start ( integer ) timestamp with time zone. Extension: The server process is waiting for activity in an extension module. See, One row for each table in the current database, showing statistics about I/O on that specific table. My application is using Postgres as DBMS, the version of Postgres that i'm using is 10.3 with the extension Postgis installed. Waiting to read or update the state of prepared transactions. Returns the time when the backend's current transaction was started. idle in transaction (aborted): This state is similar to idle in transaction, except one of the statements in the transaction caused an error. proc: Waiting to read or update the fast-path lock information. Waiting for I/O on an async (notify) buffer. sync: This standby server is synchronous. Avoid PostgreSQL LWLock:buffer_content locks in Amazon Aurora: Tips and best practices. Waiting to read or update the fast-path lock information. Alternatively, one can build custom views using the underlying cumulative statistics functions, as discussed in Section28.2.24. pg_stat_get_backend_pid ( integer ) integer, pg_stat_get_backend_start ( integer ) timestamp with time zone. 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. Indexes can be used by simple index scans, bitmap index scans, and the optimizer. Waiting for a write of logical rewrite mappings. The pg_stat_ssl view will contain one row per backend or WAL sender process, showing statistics about SSL usage on this connection. See Section30.5 for more information about the internal WAL function XLogWrite. Current WAL sender state. Waiting while sending synchronization requests to the checkpointer, because the request queue is full. Waiting during base backup when throttling activity. Topics Relevant engine versions Context Causes Actions Relevant engine versions Waiting to allocate or free a replication slot. 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. The functions for per-function statistics take a function OID. This and other streaming counters for this slot can be used to tune logical_decoding_work_mem. This counter is incremented each time a transaction is spilled, and the same transaction may be spilled multiple times. This effect can mean that you have a small shared buffers setting. Waiting for a write of a two phase state file. Waiting in main loop of WAL writer process. 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. Discards the current statistics snapshot or cached information. A backend process wants to read a page into shared memory. pg_stat_reset_replication_slot ( text ) void. Waiting to associate a data block with a buffer in the buffer pool. Waiting to ensure that the table it has selected for a vacuum still needs vacuuming. Waiting for a timeline history file received via streaming replication 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 to update the relation map file used to store catalog to filenode mapping. Its purpose is for the same page to be read into the shared buffer. Here is an example of how wait events can be viewed. Waiting to setup, drop or use replication origin. There are also several other views, listed in Table28.2, available to show the results of statistics collection. See, One row for each index in the current database, showing statistics about I/O on that specific index. 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 for other process to be attached in shared message queue. Possible values are: catchup: This WAL sender's connected standby is catching up with the primary. Presently, accesses to tables and indexes in both disk-block and individual-row terms are counted. See, Only one row, showing statistics about the WAL receiver from that receiver's connected server. Indexes can be used by simple index scans, bitmap index scans, and the optimizer. When the buffer manager receives a request, PostgreSQL uses the buffer_tag of the desired page. TCP port number that the client is using for communication with this backend, or -1 if a Unix socket is used. The track_functions parameter controls exactly which functions are tracked. 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. (For example, in psql you could issue \d+ pg_stat_activity.) 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 a write while creating the data directory lock file. What we have discussed in this episode of 5mins of Postgres. Waiting for any activity when processing replies from WAL receiver in WAL sender process. The pg_stat_slru view will contain one row for each tracked SLRU cache, showing statistics about access to cached pages. Waiting for I/O on a multixact offset buffer. Waiting to read or write a data page in memory. Resets statistics for a single table or index in the current database or shared across all databases in the cluster to zero. Waiting in WAL receiver to receive data from remote server. The pg_stat_replication_slots view will contain one row per logical replication slot, showing statistics about its usage. The parameter track_wal_io_timing enables monitoring of WAL write times. Waiting to read or update shared multixact state. The functions for per-function statistics take a function OID. This event type indicates a process waiting for activity in its main processing loop. If this field is null, it indicates that the client is connected via a Unix socket on the server machine. wait_event will identify the specific wait point. Waiting to access the transaction status SLRU cache. PostgreSQL's cumulative statistics system supports collection and reporting of information about server activity. Thanks for letting us know we're doing a good job! Wait Events of Type Extension. Number of temporary files created by queries in this database. Returns the process ID of the server process attached to the current session. pg_stat_get_backend_wait_event_type ( integer ) text. Waiting in main loop of autovacuum launcher process. Lag times work automatically for physical replication. Waiting to read or update a process' fast-path lock information. Waiting in main loop of logical apply process. Waiting for I/O on a clog (transaction status) buffer. Waiting for WAL from a stream at recovery. Table28.6. The pg_stat_ssl view will contain one row per backend or WAL sender process, showing statistics about SSL usage on this connection. Waiting to get a snapshot or clearing a transaction id at transaction end. See, One row per subscription, showing statistics about errors. Process ID of the parallel group leader, if this process is a parallel query worker. See, One row for each index in the current database, showing statistics about I/O on that specific index. See, Only one row, showing statistics about the WAL receiver from that receiver's connected server. block. Waiting for a relation data file to be extended. 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. operations, Large or bloated indexes that require the engine to read more pages than necessary into the shared buffer pool, Lack of indexes that forces the DB engine to read more pages from the tables than necessary, Checkpoints occurring too frequently or needing to flush too many modified pages, Sudden spikes for database connections trying to perform operations on the same page. The overhead of a file is much more than wasting the remainder of a page. When recovery is performed at server start (e.g., after immediate shutdown, server crash, and point-in-time recovery), all statistics counters are reset. 214 . 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 for an elected Parallel Hash participant to allocate a hash table. The pg_stat_bgwriter view will always have a single row, containing global data for the cluster. True if GSSAPI authentication was used for this connection. Waiting for logical rewrite mappings to reach durable storage. IPC: The server process is waiting for some activity from another process in the server. For client backends, this is the time the client connected to the server. idle in transaction: The backend is in a transaction, but is not currently executing a query. A database-wide ANALYZE is recommended after the statistics have been reset. The parameter track_counts controls whether cumulative statistics are collected about table and index accesses. Waiting for WAL buffers to be written to disk. Each buffer header also contains an LWLock, the "buffer content lock", that *does* represent the right to access the data: in the buffer. - 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 Doing this helps wait_event will identify the type of lock awaited. 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. 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. 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. catchup: This WAL sender's connected standby is catching up with the primary. Waiting for a relation data file to be extended. It also tracks the total number of rows in each table, and information about vacuum and analyze actions for each table. Streaming only works with top-level transactions (subtransactions can't be streamed independently), so the counter is not incremented for subtransactions. Simple test for lock_waits log messages. The LWLock:BufferIO wait event precedes the IO:DataFileRead wait event. The pg_stat_wal_receiver view will contain only one row, showing statistics about the WAL receiver from that receiver's connected server. Waiting for background worker to shut down. When the server 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. Table28.26.pg_stat_database_conflicts View, Number of queries in this database that have been canceled due to dropped tablespaces, Number of queries in this database that have been canceled due to lock timeouts, Number of queries in this database that have been canceled due to old snapshots, Number of queries in this database that have been canceled due to pinned buffers, Number of queries in this database that have been canceled due to deadlocks. PostgreSQL Entangled in Locks: Attempts to free it - Amit Kapila - Dilip Kumar PGCon 2017 . Waiting in main loop of startup process for WAL to arrive, during streaming recovery. BufferCacheHitRatio metric dips. pg_stat_get_backend_userid ( integer ) oid. Waiting for a read of a two phase state file. Waiting for a read from a timeline history file during a walsender timeline command. 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 a logical replication remote server to send data for initial table synchronization. You can split your The pg_stat_archiver view will always have a single row, containing data about the archiver process of the cluster. See Table28.5 through Table28.13. Statistics Collection Configuration, One row per server process, showing information related to the current activity of that process, such as state and current query. Waiting to read or update background worker state. Waiting for a write to a replication slot control file. The type of event for which the backend is waiting, if any; otherwise NULL. being read from storage. Also, the collector itself emits a new report at most once per PGSTAT_STAT_INTERVAL milliseconds (500 ms unless altered while building the server). Then identify which query Returns the TCP port number that the client is using for communication. Waiting to acquire a lock on a page of a relation. The next use of statistical information will cause a new snapshot to be fetched. Waiting in main loop of WAL sender process. Waiting for a write during reorder buffer management. 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, Process ID of a backend or WAL sender process, Version of SSL in use, or NULL if SSL is not in use on this connection, Name of SSL cipher in use, or NULL if SSL is not in use on this connection, Number of bits in the encryption algorithm used, or NULL if SSL is not used on this connection, True if SSL compression is in use, false if not, or NULL if SSL is not in use on this connection, Distinguished Name (DN) field from the client certificate used, or NULL if no client certificate was supplied or if SSL is not in use on this connection. Waiting for a read when creating a new WAL segment by copying an existing one. IP address of the client connected to this WAL sender. 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 the relation map file. 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. 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. This counts top-level transactions only, and is not incremented for subtransactions. 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 for a two phase state file to reach durable storage. Waiting for a write of a timeline history file received via streaming replication. 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 read from a timeline history file during walsender timeline command. If you've got a moment, please tell us what we did right so we can do more of it. The wait_event and state columns are independent. Several predefined views, listed in Table28.1, are available to show the current state of the system. 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. Waiting in main loop of WAL receiver process. Number of disk blocks read from this index. Each shared buffer has an I/O lock that is associated with the LWLock:BufferIO wait event, each time a block (or 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 read or update background worker state. Host name of the connected client, as reported by a reverse DNS lookup of client_addr. Waiting to add a message in shared invalidation queue. If a backend is in the active state, it may or may not be waiting on some event. Waiting for I/O on a sub-transaction SLRU buffer. Text of this backend's most recent query. The pg_stat_wal view will always have a single row, containing data about WAL activity of the cluster. The server process is waiting for a heavyweight lock. The access functions for per-database statistics take a database OID as an argument to identify which database to report on. edge fitness cancellation,