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Waiting for a read from a timeline history file during walsender timeline command. idle: The backend is waiting for a new client command. Waiting for a write of a WAL page during bootstrapping. Waiting to read or update the progress of one replication origin. Waiting for SSL while attempting connection. Waiting for a two phase state file to reach durable storage. Waiting for an elected Parallel Hash participant to allocate the initial hash table. Waiting while sending synchronization requests to the checkpointer, because the request queue is full. 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. So the statistics will show static information as long as you continue the current transaction. Waiting to read or truncate multixact information. Buffer pin waits can be protracted if another process holds an open cursor which last read data from the buffer in question. NULL if this process is a parallel group leader or does not participate in parallel query. shared_buffers parameter. Waiting for logical rewrite mappings to reach durable storage during a checkpoint. The pg_stat_activity view will have one row per server process, showing information related to the current activity of that process. replication_slot_io: Waiting for I/O on a replication slot. 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. See, One row only, showing statistics about the background writer process's activity. These files are stored in the directory named by the stats_temp_directory parameter, pg_stat_tmp by default. pg_blocking_pids function. See, One row for each index in the current database, showing statistics about accesses to that specific index. buffer_mapping: Waiting to associate a data block with a buffer in the buffer pool. Waiting for an elected Parallel Hash participant to allocate a hash table. Waiting to update limits on transaction id and multixact consumption. [prev in list] [next in list] [prev in thread] [next in thread] List: postgresql-general Subject: Re: [HACKERS] [PATCH] Refactoring of LWLock tranches From: Ildus Kurbangaliev <i.kurbangaliev postgrespro ! Prevent sudden database connection spikes by using a connection pool. Waiting in main loop of logical launcher process. Waiting for a write during reorder buffer management. The LWLock:BufferIO wait event precedes the IO:DataFileRead wait event. Waiting to replace a page in WAL buffers. Occasionally i noticed that in random interval of times the dbms become slow and get stuck on a few SELECT queries. Waiting in main loop of checkpointer process. The per-table and per-index functions take a table or index OID. Waiting for a relation data file to be extended. 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 access the list of finished serializable transactions. Each individual server process flushes out accumulated statistics to shared memory just before going idle, but not more frequently than once per PGSTAT_MIN_INTERVAL milliseconds (1 second unless altered while building the server); so a query or transaction still in progress does not affect the displayed totals and the displayed information lags behind actual activity. Waiting to retrieve messages from the shared catalog invalidation queue. Waiting for a write to a replication slot control file. Waiting to acquire a lock on a non-relation database object. 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 rows returned by queries in this database, Number of rows fetched by queries 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. 39919 LWLock buffer_mapping 5119 Client ClientRead 3116 IO DataFileRead . 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. The pg_stat_database_conflicts view will contain one row per database, showing database-wide statistics about query cancels occurring due to conflicts with recovery on standby servers. Waiting for a read during recheck of the data directory lock file. Statistics Functions. Waiting in main loop of WAL sender process. Waiting to send bytes to a shared message queue. Possible values are: active: The backend is executing a query. 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. 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. But access to that shared memory requires the protection of light-weight locks, which should last for only nanoseconds or microseconds while the memory access is actually occuring. Waiting for WAL to be flushed in WAL sender process. 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. 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. The parameter track_io_timing enables monitoring of block read and write times. This field is truncated if the DN field is longer than, Number of WAL files that have been successfully archived, Name of the last WAL file successfully archived, Time of the last successful archive operation, Number of failed attempts for archiving WAL files, Name of the WAL file of the last failed archival operation, Time of the last failed archival operation, Time at which these statistics were last 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. The function pg_stat_get_backend_idset provides a convenient way to generate one row for each active backend for invoking these functions. 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. OID of this database, or 0 for objects belonging to a shared relation. See, At least one row per subscription, showing information about the subscription workers. All temporary files are counted, regardless of why the temporary file was created, and regardless of the log_temp_files setting. PostgreSQL's statistics collector is a subsystem that supports collection and reporting of information about server activity. 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 to setup, drop or use replication origin. Note that only tables, indexes, and functions in the current database can be seen with these functions. 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. PostgreSQL's cumulative statistics system supports collection and reporting of information about server activity. Returns the wait event type name if this backend is currently waiting, otherwise NULL. The pg_statio_ views are primarily useful to determine the effectiveness of the buffer cache. disabled: This state is reported if track_activities is disabled in this backend. See, One row for each index in the current database, showing statistics about I/O on that specific index. Waiting to read or update transaction commit timestamps. Returns the OID of the database this backend is connected to. The WALWriteLock wait occurs while PostgreSQL flushes WAL records to disk or during a WAL segment switch.. How to reduce this wait . Waiting to read or update shared multixact state. 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. Detailed Description . The pg_stat_slru view will contain one row for each tracked SLRU cache, showing statistics about access to cached pages. Waiting to associate a data block with a buffer in the buffer pool. Waiting to retrieve or store information about serializable transactions. Waiting for other Parallel Hash participants to finish loading a hash table. Waiting for a relation data file to be extended. Number of sequential scans initiated on this table, Number of live rows fetched by sequential scans, Number of index scans initiated on this table, Number of live rows fetched by index scans, Number of rows updated (includes HOT updated rows), Number of rows HOT updated (i.e., with no separate index update required), Estimated number of rows modified since this table was last analyzed, Estimated number of rows inserted since this table was last vacuumed, Last time at which this table was manually vacuumed (not counting VACUUM FULL), Last time at which this table was vacuumed by the autovacuum daemon, Last time at which this table was manually analyzed, last_autoanalyze timestamp with time zone, Last time at which this table was analyzed by the autovacuum daemon, Number of times this table has been manually vacuumed (not counting VACUUM FULL), Number of times this table has been vacuumed by the autovacuum daemon, Number of times this table has been manually analyzed, Number of times this table has been analyzed by the autovacuum daemon. The server process is waiting for exclusive access to a data buffer. Most such locks protect a particular data structure in shared memory. IPC: The server process is waiting for some activity from another process in the server. Type of current backend. The counter gets incremented for both top-level transactions and subtransactions. Waiting to read while creating the data directory lock file. 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 allocate or free a replication slot. 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. 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 read while creating the data directory lock file. Waiting for a read of a logical mapping during reorder buffer management. Waiting for a write when creating a new WAL segment by copying an existing one. to report a documentation issue. Total amount of data written to temporary files by queries in this database. Waiting for a read of a timeline history file. Waiting for a write to the relation map file. Cumulative statistics are collected in shared memory. The pg_stat_archiver view will always have a single row, containing data about the archiver process of the cluster. For more information, see LWLock:buffer_mapping. Waiting for a relation data file to reach durable storage. IP address of the client connected to this WAL sender. For details such as the functions' names, consult the definitions of the standard views. wait_event will identify the type of lock awaited. 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. wait_event will contain a name identifying the purpose of the lightweight lock. The server process is waiting for some condition defined by an extension module. Timeout: The server process is waiting for a timeout to expire. Waiting for a write while initializing a new WAL file. For example, to show the PIDs and current queries of all backends: Table28.20. A process can wait for the data needed from a client ( Client) or another process ( IPC ). 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.

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