Waiting to acquire an advisory user lock. 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. Choose the appropriate target Region. replication_slot_io: Waiting for I/O on a replication slot. Waiting in WAL receiver to receive data from remote server. See, One row per subscription, showing statistics about errors. PostgreSQL: Documentation: 11: 28.2. The Statistics Collector Waiting for a logical replication remote server to change state. The server process is waiting for a timeout to expire. postgres/README at master postgres/postgres GitHub Waiting to acquire a lock on a page of a relation. 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. 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 for a serialized historical catalog snapshot to reach durable storage. Waiting in main loop of syslogger process. 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 background worker state. The pg_stat_wal_receiver view will contain only one row, showing statistics about the WAL receiver from that receiver's connected server. The argument can be one of CommitTs, MultiXactMember, MultiXactOffset, Notify, Serial, Subtrans, or Xact to reset the counters for only that entry. pg_blocking_pids function. 106 . Logical decoding plugins may optionally emit tracking messages; if they do not, the tracking mechanism will simply display NULL lag. The IO:DataFileRead wait event occurs while data is The statistics collector transmits the collected information to other PostgreSQL processes through temporary files. Waiting for the page number needed to continue a parallel B-tree scan to become available. Returns the process ID of the server process attached to the current session. sync: This standby server is synchronous. quorum: This standby server is considered as a candidate for quorum standbys. async: This standby server is asynchronous. IP address of the client connected to this WAL sender. Additional Statistics Functions. PostgreSQL's cumulative statistics system supports collection and reporting of information about server activity. For tranches registered by extensions, the name is specified by extension and this will be displayed as wait_event. Sometimes it may be more convenient to obtain just a subset of this information. Waiting for a serialized historical catalog snapshot to reach durable storage. PostgreSQL utilizes lightweight locks (LWLocks) to synchronize and control access to the buffer content. Since collection of statistics adds some overhead to query execution, the system can be configured to collect or not collect information. Number of disk blocks read from this index. A transaction can also see its own statistics (not yet flushed out to the shared memory statistics) in the views pg_stat_xact_all_tables, pg_stat_xact_sys_tables, pg_stat_xact_user_tables, and pg_stat_xact_user_functions. 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 read of a timeline history file. 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 for each index in the current database, showing statistics about I/O on that specific index. It works like this: Resets all statistics counters for the current database to zero. Waiting to allocate a new transaction ID. PostgreSQL is one of the most popular open-source databases in the world and has successful implementations across several mission-critical environments across various domains, using real-time high-end OLTP applications performing millions and billions of transactions per day. Each shared buffer has an I/O lock that is associated with the LWLock:BufferIO wait event, each time a block (or Streaming only works with top-level transactions (subtransactions can't be streamed independently), so the counter is not incremented for subtransactions. By default the query text is truncated at 1024 bytes; this value can be changed via the parameter track_activity_query_size. Waiting for I/O on a serializable transaction conflict SLRU buffer. Returns a record of information about the backend with the specified process ID, or one record for each active backend in the system if NULL is specified. 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. Waiting for a write while adding a line to the data directory lock file. disabled: This state is reported if track_activities is disabled in this backend. Waiting for a timeline history file received via streaming replication to reach durable storage. Waiting to insert WAL data into a memory buffer. It also tracks the total number of rows in each table, and information about . 39919 LWLock buffer_mapping 5119 Client ClientRead 3116 IO DataFileRead With C-Hash Event Count Event Type Event Name For client backends, this is the time the client connected to the server. This lock is used to handle multiple sessions that all require access to the same Waiting for background worker to shut down. Amount of transaction data decoded for streaming in-progress transactions to the decoding output plugin while decoding changes from WAL for this slot. Waiting for a newly initialized WAL file to reach durable storage. 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 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 synchronize workers during Parallel Hash Join plan execution. Number of decoded transactions sent to the decoding output plugin for this slot. Name of this database, or NULL for shared objects. Waiting in main loop of logical launcher process. Waiting to read or update information about serializable transactions. Principal used to authenticate this connection, or NULL if GSSAPI was not used to authenticate this connection. catchup: This WAL sender's connected standby is catching up with the primary. Resets statistics for a single subscription shown in the pg_stat_subscription_stats view to zero. Waiting to read or record conflicting serializable transactions. Sometimes it may be more convenient to obtain just a subset of this information. Only directly connected standbys are listed; no information is available about downstream standby servers. Most such locks protect a particular data structure in shared memory. IP address of the client connected to this backend. 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. 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 for an elected Parallel Hash participant to decide on future batch growth. 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. Waiting to write a protocol message to a shared message queue. Alternatively, one can build custom views using the underlying cumulative statistics functions, as discussed in Section28.2.24. See, Only one row, showing statistics about blocks prefetched during recovery. If you've got a moment, please tell us how we can make the documentation better. Then identify which query David Christensen on Twitter. Waiting for any activity when processing replies from WAL receiver in WAL sender process. wait_event will contain a name identifying the purpose of the lightweight lock. Possible values are: catchup: This WAL sender's connected standby is catching up with the primary. The functions for per-function statistics take a function OID. potential: This standby server is now asynchronous, but can potentially become synchronous if one of current synchronous ones fails. Waiting in main loop of logical apply process. Waiting for confirmation from remote server during synchronous replication. Waiting in main loop of background writer process. Best practices for Amazon RDS for PostgreSQL cross-Region read replicas to keep index reordering low and reduces its impact. Waiting to access the multixact member SLRU cache. Time elapsed between flushing recent WAL locally and receiving notification that this standby server has written, flushed and applied it. For better performance, stats_temp_directory can be pointed at a RAM-based file system, decreasing physical I/O requirements. Returns the time when the backend's current transaction was started. But processes can also await other events: Waits for input/output ( IO) occur when a process needs to read or write data. The fields returned are a subset of those in the pg_stat_activity view. So the statistics will show static information as long as you continue the current transaction. Also, the collector itself emits a new report at most once per PGSTAT_STAT_INTERVAL milliseconds (500 ms unless altered while building the server). See, One row for each table in the current database, showing statistics about I/O on that specific table. Waiting to read or update the state of prepared transactions. Waiting to replace a page in WAL buffers. If you see anything in the documentation that is not correct, does not match See, One row only, showing statistics about the background writer process's activity. Waiting to add or examine predicate lock information. The parameter track_io_timing enables monitoring of block read and write times. purpose is for the same page to be read into the shared buffer. LWLock:BufferIO - Amazon Relational Database Service AWSDocumentationAmazon RDS and Aurora DocumentationUser Guide Relevant engine versionsContextCausesActions LWLock:BufferIO 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. Alternatively, one can build custom views using the underlying statistics functions, as discussed in Section28.2.3. Heavyweight locks, also known as lock manager locks or simply locks, primarily protect SQL-visible objects such as tables. Waiting for a write of a WAL page during bootstrapping. The total number of rows in each table, and information about vacuum and analyze actions for each table are also counted. Waiting to read or update information about the state of synchronous replication. In such cases, an older set of per-backend statistics access functions can be used; these are shown in Table28.20. Waiting for logical rewrite mappings to reach durable storage during a checkpoint. Lock: The backend is waiting for a heavyweight lock. Waiting for a read during a file copy operation. Waiting for a read from a timeline history file during walsender timeline command. (See Chapter20 for details about setting configuration parameters.). It also tracks the total number of rows in each table, and information about vacuum and analyze actions for each table. Waiting for a read from the control file. A process can wait for the data needed from a client ( Client) or another process ( IPC ). events. 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). Waiting for activity from child process when executing. See, One row for each table in the current database, showing statistics about accesses to that specific table. Autovacuum worker or launcher waiting to update or read the current state of autovacuum workers. Synchronous state of this standby server. This can be used to gauge the delay that synchronous_commit level remote_apply incurred while committing if this server was configured as a synchronous standby. Waiting in main loop of syslogger process. Restrict the maximum number of connections to the database as a best practice. See. Waiting to elect a Parallel Hash participant to allocate a hash table. This is consistent with the goal of measuring synchronous commit and transaction visibility delays for recent write transactions. Waiting for a read of the relation map file. Waiting to read or write a data page in memory. See. Monitoring systems should choose whether to represent this as missing data, zero or continue to display the last known value. Waiting in main loop of WAL receiver process. This event type indicates a process waiting for activity in its main processing loop. Number of in-progress transactions streamed to the decoding output plugin after the memory used by logical decoding to decode changes from WAL for this slot has exceeded logical_decoding_work_mem. These numbers do not act as stated above; instead they update continuously throughout the transaction. BK_1935: "IObuffer_locks,ControlLock()"IOControlLockControlLockIOSlruSharedData 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 read or update the last value set for a transaction commit timestamp. [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 ! 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. idle in transaction (aborted): This state is similar to idle in transaction, except one of the statements in the transaction caused an error.

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