(To prevent ordinary users from hiding their activity from the administrator, only superusers are allowed to change these parameters with SET.). Waiting for WAL to reach durable storage during bootstrapping. Waiting for a read when creating a new WAL segment by copying an existing one. Waiting for a timeline history file received via streaming replication to reach durable storage. Calling, Reset statistics for a single table or index in the current database to zero (requires superuser privileges by default, but EXECUTE for this function can be granted to others), Reset statistics for a single function in the current database to zero (requires superuser privileges by default, but EXECUTE for this function can be granted to others), Set of currently active backend ID numbers (from 1 to the number of active backends), Time when the most recent query was started, IP address of the client connected to this backend, TCP port number that the client is using for communication, Wait event type name if backend is currently waiting, otherwise NULL. Did this page help you? 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. Waiting for a read during a file copy operation. Waiting to allocate or assign a transaction id. Waiting for logical replication remote server to change state. Waiting for a write to the relation map file. The next use of statistical information will cause a new snapshot to be fetched. PostgreSQL utilizes lightweight locks (LWLocks) to synchronize and control access to the buffer content. Waiting for a write of mapping data during a logical rewrite. See, One row only, showing statistics about WAL activity. For client backends, this is the time the client connected to the server. Waiting to read or update dynamic shared memory state. Waiting for a replication slot to become inactive to be dropped. See, Only one row, showing statistics about blocks prefetched during recovery. 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. 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. Waiting for action on logical replication worker to finish. Additional functions related to the cumulative statistics system are listed in Table28.34. By default the query text is truncated at 1024 bytes; this value can be changed via the parameter track_activity_query_size. Waiting for a buffered file to be truncated. Waiting for an update to the control file to reach durable storage. 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. Waiting for parallel query dynamic shared memory allocation. Waiting to acquire a speculative insertion lock. Waiting to add or examine locks for backends, or waiting to join or exit a locking group (used by parallel query). See, Only one row, showing statistics about the WAL receiver from that receiver's connected server. Its Streaming only works with top-level transactions (subtransactions can't be streamed independently), so the counter is not incremented for subtransactions. Re: Improve WALRead() to suck data directly from WAL buffers when So the statistics will show static information as long as you continue the current transaction. Occasionally i noticed that in random interval of times the dbms become slow and get stuck on a few SELECT queries. 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. quorum: This standby server is considered as a candidate for quorum standbys. The pg_stat_ssl view will contain one row per backend or WAL sender process, showing statistics about SSL usage on this connection. This standby's xmin horizon reported by hot_standby_feedback. PostgreSQL utilizes lightweight locks (LWLocks) to synchronize and control access to the buffer content. Waiting for other Parallel Hash participants to finish loading a hash table. Waiting for other Parallel Hash participants to finish hashing the inner relation. Choose the appropriate target Region. Postgres Source Code Docs: Locking Overview. It can also count calls to user-defined functions and the total time spent in each one. Waiting for I/O on a transaction status SLRU buffer. Waiting while sending synchronization requests to the checkpointer, because the request queue is full. If a backend is in the active state, it may or may not be waiting on some event. Waiting for background worker to shut down. For example, to show the PIDs and current queries of all backends: Table28.35. 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. Waiting for a relation data file to reach durable storage. 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. Waiting for a logical replication remote server to send data for initial table synchronization. If the current query is the first of its transaction, this column is equal to the query_start column. I'd like to know more about what these locks could imply if anything. The LWLock that this article will introduce is a lightweight lock (Lightweight Lock) based on SpinLock. Time at which these statistics were last reset. to report a documentation issue. IP address of the client connected to this backend. - 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 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. See Table28.4. Waiting for SLRU data to reach durable storage following a page write. Most such locks protect a particular data structure in shared memory. See, One row only, showing statistics about the background writer process's activity. Waiting for WAL buffers to be written to disk. 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. 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. Waiting to access the list of predicate locks held by the current serializable transaction during a parallel query. This facility is independent of the cumulative statistics system. Waiting for group leader to clear transaction id at transaction end. The function pg_stat_get_backend_idset provides a convenient way to generate one row for each active backend for invoking these functions. Possible values are: Wait event name if backend is currently waiting, otherwise NULL. Waiting for recovery conflict resolution for dropping a tablespace. See, One row for each table in the current database, showing statistics about accesses to that specific table. wait_event will identify the specific wait point. Waiting to synchronize workers during Parallel Hash Join plan execution. Waiting in WAL receiver to establish connection to remote server. Locks in PostgreSQL: 4. Locks in memory : Postgres Professional wait_event will identify the specific wait point. Several predefined views, listed in Table28.1, are available to show the current state of the system. Per-Backend Statistics Functions, pg_stat_get_backend_idset () setof integer. Simple test for lock_waits log messages. 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. 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. Resets some cluster-wide statistics counters to zero, depending on the argument. Waiting for a write during a file copy operation. pg_stat_reset_single_function_counters ( oid ) void. streaming: This WAL sender is streaming changes after its connected standby server has caught up with the primary. Waiting for recovery conflict resolution for a vacuum cleanup. Please refer to your browser's Help pages for instructions. Buffer pin waits can be protracted if another process holds an open cursor that last read data from the buffer in question. Waiting for I/O on a multixact_member buffer. See, One row per subscription, showing statistics about errors. 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. 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 write of a timeline history file received via streaming replication. However, current-query information collected by track_activities is always up-to-date. Waiting for a read of a serialized historical catalog snapshot. However, they are also used to ensure mutual exclusion for certain internal operations such as relation extension. Waiting to read or update the current state of autovacuum workers. Number of backends currently connected to this database. (Conflicts occur only on standby servers; see, Number of temporary files created by queries in this database. please use shared_buffers parameter. Pointers to free buffers and to the next victim are protected by one buffer strategy lock spinlock. OID of this database, or 0 for objects belonging to a shared relation. For client backends, this is the time the client connected to the server. Waiting for a WAL file to reach durable storage. Waiting for a barrier event to be processed by all backends. The parameter track_activities enables monitoring of the current command being executed by any server process. Waiting to read or update background worker state. Waiting for SLRU data to reach durable storage during a checkpoint or database shutdown. backup: This WAL sender is sending a backup. Waiting in main loop of checkpointer process. See, One row per connection (regular and replication), showing information about SSL used on this connection. idle: The backend is waiting for a new client command. Thus, the server expects something to happen that is independent of its internal processes. 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. 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. BufferPin: The server process is waiting to access to a data buffer during a period when no other process can be examining that buffer. 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. This is controlled by configuration parameters that are normally set in postgresql.conf. Waiting for another process to be attached to a shared message queue. Using pg_stat_reset() also resets counters that autovacuum uses to determine when to trigger a vacuum or an analyze. Waiting to elect a Parallel Hash participant to allocate more batches. The new locking scheme avoids grabbing system-wide exclusive locks in common code paths.