The last article introduced SpinLock in PostgreSQL. Port number of the PostgreSQL instance this WAL receiver is connected to. This and other spill counters can be used to gauge the I/O which occurred during logical decoding and allow tuning logical_decoding_work_mem. 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. idle in transaction (aborted): This state is similar to idle in transaction, except one of the statements in the transaction caused an error. See Section30.5 for more information about the internal WAL function XLogWrite. OID of the database this backend is connected to, Name of the database this backend is connected to. Waiting for a write of a serialized historical catalog snapshot. Waiting to read or update the control file or creation of a new WAL file. These numbers do not act as stated above; instead they update continuously throughout the transaction. quorum: This standby server is considered as a candidate for quorum standbys. 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 read or update shared multixact state. Total number of WAL full page images generated, Number of times WAL data was written to disk because WAL buffers became full. This standby's xmin horizon reported by hot_standby_feedback. See, One row per subscription, showing statistics about errors. The pg_stat_user_indexes and pg_stat_sys_indexes views contain the same information, but filtered to only show user and system indexes respectively. Waiting in main loop of background writer process. 105 #define BUFFER_MAPPING_LWLOCK_OFFSET NUM_INDIVIDUAL_LWLOCKS. The server process is waiting for exclusive access to a data buffer. For an asynchronous standby, the replay_lag column approximates the delay before recent transactions became visible to queries. However, current-query information collected by track_activities is always up-to-date. Waiting to access predicate lock information used by serializable transactions. Waiting for a read while adding a line to the data directory lock file. Please refer to your browser's Help pages for instructions. Waiting for logical rewrite mappings to reach durable storage. Waiting to acquire a speculative insertion lock. Waiting for confirmation from remote server during synchronous replication. Waiting to read or update the last value set for the transaction timestamp. The server process is waiting for activity on a socket connected to a user application. Write-Ahead Logging (WAL) is a standard method for ensuring data integrity. 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 The pg_stat_ssl view will contain one row per backend or WAL sender process, showing statistics about SSL usage on this connection. 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. idle in transaction (aborted): This state is similar to idle in transaction, except one of the statements in the transaction caused an error. Waiting during base backup when throttling activity. Prevent sudden database connection spikes by using a connection pool. The function pg_stat_get_backend_idset provides a convenient way to generate one row for each active backend for invoking these functions. Waiting for mapping data to reach durable storage during a logical rewrite. idle in transaction: The backend is in a transaction, but is not currently executing a query. Waiting for a write to the relation map file. It also tracks the total number of rows in each table, and information about . Waiting to read data from the client while establishing a GSSAPI session. Waiting for the relation map file to reach durable storage. Waiting to read while creating the data directory lock file. Waiting to access the list of predicate locks held by the current serializable transaction during a parallel query. Cumulative statistics are collected in shared memory. 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 read of the relation map file. Waiting for the group leader to update transaction status at end of a parallel operation. Waiting for an asynchronous prefetch from a relation data file. Waiting to update limits on transaction id and multixact consumption. Waiting to fill a dynamic shared memory backing file with zeroes. Waiting for data to reach durable storage while creating the data directory lock file. Amount of transaction data decoded for streaming in-progress transactions to the decoding output plugin while decoding changes from WAL for this slot. See, One row for each tracked function, showing statistics about executions of that function. In rows about other sessions, many columns will be null. Waiting for background worker to shut down. 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 for a read of a serialized historical catalog snapshot. Waiting for WAL buffers to be written to disk. 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. Waiting to acquire a lock on a non-relation database object. Several predefined views, listed in Table28.1, are available to show the current state of the system. disabled: This state is reported if track_activities is disabled in this backend. Using pg_stat_reset() also resets counters that autovacuum uses to determine when to trigger a vacuum or an analyze. The server process is idle. fastpath function call: The backend is executing a fast-path function. proc: Waiting to read or update the fast-path lock information. Waiting in main loop of logical apply process. A database-wide ANALYZE is recommended after the statistics have been reset. Waiting to acquire an exclusive lock to truncate off any empty pages at the end of a table vacuumed. See, One row per connection (regular and replication), showing information about GSSAPI authentication and encryption used on this connection. - 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 -1 if a Unix socket is used, Time when this process was started, i.e., when the client connected to this WAL sender. Current overall state of this backend. 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. Serial number of the client certificate, or NULL if no client certificate was supplied or if SSL is not in use on this connection. lock_manager Waiting to get the start location of a scan on a table for synchronized scans. 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 newly initialized WAL file to reach durable storage. Waiting for I/O on a multixact offset SLRU buffer. Therefore, while holding an exclusive lock, a process prevents other processes from acquiring a shared or exclusive lock. 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. Increase the number of wal_buffers available to the database. Waiting for WAL to reach durable storage during bootstrapping. Waiting to retrieve or store information about serializable transactions. Logical decoding plugins may optionally emit tracking messages; if they do not, the tracking mechanism will simply display NULL lag. Most such locks protect a particular data structure in shared memory. The argument can be one of CommitTs, MultiXactMember, MultiXactOffset, Notify, Serial, Subtrans, or Xact to reset the counters for only that entry. Synchronous state of this standby server. Tune max_wal_size and checkpoint_timeout based on Avoid PostgreSQL LWLock:buffer_content locks in Amazon Aurora: Tips and best practices. The easiest way to create a cross-Region replica for Amazon RDS for PostgreSQL is by completing the following steps: On the Amazon RDS console, choose your Amazon RDS for PostgreSQL source instance. Discards the current statistics snapshot or cached information. Waiting for WAL to be flushed in WAL sender process. Detailed Description . There is no order to the granting of LWLocks and in a high concurrency system this can cause contention. It is used per the rules above. Waiting for an asynchronous prefetch from a relation data file. 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. See, One row per SLRU, showing statistics of operations. 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 to setup, drop or use replication origin. When using the statistics to monitor collected data, it is important to realize that the information does not update instantaneously. Waiting to apply WAL at recovery because it is delayed. Waiting in main loop of WAL writer process. Waiting to read or update background worker state. 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. 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 a write during a file copy operation. Waiting in main loop of logical replication apply process. These files are stored in the directory named by the stats_temp_directory parameter, pg_stat_tmp by default. If a backend is in the active state, it may or may not be waiting on some event. Sometimes it may be more convenient to obtain just a subset of this information. might be causing it. BufferCacheHitRatio and LWLock:BufferIO wait Indexes can be used by simple index scans, bitmap index scans, and the optimizer. The server process is waiting for some interaction with another server process. (See Chapter20 for details about setting configuration parameters.). (Some locks have specific names; others are part of a group of locks each with a similar purpose.). wait_event will identify the specific wait point. However, they are also used to ensure mutual exclusion for certain internal operations such as relation extension. 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. Waiting for a read of a timeline history file. PostgreSQL Entangled in Locks: Attempts to free it - Amit Kapila - Dilip Kumar PGCon 2017 . (Conflicts occur only on standby servers; see pg_stat_database_conflicts for details.). Waiting for a read of a logical mapping during reorder buffer management. Waiting to elect a Parallel Hash participant to allocate more buckets. 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 WAL from a stream at recovery. pg_stat_get_backend_activity_start ( integer ) timestamp with time zone. A process acquires an LWLock in a shared mode to read from the buffer and . > However, someone with deeper knowledge of page pinning and buffer manager > internals could certainly devise a better solution. Waiting for SLRU data to reach durable storage during a checkpoint or database shutdown. 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. pg_stat_get_backend_client_addr ( integer ) inet. Waiting to update limit on notification message storage. Waiting for a read from a timeline history file during walsender timeline command. Waiting in main loop of syslogger process. Connection string used by this WAL receiver, with security-sensitive fields obfuscated. If this field is null, it indicates either that the client is connected via a Unix socket on the server machine or that this is an internal process such as autovacuum. Waiting in WAL receiver to establish connection to remote server. The pg_stat_wal_receiver view will contain only one row, showing statistics about the WAL receiver from that receiver's connected server. Time at which these statistics were last reset. Waiting for a write while adding a line to the data directory lock file. This documentation is for an unsupported version of PostgreSQL. Waiting to retrieve messages from the shared catalog invalidation queue. Waiting for an elected Parallel Hash participant to decide on future batch growth. events. Waiting to retrieve or remove messages from shared invalidation queue. Waiting for data to reach durable storage while adding a line to the data directory lock file. See, One row for each index in the current database, showing statistics about accesses to that specific index. Waiting for a read from a timeline history file during a walsender timeline command. See, One row only, showing statistics about the background writer process's activity. Waiting in main loop of WAL sender process. catchup: This WAL sender's connected standby is catching up with the primary. Time when this process' current transaction was started, or null if no transaction is active. Waiting to read while creating the data directory lock file. Waiting to read or update replication slot state. Lag times work automatically for physical replication. Waiting for the termination of another backend. Number of deadlocks detected in this database. If enabled, calls to user-defined functions and the total time spent in each one are counted as well. Table28.12.pg_stat_database_conflicts View. Number of data page checksum failures detected in this database (or on a shared object), or NULL if data checksums are not enabled. active: The backend is executing a query. Doing this helps IP address of the client connected to this backend. This is consistent with the goal of measuring synchronous commit and transaction visibility delays for recent write transactions. 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. See, One row for each table in the current database, showing statistics about accesses to that specific table. Resetting these counters can cause autovacuum to not perform necessary work, which can cause problems such as table bloat or out-dated table statistics.
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