27.2. 統計資訊收集器
版本:11
PostgreSQL 的統計資訊收集器是一個子系統,支援收集和回報有關伺服器活動的資訊。目前,收集器可以以磁區和單個資料列的方式計算對資料表和索引的存取。它也追踪著每個資料表中的總資料筆數,以及有關每個資料表的清理和分析操作的資訊。它還可以計算對使用者定義函數的呼叫以及每個函數所花費的總時間。
PostgreSQL 也支援回報有關目前系統中確切正在發生情況的動態資訊,例如目前由其他伺服器程序執行的確切指令以及系統中存在哪些其他連線。此功能獨立於收集器程序。
27.2.1. Statistics Collection Configuration
由於收集統計資訊會增加查詢執行的成本,因此可以將系統配置為收集或不收集資訊。這由通常由 postgresql.conf 中的配置參數所控制。 (有關設定配置參數的詳細資訊,請參閱第 19 章。)
The parameter track_activities enables monitoring of the current command being executed by any server process.
The parameter track_counts controls whether statistics are collected about table and index accesses.
The parameter track_functions enables tracking of usage of user-defined functions.
The parameter track_io_timing enables monitoring of block read and write times.
Normally these parameters are set in postgresql.conf
so that they apply to all server processes, but it is possible to turn them on or off in individual sessions using the SET command. (To prevent ordinary users from hiding their activity from the administrator, only superusers are allowed to change these parameters with SET
.)
The statistics collector transmits the collected information to other PostgreSQL processes through temporary files. These files are stored in the directory named by the stats_temp_directory parameter, pg_stat_tmp
by default. For better performance, stats_temp_directory
can be pointed at a RAM-based file system, decreasing physical I/O requirements. 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. When recovery is performed at server start (e.g. after immediate shutdown, server crash, and point-in-time recovery), all statistics counters are reset.
27.2.2. Viewing Statistics
Several predefined views, listed in Table 27.1, are available to show the current state of the system. There are also several other views, listed in Table 27.2, available to show the results of statistics collection. Alternatively, one can build custom views using the underlying statistics functions, as discussed in Section 27.2.3.
When using the statistics to monitor collected data, it is important to realize that the information does not update instantaneously. Each individual server process transmits new statistical counts to the collector just before going idle; so a query or transaction still in progress does not affect the displayed totals. Also, the collector itself emits a new report at most once per PGSTAT_STAT_INTERVAL
milliseconds (500 ms unless altered while building the server). So the displayed information lags behind actual activity. However, current-query information collected by track_activities
is always up-to-date.
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. So the statistics will show static information as long as you continue the current transaction. 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. 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. But if you want to see new results with each query, be sure to do the queries outside any transaction block. Alternatively, you can invoke pg_stat_clear_snapshot
(), which will discard the current transaction's statistics snapshot (if any). The next use of statistical information will cause a new snapshot to be fetched.
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
. These numbers do not act as stated above; instead they update continuously throughout the transaction.
Some of the information in the dynamic statistics views shown in Table 27.1 is security restricted. Ordinary users can only see all the information about their own sessions (sessions belonging to a role that they are a member of). In rows about other sessions, many columns will be null. Note, however, that the existence of a session and its general properties such as its sessions user and database are visible to all users. Superusers and members of the built-in role pg_read_all_stats
(see also Section 21.5) can see all the information about all sessions.
Table 27.1. Dynamic Statistics Views
View Name | Description |
| One row per server process, showing information related to the current activity of that process, such as state and current query. See pg_stat_activity for details. |
| One row per WAL sender process, showing statistics about replication to that sender's connected standby server. See pg_stat_replication for details. |
| Only one row, showing statistics about the WAL receiver from that receiver's connected server. See pg_stat_wal_receiver for details. |
| At least one row per subscription, showing information about the subscription workers. See pg_stat_subscription for details. |
| One row per connection (regular and replication), showing information about SSL used on this connection. See pg_stat_ssl for details. |
| One row per connection (regular and replication), showing information about GSSAPI authentication and encryption used on this connection. See pg_stat_gssapi for details. |
| One row for each backend running |
| One row for each backend (including autovacuum worker processes) running |
| One row for each backend running |
Table 27.2. Collected Statistics Views
View Name | Description |
| One row only, showing statistics about the WAL archiver process's activity. See pg_stat_archiver for details. |
| One row only, showing statistics about the background writer process's activity. See pg_stat_bgwriter for details. |
| One row per database, showing database-wide statistics. See pg_stat_database for details. |
| One row per database, showing database-wide statistics about query cancels due to conflict with recovery on standby servers. See pg_stat_database_conflicts for details. |
| One row for each table in the current database, showing statistics about accesses to that specific table. See pg_stat_all_tables for details. |
| Same as |
| Same as |
| Similar to |
| Same as |
| Same as |
| One row for each index in the current database, showing statistics about accesses to that specific index. See pg_stat_all_indexes for details. |
| Same as |
| Same as |
| One row for each table in the current database, showing statistics about I/O on that specific table. See pg_statio_all_tables for details. |
| Same as |
| Same as |
| One row for each index in the current database, showing statistics about I/O on that specific index. See pg_statio_all_indexes for details. |
| Same as |
| Same as |
| One row for each sequence in the current database, showing statistics about I/O on that specific sequence. See pg_statio_all_sequences for details. |
| Same as |
| Same as |
| One row for each tracked function, showing statistics about executions of that function. See pg_stat_user_functions for details. |
| Similar to |
The per-index statistics are particularly useful to determine which indexes are being used and how effective they are.
The pg_statio_
views are primarily useful to determine the effectiveness of the buffer cache. 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. 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. Users interested in obtaining more detailed information on PostgreSQL I/O behavior are advised to use the PostgreSQL statistics collector in combination with operating system utilities that allow insight into the kernel's handling of I/O.
Table 27.3. pg_stat_activity
View
pg_stat_activity
ViewColumn | Type | Description |
|
| OID of the database this backend is connected to |
|
| Name of the database this backend is connected to |
|
| Process ID of this backend |
|
| OID of the user logged into this backend |
|
| Name of the user logged into this backend |
|
| Name of the application that is connected to this backend |
|
| IP address of the client connected to this backend. 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. |
|
| Host name of the connected client, as reported by a reverse DNS lookup of |
|
| TCP port number that the client is using for communication with this backend, or |
|
| Time when this process was started. For client backends, this is the time the client connected to the server. |
|
| Time when this process' current transaction was started, or null if no transaction is active. 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 |
|
| Time when the |
|
| The type of event for which the backend is waiting, if any; otherwise NULL. Possible values are:
|
|
| Wait event name if backend is currently waiting, otherwise NULL. See Table 27.4for details. |
|
| Current overall state of this backend. Possible values are:
|
|
| Top-level transaction identifier of this backend, if any. |
|
| The current backend's |
|
| Text of this backend's most recent query. If |
|
| Type of current backend. Possible types are |
The pg_stat_activity
view will have one row per server process, showing information related to the current activity of that process.
Note
The wait_event
and state
columns are independent. If a backend is in the active
state, it may or may not be waiting
on some event. 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.
Table 27.4. wait_event
Description
wait_event
DescriptionWait Event Type | Wait Event Name | Description |
|
| Waiting to find or allocate space in shared memory. |
| Waiting to allocate or assign an OID. | |
| Waiting to allocate or assign a transaction id. | |
| Waiting to get a snapshot or clearing a transaction id at transaction end. | |
| Waiting to retrieve or remove messages from shared invalidation queue. | |
| Waiting to add a message in shared invalidation queue. | |
| Waiting to replace a page in WAL buffers. | |
| Waiting for WAL buffers to be written to disk. | |
| Waiting to read or update the control file or creation of a new WAL file. | |
| Waiting to perform checkpoint. | |
| Waiting to read or update transaction status. | |
| Waiting to read or update sub-transaction information. | |
| Waiting to read or update shared multixact state. | |
| Waiting to read or update multixact offset mappings. | |
| Waiting to read or update multixact member mappings. | |
| Waiting to read or write relation cache initialization file. | |
| Waiting to manage fsync requests. | |
| Waiting to read or update the state of prepared transactions. | |
| Waiting to create or drop the tablespace. | |
| Waiting to read or update vacuum-related information for a B-tree index. | |
| Waiting to manage space allocation in shared memory. | |
| Autovacuum worker or launcher waiting to update or read the current state of autovacuum workers. | |
| Waiting to ensure that the table it has selected for a vacuum still needs vacuuming. | |
| Waiting to get the start location of a scan on a table for synchronized scans. | |
| Waiting to update the relation map file used to store catalog to filenode mapping. | |
| Waiting to read or update shared notification state. | |
| Waiting to read or update notification messages. | |
| Waiting to retrieve or store information about serializable transactions. | |
| Waiting to access the list of finished serializable transactions. | |
| Waiting to perform an operation on a list of locks held by serializable transactions. | |
| Waiting to read or record conflicting serializable transactions. | |
| Waiting to read or update information about synchronous replicas. | |
| Waiting to read or update background worker state. | |
| Waiting to read or update dynamic shared memory state. | |
| Waiting to update the | |
| Waiting to allocate or free a replication slot. | |
| Waiting to read or update replication slot state. | |
| Waiting to read or update transaction commit timestamps. | |
| Waiting to read or update the last value set for the transaction timestamp. | |
| Waiting to setup, drop or use replication origin. | |
| Waiting to read or truncate multixact information. | |
| Waiting to read or update old snapshot control information. | |
| Waiting for action on logical replication worker to finish. | |
| Waiting to execute | |
| Waiting for I/O on a clog (transaction status) buffer. | |
| Waiting for I/O on commit timestamp buffer. | |
| Waiting for I/O a subtransaction buffer. | |
| Waiting for I/O on a multixact offset buffer. | |
| Waiting for I/O on a multixact_member buffer. | |
| Waiting for I/O on an async (notify) buffer. | |
| Waiting for I/O on an oldserxid buffer. | |
| Waiting to insert WAL into a memory buffer. | |
| Waiting to read or write a data page in memory. | |
| Waiting for I/O on a data page. | |
| Waiting to read or update the replication progress. | |
| Waiting for I/O on a replication slot. | |
| Waiting to read or update the fast-path lock information. | |
| Waiting to associate a data block with a buffer in the buffer pool. | |
| Waiting to add or examine locks for backends, or waiting to join or exit a locking group (used by parallel query). | |
| Waiting to add or examine predicate lock information. | |
| Waiting to perform an operation on a serializable transaction in a parallel query. | |
| Waiting for parallel query dynamic shared memory allocation lock. | |
| Waiting for TBM shared iterator lock. | |
| Waiting to choose the next subplan during Parallel Append plan execution. | |
| Waiting to allocate or exchange a chunk of memory or update counters during Parallel Hash plan execution. | |
|
| Waiting to acquire a lock on a relation. |
| Waiting to extend a relation. | |
| Waiting to acquire a lock on page of a relation. | |
| Waiting to acquire a lock on a tuple. | |
| Waiting for a transaction to finish. | |
| Waiting to acquire a virtual xid lock. | |
| Waiting to acquire a speculative insertion lock. | |
| Waiting to acquire a lock on a non-relation database object. | |
| Waiting to acquire a user lock. | |
| Waiting to acquire an advisory user lock. | |
|
| Waiting to acquire a pin on a buffer. |
|
| Waiting in main loop of the archiver process. |
| Waiting in main loop of autovacuum launcher process. | |
| Waiting in background writer process, hibernating. | |
| Waiting in main loop of background writer process background worker. | |
| Waiting in main loop of checkpointer process. | |
| Waiting in main loop of logical apply process. | |
| Waiting in main loop of logical launcher process. | |
| Waiting in main loop of the statistics collector process. | |
| Waiting for WAL from a stream at recovery. | |
| Waiting when WAL data is not available from any kind of sources (local, archive or stream) before trying again to retrieve WAL data, at recovery. | |
| Waiting in main loop of syslogger process. | |
| Waiting in main loop of WAL receiver process. | |
| Waiting in main loop of WAL sender process. | |
| Waiting in main loop of WAL writer process. | |
|
| Waiting to read data from the client. |
| Waiting to write data to the client. | |
| Waiting to read data from the client while establishing the GSSAPI session. | |
| Waiting in WAL receiver to establish connection to remote server. | |
| Waiting in WAL receiver to receive data from remote server. | |
| Waiting for SSL while attempting connection. | |
| Waiting for startup process to send initial data for streaming replication. | |
| Waiting for WAL to be flushed in WAL sender process. | |
| Waiting for any activity when processing replies from WAL receiver in WAL sender process. | |
|
| Waiting in an extension. |
|
| Waiting for background worker to shut down. |
| Waiting for background worker to start up. | |
| Waiting for the page number needed to continue a parallel B-tree scan to become available. | |
| Waiting for a checkpoint to complete. | |
| Waiting for a checkpoint to start. | |
| Waiting for group leader to update transaction status at transaction end. | |
| Waiting for activity from child process when executing | |
| Waiting for an elected Parallel Hash participant to allocate a hash table. | |
| Electing a Parallel Hash participant to allocate a hash table. | |
| Waiting for other Parallel Hash participants to finish loading a hash table. | |
| Waiting for an elected Parallel Hash participant to allocate the initial hash table. | |
| Electing a Parallel Hash participant to allocate the initial hash table. | |
| Waiting for other Parallel Hash participants to finish hashing the inner relation. | |
| Waiting for other Parallel Hash participants to finish partitioning the outer relation. | |
| Waiting for an elected Parallel Hash participant to allocate more batches. | |
| Electing a Parallel Hash participant to decide on future batch growth. | |
| Electing a Parallel Hash participant to allocate more batches. | |
| Waiting for an elected Parallel Hash participant to decide on future batch growth. | |
| Waiting for other Parallel Hash participants to finishing repartitioning. | |
| Waiting for an elected Parallel Hash participant to finish allocating more buckets. | |
| Electing a Parallel Hash participant to allocate more buckets. | |
| Waiting for other Parallel Hash participants to finish inserting tuples into new buckets. | |
| Waiting for logical replication remote server to send data for initial table synchronization. | |
| Waiting for logical replication remote server to change state. | |
| Waiting for other process to be attached in shared message queue. | |
| Waiting to write a protocol message to a shared message queue. | |
| Waiting to receive bytes from a shared message queue. | |
| Waiting to send bytes to a shared message queue. | |
| Waiting for parallel bitmap scan to become initialized. | |
| Waiting for parallel | |
| Waiting for parallel workers to finish computing. | |
| Waiting for group leader to clear transaction id at transaction end. | |
| Waiting for standby promotion. | |
| Waiting for a replication origin to become inactive to be dropped. | |
| Waiting for a replication slot to become inactive to be dropped. | |
| Waiting for a snapshot for a | |
| Waiting for confirmation from remote server during synchronous replication. | |
|
| Waiting during base backup when throttling activity. |
| Waiting in process that called | |
| Waiting to apply WAL at recovery because it is delayed. | |
|
| Waiting for a read from a buffered file. |
| Waiting for a write to a buffered file. | |
| Waiting for a read from the control file. | |
| Waiting for the control file to reach stable storage. | |
| Waiting for an update to the control file to reach stable storage. | |
| Waiting for a write to the control file. | |
| Waiting for a write to update the control file. | |
| Waiting for a read during a file copy operation. | |
| Waiting for a write during a file copy operation. | |
| Waiting for a relation data file to be extended. | |
| Waiting for a relation data file to reach stable storage. | |
| Waiting for an immediate synchronization of a relation data file to stable storage. | |
| Waiting for an asynchronous prefetch from a relation data file. | |
| Waiting for a read from a relation data file. | |
| Waiting for changes to a relation data file to reach stable storage. | |
| Waiting for a relation data file to be truncated. | |
| Waiting for a write to a relation data file. | |
| Waiting to write zero bytes to a dynamic shared memory backing file. | |
| Waiting for a read while adding a line to the data directory lock file. | |
| Waiting for data to reach stable storage while adding a line to the data directory lock file. | |
| Waiting for a write while adding a line to the data directory lock file. | |
| Waiting to read while creating the data directory lock file. | |
| Waiting for data to reach stable storage while creating the data directory lock file. | |
| Waiting for a write while creating the data directory lock file. | |
| Waiting for a read during recheck of the data directory lock file. | |
| Waiting for logical rewrite mappings to reach stable storage during a checkpoint. | |
| Waiting for mapping data to reach stable storage during a logical rewrite. | |
| Waiting for a write of mapping data during a logical rewrite. | |
| Waiting for logical rewrite mappings to reach stable storage. | |
| Waiting for truncate of mapping data during a logical rewrite. | |
| Waiting for a write of logical rewrite mappings. | |
| Waiting for a read of the relation map file. | |
| Waiting for the relation map file to reach stable storage. | |
| Waiting for a write to the relation map file. | |
| Waiting for a read during reorder buffer management. | |
| Waiting for a write during reorder buffer management. | |
| Waiting for a read of a logical mapping during reorder buffer management. | |
| Waiting for a read from a replication slot control file. | |
| Waiting for a replication slot control file to reach stable storage while restoring it to memory. | |
| Waiting for a replication slot control file to reach stable storage. | |
| Waiting for a write to a replication slot control file. | |
| Waiting for SLRU data to reach stable storage during a checkpoint or database shutdown. | |
| Waiting for a read of an SLRU page. | |
| Waiting for SLRU data to reach stable storage following a page write. | |
| Waiting for a write of an SLRU page. | |
| Waiting for a read of a serialized historical catalog snapshot. | |
| Waiting for a serialized historical catalog snapshot to reach stable storage. | |
| Waiting for a write of a serialized historical catalog snapshot. | |
| Waiting for a timeline history file received via streaming replication to reach stable storage. | |
| Waiting for a write of a timeline history file received via streaming replication. | |
| Waiting for a read of a timeline history file. | |
| Waiting for a newly created timeline history file to reach stable storage. | |
| Waiting for a write of a newly created timeline history file. | |
| Waiting for a read of a two phase state file. | |
| Waiting for a two phase state file to reach stable storage. | |
| Waiting for a write of a two phase state file. | |
| Waiting for WAL to reach stable storage during bootstrapping. | |
| Waiting for a write of a WAL page during bootstrapping. | |
| Waiting for a read when creating a new WAL segment by copying an existing one. | |
| Waiting a new WAL segment created by copying an existing one to reach stable storage. | |
| Waiting for a write when creating a new WAL segment by copying an existing one. | |
| Waiting for a newly initialized WAL file to reach stable storage. | |
| Waiting for a write while initializing a new WAL file. | |
| Waiting for a read from a WAL file. | |
| Waiting for a read from a timeline history file during walsender timeline command. | |
| Waiting for a WAL file to reach stable storage. | |
| Waiting for data to reach stable storage while assigning WAL sync method. | |
| Waiting for a write to a WAL file. |
Note
For tranches registered by extensions, the name is specified by extension and this will be displayed as wait_event
. It is quite possible that user has registered the tranche in one of the backends (by having allocation in dynamic shared memory) in which case other backends won't have that information, so we display extension
for such cases.
Here is an example of how wait events can be viewed
Table 27.5. pg_stat_replication
View
Column | Type | Description |
|
| Process ID of a WAL sender process |
|
| OID of the user logged into this WAL sender process |
|
| Name of the user logged into this WAL sender process |
|
| Name of the application that is connected to this WAL sender |
|
| IP address of the client connected to this WAL sender. If this field is null, it indicates that the client is connected via a Unix socket on the server machine. |
|
| Host name of the connected client, as reported by a reverse DNS lookup of |
|
| 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 |
|
| This standby's |
|
| Current WAL sender state. 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). This can be used to gauge the delay that |
|
| Time elapsed between flushing recent WAL locally and receiving notification that this standby server has written and flushed it (but not yet applied it). This can be used to gauge the delay that |
|
| Time elapsed between flushing recent WAL locally and receiving notification that this standby server has written, flushed and applied it. This can be used to gauge the delay that |
|
| Priority of this standby server for being chosen as the synchronous standby in a priority-based synchronous replication. This has no effect in a quorum-based synchronous replication. |
|
| Synchronous state of this standby server. Possible values are:
|
|
| Send time of last reply message received from standby server |
The pg_stat_replication
view will contain one row per WAL sender process, showing statistics about replication to that sender's connected standby server. Only directly connected standbys are listed; no information is available about downstream standby servers.
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. 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. For an asynchronous standby, the replay_lag
column approximates the delay before recent transactions became visible to queries. 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.
Lag times work automatically for physical replication. Logical decoding plugins may optionally emit tracking messages; if they do not, the tracking mechanism will simply display NULL lag.
Note
The reported lag times are not predictions of how long it will take for the standby to catch up with the sending server assuming the current rate of replay. Such a system would show similar times while new WAL is being generated, but would differ when the sender becomes idle. In particular, when the standby has caught up completely, pg_stat_replication
shows the time taken to write, flush and replay the most recent reported WAL location rather than zero as some users might expect. This is consistent with the goal of measuring synchronous commit and transaction visibility delays for recent write transactions. To reduce confusion for users expecting a different model of lag, the lag columns revert to NULL after a short time on a fully replayed idle system. Monitoring systems should choose whether to represent this as missing data, zero or continue to display the last known value.
Table 27.6. pg_stat_wal_receiver
View
pg_stat_wal_receiver
ViewColumn | Type | Description |
|
| Process ID of the WAL receiver process |
|
| 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 |
|
| Host of the PostgreSQL instance this WAL receiver is connected to. This can be a host name, an IP address, or a directory path if the connection is via Unix socket. (The path case can be distinguished because it will always be an absolute path, beginning with |
|
| Port number of the PostgreSQL instance this WAL receiver is connected to. |
|
| Connection string used by this WAL receiver, with security-sensitive fields obfuscated. |
The pg_stat_wal_receiver
view will contain only one row, showing statistics about the WAL receiver from that receiver's connected server.
Table 27.7. pg_stat_subscription
View
pg_stat_subscription
ViewColumn | Type | Description |
|
| OID of the subscription |
|
| Name of the subscription |
|
| 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 |
|
| Send time of last message received from origin WAL sender |