By default, a standby server restores WAL records from the sending server as soon as possible. It may be useful to have a time-delayed copy of the data, offering opportunities to correct data loss errors. This parameter allows you to delay recovery by a specified amount of time. For example, if you set this parameter to
5min, the standby will replay each transaction commit only when the system time on the standby is at least five minutes past the commit time reported by the primary. If this value is specified without units, it is taken as milliseconds. The default is zero, adding no delay.
It is possible that the replication delay between servers exceeds the value of this parameter, in which case no delay is added. Note that the delay is calculated between the WAL time stamp as written on primary and the current time on the standby. Delays in transfer because of network lag or cascading replication configurations may reduce the actual wait time significantly. If the system clocks on primary and standby are not synchronized, this may lead to recovery applying records earlier than expected; but that is not a major issue because useful settings of this parameter are much larger than typical time deviations between servers.
The delay occurs only on WAL records for transaction commits. Other records are replayed as quickly as possible, which is not a problem because MVCC visibility rules ensure their effects are not visible until the corresponding commit record is applied.
The delay occurs once the database in recovery has reached a consistent state, until the standby is promoted or triggered. After that the standby will end recovery without further waiting.
WAL records must be kept on the standby until they are ready to be applied. Therefore, longer delays will result in a greater accumulation of WAL files, increasing disk space requirements for the standby's pg_wal directory.
This parameter is intended for use with streaming replication deployments; however, if the parameter is specified it will be honored in all cases except crash recovery. hot_standby_feedback will be delayed by use of this feature which could lead to bloat on the primary; use both together with care.
Synchronous replication is affected by this setting when synchronous_commit is set to
COMMIT will need to wait to be applied.
This parameter can only be set in the postgresql.conf file or on the server command line.