max_standby_streaming_delay

Definition PG 18 manual

When hot standby is active, this parameter determines how long the standby server should wait before canceling standby queries that conflict with about-to-be-applied WAL entries, as described in Section 26.4.2. max_standby_streaming_delay applies when WAL data is being received via streaming replication. If this value is specified without units, it is taken as milliseconds. The default is 30 seconds. A value of -1 allows the standby to wait forever for conflicting queries to complete. This parameter can only be set in the postgresql.conf file or on the server command line.

Note that max_standby_streaming_delay is not the same as the maximum length of time a query can run before cancellation; rather it is the maximum total time allowed to apply WAL data once it has been received from the primary server. Thus, if one query has resulted in significant delay, subsequent conflicting queries will have much less grace time until the standby server has caught up again.

How it works

Sets the maximum delay before canceling queries when a hot standby server is processing streamed WAL data. A configuration reload applies a new value; existing work already in flight is not retroactively changed.

While replaying streamed WAL, recovery may wait up to this accumulated delay for conflicting queries before cancellation. Larger values favor read continuity but raise replication lag and recovery-point exposure; hot_standby_feedback addresses some snapshot conflicts with primary-side bloat risk.

Monitor and change max_standby_streaming_delay together with max_standby_archive_delay, hot_standby, hot_standby_feedback. Validate on the relevant server role and real workload, then use its sighup context to choose session change, reload, or restart; a historical boot default is not the current effective value.

Operational considerations

Changing it on the wrong primary, standby, sender, or subscriber role.

Watching only configured bytes/time instead of actual lag, slot position, and worker state.

Failing over to a node that lacks the old primary's capacity or prerequisites.

Using infinite waits or WAL retention to hide a failed consumer.

Workload guidance

OLAP: Read standbys and logical subscribers often see long queries or large transactions. Put explicit bounds on replay/apply and monitor lag, worker saturation, slot restart_lsn, and conflict cancellations.

OLTP: Size max_standby_streaming_delay from topology, failover roles, slot/subscription count, and reconnect headroom. Test worst-case primary latency, standby replay, and disk retention before production.

SMALL: Configure only replication capacity that is actually used. Even a small topology needs bounded timeouts and slot lifecycle; unlimited retention is not reliability.