PostgreSQL Replication Lag – WALs Streaming but Not Being Applied

Hello,

I would appreciate some guidance on an issue we are experiencing with
PostgreSQL streaming replication.

The standby appears to be *receiving/streaming WALs and is up to date with
the primary*, but the WAL records are not being applied to the standby,
resulting in an increasing replication/replay lag.

We have confirmed that:

- WALs are being streamed from the primary to the standby.
- The WAL receiver appears to be active.
- The standby is receiving the WALs, but the replay/apply process is not
keeping up.
- pg_stat_database_conflicts shows no conflicts.
- pg_is_wal_replay_paused() returns false.
- recovery_min_apply_delay is 0.
- hot_standby_feedback is disabled.

[image: Screenshot 2026-08-18 at 20.52.58.png]

*My questions are:*

1. What could cause WALs to be successfully streamed to the standby but
not applied/replayed?
2. What are the best steps to identify what the WAL replay process is
waiting on?
3. Is restarting the replication/standby the only practical solution, or
is there a way to safely resume WAL replay without restarting?
4. Are there specific PostgreSQL views, wait events, or system metrics
that you would recommend checking to identify the bottleneck?

I would appreciate any guidance on troubleshooting this and identifying the
underlying cause rather than simply restarting the replication whenever
this occurs.

*NB: This happens on a daily basis!*

Thank you in advance.

Regards,
Musila