Re: PostgreSQL Replication Lag – WALs Streaming but Not Being Applied

On Tue, Aug 18, 2026 at 09:03:39PM +0300, Simon Musila - Localhost wrote:
> *My questions are:*
> 1. What could cause WALs to be successfully streamed to the standby but
> not applied/replayed?

Concurrent "traffic" (queries). Or special operations in wal that take
"a while" to apply.

> 2. What are the best steps to identify what the WAL replay process is
> waiting on?

I'd start with:
a. `ps uwf -u postgres` shell command (as long as the underlying os is
linux-ish.
b. `select * from pg_stat_activity`
c. `sudo strace -f -tt -T -s 256 -p PID-OF-PG-STARTUP-PROCESS` - again,
on linux hosts
d. pg_waldump of the wal file that takes long time, and check what is
going on there

Best regards,

depesz