On Tue, Sep 18, 2018 at 04:10:17PM +0000, PG Doc comments form wrote:
> Documentation for restore_command in recovery.conf only says, "This
> parameter is required for archive recovery, but optional for streaming
> replication," but it doesn't say specifically when the restore_command
> string will be used and why. My guess is that as long as the WAL files
> are still available on the master (primary) host, the recovery will
> fetch them via the replication slot, but if a WAL is missing on the
> primary, only then will the restore_command be used. But this isn't
> explicitly stated. It could be that the restore_command is always
> used, even if the WAL file is still available on the primary.
The logic around the source used to fetch WAL segments is defined in
WaitForWALToBecomeAvailable/xlog.c. A standby would give priority to
what is present in the local pg_wal or in the WAL archive before
switching to streaming. If fetching a segment from the archive or the
local pg_wal fails, then a switch to streaming happens (of course if
primary_conninfo is defined). There is nothing really linked to
replication slots in this case.
--
Michael
