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
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