Re: pg_xlogfile_name_offset() et al and recovery

On 2016/05/19 17:34, Amit Langote wrote:
> Currently in HEAD and 9.6, one can issue a non-exclusive backup on
> standby, so this is OK:
> 
> select pg_is_in_recovery();
>  pg_is_in_recovery
> -------------------
>  t
> (1 row)
> 
> select pg_start_backup('sby-bkp-test', 'f', 'f');
>  pg_start_backup
> -----------------
>  0/5000220
> (1 row)
> 
> However the following happens:
> 
> select pg_xlogfile_name_offset(pg_start_backup('sby-bkp-test', 'f', 'f'));
> ERROR:  recovery is in progress
> HINT:  pg_xlogfile_name_offset() cannot be executed during recovery.
> 
> Should this restriction be relaxed or am I missing something?

Answering my own question:

While reading the thread "BUG #14230: Wrong timeline returned by
pg_stop_backup on a standby", I came to know that ThisTimelineId is
invalid on standby.  And because pg_xlogfile_name_offset() uses the same
to compute its result, it makes sense to prevent it from being used on a
standby.

Thanks,
Amit