Re: pg_xlogfile_name_offset() et al and recovery

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От Simon Riggs
Тема Re: pg_xlogfile_name_offset() et al and recovery
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Msg-id CANP8+jKXcRSKXnxkvKWXSKucSFRDTEGArXUUoSkJ7x9jyASRww@mail.gmail.com
обсуждение исходный текст
Ответ на Re: pg_xlogfile_name_offset() et al and recovery  (Michael Paquier <michael.paquier@gmail.com>)
Ответы Re: pg_xlogfile_name_offset() et al and recovery  (Michael Paquier <michael.paquier@gmail.com>)
Re: pg_xlogfile_name_offset() et al and recovery  (Robert Haas <robertmhaas@gmail.com>)
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On 7 July 2016 at 08:26, Michael Paquier <michael.paquier@gmail.com> wrote:
On Thu, Jul 7, 2016 at 3:59 PM, Amit Langote
<Langote_Amit_f8@lab.ntt.co.jp> wrote:
> 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.

To be honest, I have always found that this restriction was hard to
justify on a function that basically performs a static calculation.

I know its annoying behaviour, but this is not an immutable function i.e. not a static calculation.
The timeline is not so much invalid as variable over time.

The behaviour of that function is defined in backbranches, so I suggest we should not alter that now.

What we can do is have another function that makes it clearer that the answer is variable.
   pg_xlogfile_name_offset(offset, timelineid) 
always succeeds on standby but needs to be told what timelineid to use

then we have another function
  pg_last_xact_replay_timeline() that allows you to find out the current timeline


The actual problem here is somewhat more convoluted, because we would like to know the timeline of the basebackup at start and stop. That information is not easily available from the backup label data returned by pg_stop_backup().
We would like to do something like this...

select pg_xlogfile_name_offset(l.lsn, l.tli) from pg_stop_backup(false) l;

So I suggest we add another column to the output of pg_stop_backup() to return the tli value directly.

--
Simon Riggs                http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

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