On 2013-06-09 17:50:13 +0800, Craig Ringer wrote:
> On 05/31/2013 03:41 PM, Fabien COELHO wrote:
> >
> >>> However I'm not sure that pg_stat_replication currently has the
> >>> necessary information on either side to measure the lag (in time
> >>> transactions, but how do I know when a transaction was committed? or
> >>> number of transactions?).
> >>
> >> The BDR codebase now has a handy function to report when a transaction
> >> was committed, pg_get_transaction_committime(xid) .
> >
> > This looks handy for monitoring a replication setup.
> > It should really be in core...
> >
> > Any plans? Or is there other ways to get this kind of information in
> > core?
> pg_get_transaction_committime isn't trivial to just add to core because
> it requires a commit time to be recorded with commit records in the
> transaction logs, among other changes.
The commit records actually already have that information available
(c.f. xl_xact_commit(_compact) in xact.h), the problem is having a
datastructure which collects all that.
That's why the committs (written by Alvaro) added an slru mapping xids
to timestamps. And yes, we want to submit that sometime.
The pg_xlog_wait_remote_apply(), pg_xlog_wait_remote_receive() functions
however don't need any additional infrastructure, so I think those are
easier and less controversial to add.
Greetings,
Andres Freund
-- Andres Freund http://www.2ndQuadrant.com/PostgreSQL Development, 24x7 Support, Training &
Services
