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Re: Re: Hot Standby query cancellation and Streaming Replication integration

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Greg Stark <gsstark(at)mit(dot)edu>
Cc: Greg Smith <greg(at)2ndquadrant(dot)com>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Re: Hot Standby query cancellation and Streaming Replication integration
Date: 2010-02-26 18:53:16
Message-ID: 1916.1267210396@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-hackers
Greg Stark <gsstark(at)mit(dot)edu> writes:
> In the model you describe any long-lived queries on the slave cause
> tables in the master to bloat with dead records.

Yup, same as they would do on the master.

> I think this model is on the roadmap but it's not appropriate for
> everyone and I think one of the benefits of having delayed it is that
> it forces us to get the independent model right before throwing in
> extra complications. It would be too easy to rely on the slave
> feedback as an answer for hard questions about usability if we had it
> and just ignore the question of what to do when it's not the right
> solution for the user.

I'm going to make an unvarnished assertion here.  I believe that the
notion of synchronizing the WAL stream against slave queries is
fundamentally wrong and we will never be able to make it work.
The information needed isn't available in the log stream and can't be
made available without very large additions (and consequent performance
penalties).  As we start getting actual beta testing we are going to
uncover all sorts of missed cases that are not going to be fixable
without piling additional ugly kluges on top of the ones Simon has
already crammed into the system.  Performance and reliability will both
suffer.

I think that what we are going to have to do before we can ship 9.0
is rip all of that stuff out and replace it with the sort of closed-loop
synchronization Greg Smith is pushing.  It will probably be several
months before everyone is forced to accept that, which is why 9.0 is
not going to ship this year.

			regards, tom lane

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