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Re: Hot Standby query cancellation and Streaming Replication integration

From: Greg Smith <greg(at)2ndquadrant(dot)com>
To: Greg Stark <gsstark(at)mit(dot)edu>
Cc: Bruce Momjian <bruce(at)momjian(dot)us>, Josh Berkus <josh(at)agliodbs(dot)com>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: Hot Standby query cancellation and Streaming Replication integration
Date: 2010-02-28 06:18:00
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Lists: pgsql-hackers
Greg Stark wrote:
> On Sun, Feb 28, 2010 at 5:28 AM, Greg Smith <greg(at)2ndquadrant(dot)com> wrote:
>> The idea of the workaround is that if you have a single long-running query
>> to execute, and you want to make sure it doesn't get canceled because of a
>> vacuum cleanup, you just have it connect back to the master to keep an open
>> snapshot the whole time.
> Also, I'm not sure this actually works. When your client makes this
> additional connection to the master it's connecting at some
> transaction in the future from the slave's point of view. The master
> could have already vacuumed away some record which the snapshot the
> client gets on the slave will have in view.

Right, and there was an additional comment in the docs alluding to some 
sleep time on the master that intends to try and improve thins.  If you 
knew how long archive_timeout was you could try to sleep longer than it 
to try and increase your odds of avoiding an ugly spot.  But there are 
race conditions galore possible here, particularly if your archiver or 
standby catchup is backlogged.

> Still it's a handy practical trick even if it isn't 100% guaranteed to
> work. But I don't think it provides the basis for something we can
> bake in.

Agreed on both counts, which is why it's in the current docs as a 
workaround people can consider, but not what I've been advocating as the 
right way to proceed.

Greg Smith  2ndQuadrant US  Baltimore, MD
PostgreSQL Training, Services and Support

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