Re: multixacts woes

On 05/08/2015 11:27 AM, Andres Freund wrote:
> Hi,
>
> On 2015-05-08 14:15:44 -0400, Robert Haas wrote:
>> 3. It seems to me that there is a danger that some users could see
>> extremely frequent anti-mxid-member-wraparound vacuums as a result of
>> this work. Granted, that beats data corruption or errors, but it
>> could still be pretty bad.
>
> It's certainly possible to have workloads triggering that, but I think
> it's relatively uncommon. I in most cases I've checked the multixact
> consumption rate is much lower than the xid consumption. There are some
> exceptions, but often that's pretty bad code.

I have a couple workloads in my pool which do consume mxids faster than
xids, due to (I think) execeptional numbers of FK conflicts. It's
definitely unusual, though, and I'm sure they'd rather have corruption
protection and endure some more vacuums. If we do this, though, it
might be worthwhile to backport the multixact age function, so that
affected users can check and schedule mxact wraparound vacuums
themselves, something you currently can't do on 9.3.

--
Josh Berkus
PostgreSQL Experts Inc.
http://pgexperts.com