From: | "Tsunakawa, Takayuki" <tsunakawa(dot)takay(at)jp(dot)fujitsu(dot)com> |
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To: | 'Robert Haas' <robertmhaas(at)gmail(dot)com> |
Cc: | Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com>, "pgsql-hackers(at)postgresql(dot)org" <pgsql-hackers(at)postgresql(dot)org> |
Subject: | RE: Temporary tables prevent autovacuum, leading to XID wraparound |
Date: | 2018-01-31 00:04:32 |
Message-ID: | 0A3221C70F24FB45833433255569204D1F8AABEC@G01JPEXMBYT05 |
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From: Robert Haas [mailto:robertmhaas(at)gmail(dot)com]
> Unfortunately, I think a full solution to the problem of allocating AV
> workers to avoid wraparound is quite complex.
Yes, that easily puts my small brain into an infinite loop...
> Given all of the foregoing this seems like a very hard problem. I can't
> even articulate a clear set of rules for what our priorities should be,
> and it seems that such rules would depend on the rate at which we're consuming
> XIDs, how close we are in each database to a wraparound shutdown, what tables
> exist in each database, how big the not-all-frozen part of each one is,
> how big their indexes are, how much they're holding back relfrozenxid, and
> which ones already have workers, among other things. I think it's quite
> possible that we can come up with something that's better than what we have
> now without embarking on a huge project, but it's not going to be anywhere
> near perfect because this is really complicated, and there's a real risk
> that we'll just making some cases better and others worse rather than
> actually coming out ahead overall.
So a simple improvement would be to assign workers fairly to databases facing a wraparound risk, as Sawada-san suggested.
One ultimate solution should be the undo-based MVCC that makes vacuuming unnecessary, which you proposed about a year ago...
Regards
Takayuki Tsunakawa
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