| From: | Andres Freund <andres(at)anarazel(dot)de> |
|---|---|
| To: | Peter Geoghegan <pg(at)bowt(dot)ie> |
| Cc: | Alvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org>, "Wood, Dan" <hexpert(at)amazon(dot)com>, pgsql-committers <pgsql-committers(at)postgresql(dot)org>, "Wong, Yi Wen" <yiwong(at)amazon(dot)com>, Michael Paquier <michael(dot)paquier(at)gmail(dot)com>, Robert Haas <robertmhaas(at)gmail(dot)com>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, PostgreSQL Hackers <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: [COMMITTERS] pgsql: Fix freezing of a dead HOT-updated tuple |
| Date: | 2017-11-10 00:17:18 |
| Message-ID: | 20171110001718.rpgx4lf6xxi4ge3l@alap3.anarazel.de |
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On 2017-11-09 16:02:17 -0800, Peter Geoghegan wrote:
> > What I'm currently wondering about is how much we need to harden
> > postgres against such existing corruption. If e.g. the hot chains are
> > broken somebody might have reindexed thinking the problem is fixed - but
> > if they then later vacuum everything goes to shit again, with dead rows
> > reappearing.
>
> I don't follow you here. Why would REINDEXing make the rows that
> should be dead disappear again, even for a short period of time?
It's not the REINDEX that makes them reappear. It's the second
vacuum. The reindex part was about $user trying to fix the problem...
As you need two vacuums with appropriate cutoffs to hit the "rows
revive" problem, that'll often in practice not happen immediately.
> Actually, on second thought, I take that back -- I don't think that
> REINDEXing will even finish once a HOT chain is broken by the bug.
> IndexBuildHeapScan() actually does quite a good job of making sure
> that HOT chains are sane, which is how the enhanced amcheck notices
> the bug here in practice.
I think that's too optimistic.
Greetings,
Andres Freund
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