Horton

Re: [HACKERS] pgsql: Fix freezing of a dead HOT-updated tuple

Peter Geoghegan <pg@bowt.ie>

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From: Peter Geoghegan <pg@bowt.ie>
To: Andres Freund <andres@anarazel.de>
Cc: Alvaro Herrera <alvherre@alvh.no-ip.org>, "Wood, Dan" <hexpert@amazon.com>, pgsql-committers <pgsql-committers@postgresql.org>, "Wong, Yi Wen" <yiwong@amazon.com>, Michael Paquier <michael.paquier@gmail.com>, Robert Haas <robertmhaas@gmail.com>, Tom Lane <tgl@sss.pgh.pa.us>, PostgreSQL Hackers <pgsql-hackers@postgresql.org>
Date: 2017-11-10T01:13:40Z
Lists: pgsql-hackers 
On Thu, Nov 9, 2017 at 4:53 PM, Andres Freund <andres@anarazel.de> wrote:
> Primarily because it's not an anti-corruption tool. I'd be surprised if
> there weren't ways to corrupt the page using these corruptions that
> aren't detected by it.

It's very hard to assess the risk of missing something that's actually
detectable with total confidence, but I think that the check is actually
very thorough.

> But even if it were, I don't think there's
> enough information to do so in the general case. You very well can end
> up with pages where subsequent hot pruning has removed a good bit of the
> direct evidence of this bug.

Sure, but maybe those are cases that can't get any worse anyway. So the
question of avoiding making it worse doesn't arise.

> But I'm not really sure why the error detection capabilities of matter
> much for the principal point I raised, which is how much work we need to
> do to not further worsen the corruption.

You're right. Just trying to put the risk in context, and to understand the
extent of the concern that you have.

-- 
Peter Geoghegan

Commits

  1. Backport addition of rs_old_rel to rewriteheap's state.

  2. Perform a lot more sanity checks when freezing tuples.

  3. Revert bogus fixes of HOT-freezing bug

  4. Fix traversal of half-frozen update chains

  5. Fix freezing of a dead HOT-updated tuple

  6. During index build, check and elog (not just Assert) for broken HOT chain.

  7. Fix WAL replay of locking an updated tuple

  8. Change the way we mark tuples as frozen.