Horton

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

Andres Freund <andres@anarazel.de>

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From: Andres Freund <andres@anarazel.de>
To: Alvaro Herrera <alvherre@2ndQuadrant.com>
Cc: "Wood, Dan" <hexpert@amazon.com>, Peter Geoghegan <pg@bowt.ie>, 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>, pgsql-hackers@postgresql.org
Date: 2017-12-07T20:17:06Z
Lists: pgsql-hackers 
Hi,

On 2017-12-06 13:21:15 -0300, Alvaro Herrera wrote:
> I think you've done a stellar job of identifying what the actual problem
> was.  I like the new (simpler) coding of that portion of
> HeapTupleSatisfiesVacuum.

Thanks!

> freeze-the-dead is not listed in isolation_schedule; an easy fix.

Yea, I'd sent an update about that, stupidly forgot git amend the
commit...


> I confirm that the test crashes with an assertion failure without the
> code fix, and that it doesn't with it.
> 
> I think the comparison to OldestXmin should be reversed:
> 
> 			if (!TransactionIdPrecedes(xmax, OldestXmin))
> 				return HEAPTUPLE_RECENTLY_DEAD;
> 
> 			return HEAPTUPLE_DEAD;
> 
> This way, an xmax that has exactly the OldestXmin value will return
> RECENTLY_DEAD rather DEAD, which seems reasonable to me (since
> OldestXmin value itself is supposed to be still possibly visible to
> somebody).

Yes, I think you're right. That's a bug.


> Your commit message does a poor job of acknowledging prior work on
> diagnosing the problem starting from Dan's initial test case and patch.

Yea, you're right. I was writing it with 14h of jetlag, apparently that
does something to your brain...

Greetings,

Andres Freund

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.