Re: Re: BUG #12990: Missing pg_multixact/members files (appears to have wrapped, then truncated)

Thomas Munro wrote:

> I think I see why, and I think it's a bug:  if you vacuum freeze all
> your databases, MultiXactState->oldestMultiXactId finishes up equal to
> MultiXactState->nextMXact.

Uhm, that's rather silly.

> But that's not actually a multixact that exists yet, so when when
> DetermineSafeOldestOffset calls find_multixact_start, it reads a
> garbage offset (all zeros in practice since pages start out zeroed)
> and produces a garbage value for offsetStopLimit which might
> incorrectly stop you from creating any more multixacts even though
> member space is entirely empty (but it depends on where your
> nextOffset happens to be at the time).

Right.

> I think the fix is something like "if nextMXact == oldestMultiXactId,
> then there are no active multixacts, so the offsetStopLimit should be
> set to nextOffset - (a segment's worth)".

Makes sense.

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Álvaro Herrera                http://www.2ndQuadrant.com/
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