Re: MultiXact pessmization in 9.3

Andres Freund wrote:

> While looking at the multixact truncation code (mail nearby), I've
> noticed that there's a significant difference in the way multixact
> members are accessed since fkey locks were introduced:
>
> <9.3 when heap_lock_tuple finds a XMAX_IS_MULTI tuple it executes
> MultiXactIdWait() which in turn uses GetMultiXactIdMembers() to get the
> xids to wait for. But it skips the lookup if the mxid we lookup is older
> than OldestVisibleMXactId.
>
> 9.3+ instead always looks up the members because GetMultiXactIdMembers()
> is also used in cases where we need to access old xids (to check whether
> members have commited in non-key updates).

But HeapTupleSatisfiesUpdate(), called at the top of heap_lock_tuple,
has already called MultiXactIdIsRunning() (which calls GetMembers)
before that's even considered. So the call heap_lock_tuple should have
members obtained from the multixact.c cache.

Am I misunderstanding which code path you mean?

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