Re: Make HeapTupleSatisfiesMVCC more concurrent

Tom Lane wrote:
> Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> writes:

> > I am wondering that is there any harm in calling TransactionIdDidAbort()
> > in slow path before calling SubTransGetTopmostTransaction(), that can
> > also maintain consistency of checks in both the functions?
>
> I think this is probably a bad idea. It adds a pg_clog lookup that we
> would otherwise not do at all, in hopes of avoiding a pg_subtrans lookup.
> It's not exactly clear that that's a win even if we successfully avoid
> the subtrans lookup (which we often would not). And even if it does win,
> that would only happen if the other transaction has aborted, which isn't
> generally the case we prefer to optimize for.

It's probably key to this idea that TransactionIdDidAbort returns in a
single slru lookup, whereas SubTransGetTopmostTransaction needs to
iterate possibly many layers of subxacts. But the point about this
being a win only for aborted xacts makes it probably pointless, I agree.

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