On 20 February 2017 at 17:32, Robert Haas <robertmh...@gmail.com> wrote:

>>> Have you checked whether this
>>> patch makes any noticeable performance difference?
>> No, but then we're reducing the number of calls to PgXact directly;
>> there is no heuristic involved, its just a pure saving.
> Well, it's adding a branch where there wasn't one.

A branch that is avoided in almost all cases, so easy to predict.

> Maybe that costs
> essentially nothing and the saved write to shared memory saves
> something noticeable, but for all I know it's the reverse.  If I had
> to guess, it would be that neither the costs nor the savings from this
> are in the slightest way noticeable on a macrobenchmark, and therefore
> there's not much point in changing it, but that could be 100% wrong.

Given Andres' earlier measurements, it seems worth testing to me.

Hopefully someone can recheck. Thanks in advance.

Simon Riggs                http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

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