Re: Avoid sorting when doing an array_agg

On Sat, Dec 3, 2016 at 5:20 PM, Tomas Vondra
<tomas.vondra@2ndquadrant.com> wrote:
> So the sort is probably slow because of CPU, as it compares strings. In
> some locales that may be very expensive - not sure which locale is used
> in this case, as it was not mentioned.

I wonder what it would take to teach the optimizer to consider the
possibility of a "collation strength reduction". In other words, for
aggregates that perform a sort (or for aggregates that rely on the
presence of a sort node without there being some other dependency on
the sort node), it should be possible for the optimizer to determine
that it would be just fine to use the C locale, since the user isn't
entitled to assume anything about the exact sort order. There are of
course cases where this can make a huge difference.

--
Peter Geoghegan


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