Re: Avoid sorting when doing an array_agg

2016-12-04 23:12 GMT+01:00 Peter Geoghegan <pg(at)bowt(dot)ie>:

> On Sat, Dec 3, 2016 at 5:20 PM, Tomas Vondra
> <tomas(dot)vondra(at)2ndquadrant(dot)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.
>

it is pretty good idea.

Regards

Pavel

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> Peter Geoghegan
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