Re: Making the C collation less inclined to abort abbreviation

On Sun, Nov 29, 2015 at 4:02 PM, Peter Geoghegan <pg(at)heroku(dot)com> wrote:
> The C collation is treated exactly the same as other collations when
> considering whether the generation of abbreviated keys for text should
> continue. This doesn't make much sense. With text, the big cost that
> we are concerned about going to waste should abbreviated keys not
> capture sufficient entropy is the cost of n strxfrm() calls. However,
> the C collation doesn't use strxfrm() -- it uses memcmp(), which is
> far cheaper.
>
> With other types, like numeric and now UUID, the cost of generating an
> abbreviated key is significantly lower than text when using collations
> other than the C collation. Their cost models reflect this, and abort
> abbreviation far less aggressively than text's, even though the
> trade-off is very similar when text uses the C collation.
>
> Attached patch fixes this inconsistency by making it significantly
> less likely that abbreviation will be aborted when the C collation is
> in use. The behavior with other collations is unchanged. This should
> be backpatched to 9.5 as a bugfix, IMV.

Could you provide a test case where this change is a winner for the C
locale but a loser for some other locale?

--
Robert Haas
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