Re: Per-column collation

From: Peter Eisentraut <peter_e(at)gmx(dot)net>
To: Itagaki Takahiro <itagaki(dot)takahiro(at)gmail(dot)com>
Cc: pgsql-hackers(at)postgresql(dot)org
Subject: Re: Per-column collation
Date: 2010-11-24 20:22:15
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On mån, 2010-11-22 at 11:58 +0900, Itagaki Takahiro wrote:
> * Did you see any performance regression by collation?
> I found a bug in lc_collate_is_c(); result >= 0 should be
> checked before any other checks. SearchSysCache1() here
> would be a performance regression.

That code turned out to be buggy anyway, because it was using the
"result" cache variable independent of the collation parameter.

I did some profiling with this now. The problem is that this function
lc_collate_is_c() would need to cache the "C-ness" property for any
number of collations. Depending on what call pattern you expect or want
to optimize for, you might end up caching most of the pg_collation
catalog, which is actually the mandate of SearchSysCache, but the
profile shows that SearchSysCache takes a large chunk of the additional
run time.

If I remove that branch altogether, that is, don't treat the C locale
specially at all in the nondefault collation case, then using non-C
locales as nondefault collation is almost as fast as using non-C locales
as default location. However, using the C locale as a nondefault
collation would then be quite slow (still faster that non-C locales).

The solution would perhaps be a custom, lightweight caching system, but
I haven't thought of one yet.

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