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Re: Index problem

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Patrik Kudo <kudo(at)partitur(dot)se>
Cc: pgsql-sql(at)postgresql(dot)org, girgen(at)partitur(dot)se, Peter Eisentraut <peter_e(at)gmx(dot)net>
Subject: Re: Index problem
Date: 2000-11-15 18:19:45
Message-ID: 13864.974312385@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-sql
Patrik Kudo <kudo(at)partitur(dot)se> writes:
> select * from elever where userid like 'walth%'; <-- Not OK!

> Droping and recreating the index solves the problem, but that's not
> good enough for me since the problem has reoccured on a different
> machine with a different database. vacuum and vacuum analyze does not
> report any problem with the table.

> Both times the problem occured with userid's starting with a
> "w". Postgres is running with a Swedish locale, and on FreeBSD this
> means that "w" and "v" (among a number of other letter) are treated
> equally when collating/sorting. I suppose this could be part of the
> problem.

> Is this a known problem? If so, is it fixed in 7.0.2? I've not seen
> this happen on any of our postgres 7.0.2 systems, but as I can't even
> reproduce it on the 6.5.3, that's no guarantee it's fixed...

Hmm.  I can think of two known issues that may be relevant.  First off,
there are indeed known problems with LIKE index optimization in
non-ASCII locales.  I believe that 7.0 fixes the problems for locales
that just have a non-ASCII sort order of individual characters, but
we've since realized that it can still do the wrong thing in locales
where there are special rules for handling multi-character sequences.
I don't know the Swedish rules offhand, so don't know if that's a
problem for you.

However, a LIKE optimization problem would not be fixed by dropping and
recreating the index.  This makes me think you are being bit by the
other issue: if you compile with LOCALE support then it is critical that
the postmaster *always* be started with the same LOCALE settings (at
least the same LC_COLLATE value).  If LOCALE varies that means your
text sort ordering varies, which means that indexes on text columns may
appear out-of-order, which causes index searches to miss entries they
should have found.  If you insert entries under different LOCALE
settings then you may wind up with an index that is not consistent with
*any* single LOCALE; the only cure for that is to drop and rebuild the
index.

Unfortunately, it's way too easy to get bit by this bug.  The most
common error is to start the postmaster by hand from a shell account
whose LOCALE environment is different from what's supplied when the
postmaster is started from a boot-time script.  Best bet is to set
the correct LOCALE values in a wrapper script that you use in both
cases.

We have talked about fixing this by saving the active LOCALE variables
at initdb time, and having the postmaster adopt those values whenever
it's started.  But it hasn't got done yet.  (Peter, would it be
easy to make GUC handle this?  There'd need to be some way to cause
guc.c to do a putenv() ...)

			regards, tom lane

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