From: Tom Lane <tgl@sss.pgh.pa.us>
To: Gregory Stark <stark@enterprisedb.com>
To: Guillaume Smet <guillaume.smet@gmail.com>
To: pgsql-performance <pgsql-performance@postgresql.org>
To: pgsql-hackers@postgresql.org
Subject: Re: [HACKERS] Estimation problem with a LIKE clause containing a /
Date: Thu, 08 Nov 2007 21:08:34 -0500
Message-ID: <28300.1194574114@sss.pgh.pa.us> (raw)
In-Reply-To: <25287.1194568820@sss.pgh.pa.us>
References: <1d4e0c10711070453q214f089cpd144cbb5193d3c4f@mail.gmail.com>
<17767.1194453292@sss.pgh.pa.us>
<11467.1194477248@sss.pgh.pa.us>
<1d4e0c10711080208n6c9875b4tf5882da3ced18532@mail.gmail.com>
<27405.1194542548@sss.pgh.pa.us>
<87prykpc3q.fsf@oxford.xeocode.com>
<25287.1194568820@sss.pgh.pa.us>
I wrote:
> I did do some experimentation and found that among the ASCII characters
> (ie, codes 32-126), nearly all the non-C locales on my Fedora machine
> sort Z last and z next-to-last or vice versa. Most of the remainder
> sort digits last and z or Z as the last non-digit character. Since Z is
> not that close to the end of the sort order in C locale, however, z
> seems the best bet.
With still further experimentation, it seems that doesn't work very
well, because the locales that sort digits last also seem not to
discriminate against digits in their first pass. What did seem to work
was:
* Determine which of the strings "Z", "z", "y", "9" is seen as largest
by strcoll().
* Append this string to the given input.
* Search (using the CVS-HEAD make_greater_string logic) for a string
greater than that.
This rule works for all the locales I have installed ... but I don't
have any Far Eastern locales installed. Also, my test cases are only
covering ASCII characters, and I believe many locales have some non-ASCII
letters that sort after 'Z'. I'm not sure how hard we need to try to
cover those corner cases, though. It is ultimately only an estimate...
regards, tom lane
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To: pgsql-performance@postgresql.org
Cc: tgl@sss.pgh.pa.us, stark@enterprisedb.com, guillaume.smet@gmail.com, pgsql-hackers@postgresql.org
Subject: Re: [HACKERS] Estimation problem with a LIKE clause containing a /
In-Reply-To: <28300.1194574114@sss.pgh.pa.us>
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