| From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
|---|---|
| To: | Peter Eisentraut <peter_e(at)gmx(dot)net> |
| Cc: | Marcin Mańk <marcin(dot)mank(at)gmail(dot)com>, Robert Haas <robertmhaas(at)gmail(dot)com>, Kyotaro HORIGUCHI <horiguchi(dot)kyotaro(at)oss(dot)ntt(dot)co(dot)jp>, "pgsql-hackers(at)postgresql(dot)org" <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: [v9.2] make_greater_string() does not return a string in some cases |
| Date: | 2011-09-26 14:08:54 |
| Message-ID: | 5358.1317046134@sss.pgh.pa.us |
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| Lists: | pgsql-bugs pgsql-hackers |
Peter Eisentraut <peter_e(at)gmx(dot)net> writes:
> On fre, 2011-09-23 at 20:35 +0300, Marcin Mak wrote:
>> One idea:
>> col like 'foo%' could be translated to col >= 'foo' and col <= foo || 'zzz' , where 'z' is the largest possible character. This should be good enough for calculating stats.
>> How to find such a character, i do not know.
> That's what makes this so difficult.
> If we knew the largest character, we could probably also find the
> largest-1, largest-2, etc. characters and determine the total order of
> everything.
No, it's a hundred times worse than that, because in collations other
than C there typically *is* no total order. The collation behavior of
many characters is context-sensitive, thanks to the multi-pass behavior
of typical "dictionary" algorithms.
regards, tom lane
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