Re: Patch for collation using ICU

--On fredag, maj 06, 2005 22.57.59 -0400 Bruce Momjian
<pgman(at)candle(dot)pha(dot)pa(dot)us> wrote:

>
> Is this patch ready for application?
>
> http://people.freebsd.org/~girgen/postgresql-icu/pg-802-icu-2005-05-06.d
> iff.gz
>
> The web site is:
>
> http://people.freebsd.org/~girgen/postgresql-icu/readme.html

I don't think so, not quite. I have not had any positive reports from linux
users, this is only tested in a FreeBSD environment. I'd say it needs some
more testing.

Also, apparently, ICU is installed by default in many linux distributions,
and usually it is version 2.8. Some linux users have asked me if there are
plans for a patch that works with ICU 2.8. That's probably a good idea. IBM
and the ICU folks seem to consider 3.2 to be the stable version, older
versions are hard to find on their sites, but most linux distributers seem
to consider it too bleeding edge, even gentoo. I don't know why they don't
agree.

> I do have a few questions:
>
> Why don't you use the lc_ctype_is_c() part of this test?
>
> if (pg_database_encoding_max_length() > 1 && !lc_ctype_is_c())

Um, well, I didn't think about that. :) What would be the locale in this
case? c_C.UTF-8? ;) Hmm, it is possible to have CTYPE=C and use a wide
encoding, indeed. Then the strings will be handled like byte-wide chars.
Yeah, it's a bug. I'll fix it! Thanks.

> Why is so much code added, for example, in lower()? The existing
> multibyte code is much smaller, and lots of code is added in other
> places too.

ICU uses UTF-16 internally, so all strings must be converted from the
database encoding to UTF-16. Since that means the strings need to be
copied, I took the same approach as in varlena.c:varstr_cmp(), where small
strings use the heap and only larger strings use a palloc. Comments in
varstr_cmp about performance made me use that approach.

Also, in the latest patch, I also added checks and logging for *every*
status returned from ICU. I hope this will help debugging on debian, where
previous version didn't work. That excessive status checking is hardly be
necessary once the stuff is better tested.

I think the string copying and heap/palloc choices stands for most of the
code bloat, together with the excessive status checking and logging.

> Why do you need to add a mapping of encoding names from iana to our
> names?

This was already answered by John Hansen... There's an old thread here
about the choice of the name "UNICODE" to describe an encoding, which it
doesn't. There's half a dozen unicode based encodings... UTF-8 is used by
postgresql, that would have been a better name... Similarly for most other
encodings, really. ICU expect a setlocale(3) string (i.e. IANA). PostgreSQL
can't provide it, so a mapping table is required.

I use this patch in production on one FreeBSD 4.10 server at the moment.
With the latest version, I've had no problems. Logging is swithed on for
now, and it shows no signs of ICU complaining. I'd like more reports on
Linux, though.

/Palle

>
> -------------------------------------------------------------------------
> --
>
> Palle Girgensohn wrote:
>> Hi!
>>
>> I've put together a patch for using IBM's ICU package for collation.
>>
>> If your OS does not have full support for collation ur
>> uppercase/lowercase in multibyte locales, this might be useful. If you
>> are using a multibyte character encoding in your database and want
>> collation, i.e. order by, and also lower(), upper() and initcap() to
>> work properly, this patch will do just that.
>>
>> This patch is needed for FreeBSD, since this OS has no support for
>> collation of for example unicode locales (that is, wcscoll(3) does not
>> do what you expect if you set LC_ALL=sv_SE.UTF-8, for example). AFAIK
>> the patch is *not* necessary for Linux, although IBM claims ICU
>> collation to be about twice as fast as glibc for simple western locales.
>>
>> It adds a configure switch, `--with-icu', which will set up the code to
>> use ICU instead of wchar_t and wcscoll.
>>
>> This has been tested only on FreeBSD-4.11 & FreeBSD-5-stable, where it
>> seems to run well. I've not had the time to do any comparative
>> performance tests yet, but it seems it is at least not slower than
>> using LATIN1 with sv_SE.ISO8859-1 locale, perhaps even faster.
>>
>> I'd be delighted if some more experienced postgresql hackers would
>> review this stuff. The patch is pretty compact, so it's fast reading :)
>> I'm planning to add this patch as an option (tagged "experimental") to
>> FreeBSD's postgresql port. Any ideas about whether this is a good idea
>> or not?
>>
>> Any thoughts or ideas are welcome!
>>
>> Cheers,
>> Palle
>>
>> Patch at:
>> <http://people.freebsd.org/~girgen/postgresql-icu/pg-801-icu-2005-03-14.
>> diff>
>>
>> ICU at sourceforge: <http://icu.sf.net/>
>>
>>
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>
> --
> Bruce Momjian | http://candle.pha.pa.us
> pgman(at)candle(dot)pha(dot)pa(dot)us | (610) 359-1001
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