Re: Unicode combining characters

Can I ask about the status of this?


> Hi all,
> 
> while working on a new project involving PostgreSQL and making some
> tests, I have come up with the following output from psql :
> 
>  lang | length | length |   text    |   text
> ------+--------+--------+-----------+-----------
>  isl  |      7 |      6 | _l_ta | _leit
>  isl  |      7 |      7 | _l_ta | _litum
>  isl  |      7 |      7 | _l_ta | _liti_
>  isl  |      5 |      4 | ma_ur    | mann
>  isl  |      5 |      7 | ma_ur    | m_nnum
>  isl  |      5 |      5 | ma_ur    | manna
>  isl  |      5 |      4 | _ska    | -a_i
> 
> [the misalignment is what I got, it's not a copy-paste error]
> 
> This is pasted from a UTF-8 xterm running psql under a UTF-8 locale,
> querying a database created with -E UNICODE (by the way, these are
> icelandic words :) ).
> 
> What you see above is misleading, since it's not possible to see that
> '_', '_', '_' and '_' are using combining marks, while '_' is not.
> 
> As a reminder, a combining mark in Unicode is that _ is actually
> encoded as a + ' (where ' is the acute combining mark).
> 
> Encoded in UTF-8, it's then <61 cc 81> [UTF16: 0061 0301],
> instead of <c3 a1> [UTF16: 00E1].
> 
> The "length" fields are what is returned by length(a.text) and
> length(b.text).
> 
> So, this shows two problems :
> 
> - length() on the server side doesn't handle correctly Unicode [I have
>   the same result with char_length()], and returns the number of chars
>   (as it is however advertised to do), rather the length of the
>   string.
> 
> - the psql frontend makes the same mistake.
> 
> I am using version 7.1.3 (debian sid), so it may have been corrected
> in the meantime (in this case, I apologise, but I have only recently
> started again to use PostgreSQL and I haven't followed -hackers long
> enough).
> 
> 
> => I think fixing psql shouldn't be too complicated, as the glibc
> should be providing the locale, and return the right values (is this
> the case ? and what happens for combined latin + chinese characters
> for example ? I'll have to try that later). If it's not fixed already,
> do you want me to look at this ? [it will take some time, as I haven't
> set up any development environment for postgres yet, and I'm away for
> one week from thursday].
> 
> => regarding the backend, it may be more complex, as the underlaying
> system may not provide any UTF-8 locale to use (!= from being UTF-8
> aware : an administrator may have decided that UTF-8 locales are
> useless on a server, as only root connections are made, and he wants
> only the C locale on the console - I've seen that quite often ;) ).
> 
> 
> This brings me to another subject : I will need to support the full
> Unicode collation algorithm (UCA, as described in TR#10 [1] of the
> Unicode consortium), and I will need to be able to sort according to
> locales which may not be installed on the backend server (some of
> which may not even be recognised by GNU libc, which supports already
> more than 140 locales -- artificial languages would be an example). I
> will also need to be able to normalise the unicode strings (TR#15 [2])
> so that I don't have some characters in legacy codepoints [as 00E1
> above], and others with combining marks.
> 
> There is today an implementation in perl of the needed functionality,
> in Unicode::Collate and Unicode::Normalize (which I haven't tried yet
> :( ). But as they are Perl modules, the untrusted version of perl,
> plperlu, will be needed, and it's a pity for what I consider a core
> functionality in the future (not that plperlu isn't a good thing - I
> can't wait for it ! - but that an untrusted pl language is needed to
> support normalisation and collation).
> 
> Note also that there are a lot of data associated with these
> algorithms, as you could expect.
> 
> I was wondering if some people have already thought about this, or
> already done something, or if some of you are interested in this. If
> nobody does anything, I'll do something eventually, probably before
> Christmas (I don't have much time for this, and I don't need the
> functionality right now), but if there is an interest, I could team
> with others and develop it faster :)
> 
> Anyway, I'm open to suggestions :
> 
> - implement it in C, in the core,
> 
> - implement it in C, as contributed custom functions,
> 
> - implement it in perl (by reusing Unicode:: work), in a trusted plperl,
> 
> - implement it in perl, calling Unicode:: modules, in an untrusted
>   plperl.
> 
> and then :
> 
> - provide the data in tables (system and/or user) - which should be
>   available across databases,
> 
> - load the data from the original text files provided in Unicode (and
>   other as needed), if the functionality is compiled into the server.
> 
> - I believe the basic unicode information should be standard, and the
>   locales should be provided as contrib/ files to be plugged in as
>   needed.
> 
> I can't really accept a solution which would rely on the underlaying
> libc, as it may not provide the necessary locales (or maybe, then,
> have a way to override the collating tables by user tables - actually,
> this would be certainly the best solution if it's in the core, as the
> tables will put an extra burden on the distribution and the
> installation footprint, especially if the tables are already there,
> for glibc, for perl5.6+, for other software dealing with Unicode).
> 
> The main functions I foresee are :
> 
> - provide a normalisation function to all 4 forms,
> 
> - provide a collation_key(text, language) function, as the calculation
>   of the key may be expensive, some may want to index on the result (I
>   would :) ),
> 
> - provide a collation algorithm, using the two previous facilities,
>   which can do primary to tertiary collation (cf TR#10 for a detailed
>   explanation).
> 
> I haven't looked at PostgreSQL code yet (shame !), so I may be
> completely off-track, in which case I'll retract myself and won't
> bother you again (on that subject, that is ;) )...
> 
> Comments ?
> 
> 
> Patrice.
> 
> [1] http://www.unicode.org/unicode/reports/tr10/
> 
> [2] http://www.unicode.org/unicode/reports/tr15/
> 
> -- 
> Patrice H_D_ ------------------------------- patrice _ islande.org -----
>   --  Isn't it weird  how scientists  can imagine  all the matter of the
> universe exploding out of a dot smaller than the head of a pin, but they
> can't come up with a more evocative name for it than "The Big Bang" ?
>   -- What would _you_ call the creation of the universe ?
>   -- "The HORRENDOUS SPACE KABLOOIE !"               - Calvin and Hobbes
> ------------------------------------------ http://www.islande.org/ -----
> 
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