Thread: Re: Add CASEFOLD() function.
Hi 2024年12月12日(木) 18:00 Jeff Davis <pgsql@j-davis.com>: > > Unicode case folding is a way to convert a string to a canonical case > for the purpose of case-insensitive matching. > > Users have long used LOWER() for that purpose, but there are a few edge > case problems: > > * Some characters have more than two cased forms, such as "Σ" (U+03A3), > which can be lowercased as "σ" (U+03C3) or "ς" (U+03C2). The CASEFOLD() > function converts all cased forms of the character to "σ". > > * The character "İ" (U+0130, capital I with dot) is lowercased to "i", > which can be a problem in locales that don't expect that. > > * If new lower case characters are added to Unicode, the results of > LOWER() may change. > > The CASEFOLD() function solves these problems. > > Patch attached. I took a quick look at this as it sounds useful for the described issue, and it seems to work as advertised, except the function is named "FOLDCASE()" in the patch, so I'm wondering which is intended? A quick search indicates there are no functions of either name in other databases; Python has a "casefold()" function [1] and PHP a "foldCase()" function [2], so it doesn't seem there's a de-facto standard for this. [1] https://docs.python.org/3/library/stdtypes.html#str.casefold [2] https://www.php.net/manual/en/intlchar.foldcase.php Regards Ian Barwick
On Thu, 2024-12-12 at 21:52 +0900, Ian Lawrence Barwick wrote: > and it seems to work as advertised, except the function is named > "FOLDCASE()" > in the patch, so I'm wondering which is intended? Thank you for looking into this, I went back and forth on the name, and mistyped it a few times. ICU seems to use "foldcase": https://unicode-org.github.io/icu-docs/apidoc/dev/icu4c/ucasemap_8h.html and it seems to be slightly more proper grammatically (with "fold" being the verb). However, "case folding" is common terminology in Postgres and Unicode, so "casefold" can be seen as the verb instead. I don't have a strong opinion here so I will just go with whatever seems like the popular choice. Regards, Jeff Davis
On 12/12/24 13:30, Jeff Davis wrote: > On Thu, 2024-12-12 at 21:52 +0900, Ian Lawrence Barwick wrote: >> and it seems to work as advertised, except the function is named >> "FOLDCASE()" >> in the patch, so I'm wondering which is intended? > > Thank you for looking into this, I went back and forth on the name, and > mistyped it a few times. > > ICU seems to use "foldcase": > > https://unicode-org.github.io/icu-docs/apidoc/dev/icu4c/ucasemap_8h.html > > and it seems to be slightly more proper grammatically (with "fold" > being the verb). However, "case folding" is common terminology in > Postgres and Unicode, so "casefold" can be seen as the verb instead. > > I don't have a strong opinion here so I will just go with whatever > seems like the popular choice. FWIW I prefer casefold() -- Joe Conway PostgreSQL Contributors Team RDS Open Source Databases Amazon Web Services: https://aws.amazon.com
On 12/16/24 12:49, Jeff Davis wrote:
> One question I have is whether we want this function to normalize the
> output.
>
> I believe most usecases would want the output normalized, because
> normalization differences (e.g. "a" U+0061 followed by "combining
> acute" U+0301 vs "a with acute" U+00E1) are more minor than differences
> in case.
>
> Of course, a user could wrap it with the normalize() function, but
> that's verbose and easy to forget. I'm also not sure that it can be
> made as fast as a combined function that does both.
Perhaps a one arg version that always casefolds and a two arg version
that accepts nfc, nfd, none (or something similar)?
> And a follow-up question: if it does normalize, the second parameter
> would be the requested normal form. But to accept the keyword forms
> (NFC, NFD in gram.y) rather than the string forms ('NFC', 'NFD') then
> we'd need to also need to add CASEFOLD to gram.y (like NORMALIZE). Is
> that a reasonable thing to do?
SQL 2023 seems to include the NORMALIZE syntax, but the only case
folding considered is UPPER and LOWER. As such, I think it ought to be a
function but not part of the grammar.
--
Joe Conway
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com
On Mon, 2024-12-16 at 16:27 -0500, Joe Conway wrote:
>
> SQL 2023 seems to include the NORMALIZE syntax, but the only case
> folding considered is UPPER and LOWER. As such, I think it ought to
> be a
> function but not part of the grammar.
Should the standard support something like the Unicode idea of case
folding? If so, do we need to be careful of conflicts?
Regards,
Jeff Davis
On 16.12.24 18:49, Jeff Davis wrote:
> One question I have is whether we want this function to normalize the
> output.
>
> I believe most usecases would want the output normalized, because
> normalization differences (e.g. "a" U+0061 followed by "combining
> acute" U+0301 vs "a with acute" U+00E1) are more minor than differences
> in case.
Can you explain this in further detail? I don't quite follow why this
would be required.
> Of course, a user could wrap it with the normalize() function, but
> that's verbose and easy to forget. I'm also not sure that it can be
> made as fast as a combined function that does both.
>
> And a follow-up question: if it does normalize, the second parameter
> would be the requested normal form. But to accept the keyword forms
> (NFC, NFD in gram.y) rather than the string forms ('NFC', 'NFD') then
> we'd need to also need to add CASEFOLD to gram.y (like NORMALIZE). Is
> that a reasonable thing to do?
That's maybe one reason to keep it separate.
Another might be that's not entirely clear how this should work in
encodings other than UTF-8. For example, the normalized string might
not be representable in the encoding.
On Thu, 2024-12-19 at 17:18 +0100, Peter Eisentraut wrote:
> Can you explain this in further detail? I don't quite follow why
> this
> would be required.
I am unsure now.
My initial reasoning was based on the idea that users would want to use
CASEFOLD(t) in a unique expression index as an improvement over
LOWER(t). And if you do that, you'd be surprised if some equivalent
strings ended up in the index. I don't think that's a huge problem,
because in other contexts we leave it up to the user to keep things
normalized consistently, and a CHECK(t IS NFC NORMALIZED) is a good way
to do that.
But there's a problem: full case folding doesn't preserve the normal
form, so even if the input is NFC normalized, the output might not be.
If we solve this problem, then we can just say that CASEFOLD()
preserves the normal form, consistently with how the spec defines
LOWER()/UPPER(), and I think that would be the best outcome.
I'm not sure if that problem is solvable, though, because what if the
input string is in both NFC and NFD, how do we know which normal form
to preserve?
We could tell users to use an expression index on
NORMALIZE(CASEFOLD(t)) instead, but that feels like inefficient
boilerplate.
>
> Another might be that's not entirely clear how this should work in
> encodings other than UTF-8. For example, the normalized string might
> not be representable in the encoding.
That's a good point.
Regards,
Jeff Davis