public inbox for [email protected]  

help / color / mirror / Atom feed

From: Jeff Davis <[email protected]>
To: Peter Eisentraut <[email protected]>
To: Joe Conway <[email protected]>
To: Ian Lawrence Barwick <[email protected]>
Cc: [email protected]
Subject: Re: Add CASEFOLD() function.
Date: Thu, 19 Dec 2024 09:51:32 -0800
Message-ID: <[email protected]> (raw)
In-Reply-To: <[email protected]>
References: <[email protected]>
	<CAB8KJ=jgnpbDTJS+jOQMzo7b1mPTwM0hCqQBBRgoX_kvyxWVog@mail.gmail.com>
	<[email protected]>
	<[email protected]>
	<[email protected]>
	<[email protected]>

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

view thread (30+ messages)  latest in thread

reply

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Reply to all the recipients using the --to and --cc options:
  reply via email

  To: [email protected]
  Cc: [email protected], [email protected], [email protected], [email protected]
  Subject: Re: Add CASEFOLD() function.
  In-Reply-To: <[email protected]>

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

This inbox is served by agora; see mirroring instructions
for how to clone and mirror all data and code used for this inbox