Re: Built-in CTYPE provider

On Tue, 2024-03-26 at 08:14 +0100, Peter Eisentraut wrote:
>
> Full vs. simple case mapping is more of a legacy compatibility
> question,
> in my mind.  There is some expectation/precedent that C.UTF-8 uses
> simple case mapping, but beyond that, I don't see a reason why
> someone
> would want to explicitly opt for simple case mapping, other than if
> they
> need length preservation or something, but if they need that, then
> they
> are going to be in a world of pain in Unicode anyway.

I mostly agree, though there are some other purposes for the simple
mapping:

* a substitute for case folding: lower() with simple case mapping will
work better for that purpose than lower() with full case mapping (after
we have casefold(), this won't be a problem)

* simple case mapping is conceptually simpler, and that's a benefit by
itself in some situations -- maybe the 1:1 assumption exists other
places in their application

> > There's also another reason to consider it an argument rather than
> > a
> > collation property, which is that it might be dependent on some
> > other
> > field in a row. I could imagine someone wanting to do:
> >
> >     SELECT
> >       UPPER(some_field,
> >             full => true,
> >             dotless_i => CASE other_field WHEN ...)
> >     FROM ...
>
> Can you index this usefully?  It would only work if the user query
> matches exactly this pattern?

In that example, UPPER is used in the target list -- the WHERE clause
might be indexable. The UPPER is just used for display purposes, and
may depend on some locale settings stored in another table associated
with a particular user.

Regards,
Jeff Davis