Re: insensitive collations

From: "Daniel Verite" <daniel(at)manitou-mail(dot)org>
To: "Jim Finnerty" <jfinnert(at)amazon(dot)com>
Cc: pgsql-hackers(at)postgresql(dot)org
Subject: Re: insensitive collations
Date: 2021-03-25 12:27:55
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Jim Finnerty wrote:

> Currently nondeterministic collations are disabled at the database level.

Deterministic ICU collations are also disabled.

> The cited reason was because of the lack of LIKE support and because certain
> catalog views use LIKE.

But the catalogs shouldn't use the default collation of the database.

commit 586b98fdf1aaef4a27744f8b988479aad4bd9a01 provides
some details about this:

Author: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Date: Wed Dec 19 17:35:12 2018 -0500

Make type "name" collation-aware.

The "name" comparison operators now all support collations, making them
functionally equivalent to "text" comparisons, except for the different
physical representation of the datatype. They do, in fact, mostly share
the varstr_cmp and varstr_sortsupport infrastructure, which has been
slightly enlarged to handle the case.

To avoid changes in the default behavior of the datatype, set name's
typcollation to C_COLLATION_OID not DEFAULT_COLLATION_OID, so that
by default comparisons to a name value will continue to use strcmp
semantics. (This would have been the case for system catalog columns
anyway, because of commit 6b0faf723, but doing this makes it true for
user-created name columns as well. In particular, this avoids
locale-dependent changes in our regression test results.)

In consequence, tweak a couple of places that made assumptions about
collatable base types always having typcollation DEFAULT_COLLATION_OID.
I have not, however, attempted to relax the restriction that user-
defined collatable types must have that. Hence, "name" doesn't
behave quite like a user-defined type; it acts more like a domain
with COLLATE "C". (Conceivably, if we ever get rid of the need for
catalog name columns to be fixed-length, "name" could actually become
such a domain over text. But that'd be a pretty massive undertaking,
and I'm not volunteering.)


Best regards,
Daniel Vérité
PostgreSQL-powered mailer:
Twitter: @DanielVerite

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