Horton

Re: Built-in CTYPE provider

Noah Misch <noah@leadboat.com>

View thread View on postgr.es
From: Noah Misch <noah@leadboat.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Jeff Davis <pgsql@j-davis.com>, Peter Eisentraut <peter@eisentraut.org>, Daniel Verite <daniel@manitou-mail.org>, Robert Haas <robertmhaas@gmail.com>, Jeremy Schneider <schneider@ardentperf.com>, pgsql-hackers@postgresql.org
Date: 2024-07-09T01:05:45Z
Lists: pgsql-hackers 
On Sat, Jul 06, 2024 at 04:19:21PM -0400, Tom Lane wrote:
> Noah Misch <noah@leadboat.com> writes:
> > As a released feature, NORMALIZE() has a different set of remedies to choose
> > from, and I'm not proposing one.  I may have sidetracked this thread by
> > talking about remedies without an agreement that pg_c_utf8 has a problem.  My
> > question for the PostgreSQL maintainers is this:
> 
> >   textregexeq(... COLLATE pg_c_utf8, '[[:alpha:]]') and lower(), despite being
> >   IMMUTABLE, will change behavior in some major releases.  pg_upgrade does not
> >   have a concept of IMMUTABLE functions changing, so index scans will return
> >   wrong query results after upgrade.  Is it okay for v17 to release a
> >   pg_c_utf8 planned to behave that way when upgrading v17 to v18+?
> 
> I do not think it is realistic to define "IMMUTABLE" as meaning that
> the function will never change behavior until the heat death of the
> universe.  As a counterexample, we've not worried about applying
> bug fixes or algorithm improvements that change the behavior of
> "immutable" numeric computations.

True.  There's a continuum from "releases can change any IMMUTABLE function"
to "index integrity always wins, even if a function is as wrong as 1+1=3".
I'm less concerned about the recent "Incorrect results from numeric round"
thread, even though it's proposing to back-patch.  I'm thinking about these
aggravating factors for $SUBJECT:

- $SUBJECT is planning an annual cadence of this kind of change.

- We already have ICU providing collation support for the same functions.
  Unlike $SUBJECT, ICU integration gives packagers control over when to accept
  corruption at pg_upgrade time.

- SQL Server, DB2 and Oracle do their Unicode updates in a non-corrupting way.
  (See Jeremy Schneider's reply concerning DB2 and Oracle.)

- lower() and regexp are more popular in index expressions than
  high-digit-count numeric calculations.

> I'd say a realistic policy is "immutable means we don't intend to
> change it within a major release".  If we do change the behavior,
> either as a bug fix or a major-release improvement, that should
> be release-noted so that people know they have to rebuild dependent
> indexes and matviews.

It sounds like you're very comfortable with $SUBJECT proceeding in its current
form.  Is that right?

Commits

  1. Support PG_UNICODE_FAST locale in the builtin collation provider.

  2. Support Unicode full case mapping and conversion.

  3. Fix test failures when language environment is not UTF-8.

  4. Add unicode_strtitle() for Unicode Default Case Conversion.

  5. Use version for builtin collations.

  6. Fix convert_case(), introduced in 5c40364dd6.

  7. Inline basic UTF-8 functions.

  8. Support C.UTF-8 locale in the new builtin collation provider.

  9. Fix another warning, introduced by 846311051e.

  10. Address more review comments on commit 2d819a08a1.

  11. Fix unreachable code warning from commit 2d819a08a1.

  12. Introduce "builtin" collation provider.

  13. Catalog changes preparing for builtin collation provider.

  14. Unicode case mapping tables and functions.

  15. Add Unicode property tables.

  16. Documentation update for Standard Collations.

  17. Cleanup for unicode-update build target and test.

  18. Shrink Unicode category table.

  19. Make some error strings more generic

  20. pg_upgrade: copy locale and encoding information to new cluster.

  21. Update Unicode data to Unicode 15.0.0

  22. Create a new type category for "internal use" types.