Supporting non-deterministic collations with tailoring rules.

Todd Lang <todd.lang@d2l.com>

From: Todd Lang <Todd.Lang@D2L.com>

To: "pgsql-hackers@lists.postgresql.org" <pgsql-hackers@lists.postgresql.org>

Date: 2025-09-23T14:51:28Z

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Allow setting the collation strength in ICU tailoring rules
- 905e44152a1d 19 (unreleased) landed


Reposting this here from the Discord server as requested:

When creating a collation, in https://github.com/postgres/postgres/blob/master/src/backend/utils/adt/pg_locale_icu.c#L461 it is opening the collator with the tailoring rules supplied. However, it has hardcoded the strength level UCOL_DEFAULT_STRENGTH. This has the effect of ignoring the "deterministic=false" you may have specified in your CREATE COLLATION call. If, instead of UCOL_DEFAULT_STRENGTH, the code understood the deterministic parameter and passed either UCOL_PRIMARY for "deterministic=true", and UCOL_SECONDARY for "deterministic=false", this would preserve the attempt to obtain case-insensitivity in the locale while simultaneously allowing tailoring as expected.

I have made the modification to the pg_locale_icu.c and tested it locally (simply hardcoding UCOL_SECONDARY - not checking the deterministic parameter) and it behaves as expected, though I freely admit my knowledge of ICU intersecting with Postgres is rather limited.