Re: Reduce build times of pg_trgm GIN indexes

On Wed, 21 Jan 2026 at 16:45, David Geier <geidav(dot)pg(at)gmail(dot)com> wrote:
>
> How do we usually go about such backwards-compatibility breaking
> changes?

When it concerns a bug, we mention the change in the release notes
with a warning to reindex affected indexes to be sure no known
corruption remains. See e.g. the final entry in the PG18 release
notes' migration section here:
https://www.postgresql.org/docs/18/release-18.html#RELEASE-18-MIGRATION.

> Could we have pg_upgrade reindex all GIN indexes? Would that be
> acceptable?

No. We'd handle this like any other collation/opclass fixes; we ask
users to reindex their indexes in their own time after they've
upgraded their cluster. Note that in this case it concerns an issue
with just one GIN opclass, not all GIN indexes; so even if we were to
address this in pg_upgrade it wouldn't be a correct choice to reindex
every GIN index, as only a subset of those would be affected by this
issue.

Generally speaking, pg_upgrade doesn't concern itself with the
validity of the data structures that are described by the catalogs
that it upgrades, it only concerns itself with that it correctly
transcribes the catalogs from one version to another, and that the
data files of the old cluster are transfered correctly without
changes.

Kind regards,

Matthias van de Meent
Databricks (https://www.databricks.com)