Re: Collation versioning

On Fri, 2019-11-08 at 15:04 +1300, Thomas Munro wrote:
> So we have three proposals:
> 
> 1.  Assume that pre-13 indexes that depend on collations are
> potentially corrupted and complain until they are reindexed.  This
> could be done by having pg_upgrade run ALTER INDEX ... DEPENDS ON
> COLLATION "fr_FR" VERSION '' (empty string, or some other default
> value that we don't think is going to coincide with a real version).
> 2.  Assume that pre-13 indexes are not corrupted.  In the target 13
> database, the index will be created in the catalogs with the
> provider's current version.
> 3.  We don't know if pre-13 indexes are corrupted or not, and we'll
> record that with a special value just as in proposal #1, except that
> we could show a different hint for that special version value.  It
> would tell you can you can either REINDEX, or run ALTER INDEX ...
> DEPENDS ON COLLATION "fr_FR" VERSION '34.0' if you believe the index
> to have been created with the current collation version on an older
> release of PostgreSQL that didn't track versions.

I think #1 is bad because it would frighten all users, even those who
didn't upgrade their libc at all, only PostgreSQL.  I don't think that
shouting "wolf" for no real reason will improve trust in PostgreSQL.

#2 is bad because it may hide pre-existing index corruption.

#3 is the best proposal, but there is still the need to run
ALTER INDEX on all affected indexes to keep PostgreSQL from nagging.
Perhaps the situation could be improved with a pg_upgrade option
--i-know-my-indexes-are-fine that causes a result like #2.
Together with a bold note in the release notes, this may relieve
the pain.

Yours,
Laurenz Albe