Re: SQL:2011 application time

On 11/14/24 10:14, Matthias van de Meent wrote:
> I called this issue out earlier this year: amcanunique implies
> btree-style uniqueness, and allows CREATE UNIQUE INDEX. However, that
> IndexAmRoutine field seems to be ignored for indexes that are created
> to back temporal unique constraints, which thus get
> indrel->indisunique = true. This causes interesting issues when you
> look at the index catalog and errors: there are indexes with
> indisunique using gist, but CREATE UNIQUE INDEX USING gist (...)
> throws the nice "access method "gist" does not support unique indexes"
> error.
>
> It'd be nice if there was a better internal API to describe what types
> of uniqueness each index supports, so CREATE UNIQUE INDEX could work
> with gist for WITHOUT OVERLAPS, and these WITHOUT OVERLAPS unique
> indexes could be attached to primary keys without taking O(>=
> tablesize) of effort.

I think the issue is that a specific GiST opclass may support uniqueness (if it defines the support
proc to communicate its equality stratnum), but the AM as a whole doesn't support uniqueness. So
amcanunique is false. We could make the stratnum support proc required, but that seems like it would
break too many extensions. Or maybe we could change canunique to an opclass-level property?

Probably we could support `CREATE UNIQUE INDEX USING gist (...)` *if* the opclasses involved have
stratnum support funcs. I'm happy to write/assist a patch for that. It would use exclusion
constraints behind the scenes, as we're doing with temporal PKs/UNIQUEs. But that still wouldn't
give you CONCURRENTLY (which is the main motivation), because we don't support creating exclusion
constraints concurrently yet. The work in 2014 on REINDEX CONCURRENTLY originally tried to support
exclusion constraints, but it didn't make it into the final version. I'm not sure what needs to be
done there. But one thing that seems tricky is that the *index* doesn't know about the exclusion
rules; it's all in pg_constraint.

Yours,

--
Paul ~{:-)
pj(at)illuminatedcomputing(dot)com