Re: No longer possible to query catalogs for index capabilities?

* Andrew Gierth (andrew(at)tao11(dot)riddles(dot)org(dot)uk) wrote:
> >>>>> "Tom" == Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> writes:
>
> >> With the gutting of pg_am in 9.6, there seems to be no longer any
> >> way for a query of the system catalogs to discover any of the index
> >> capabilities that were formerly columns in pg_am (notably
> >> amcanorder, amcanorderbyop, amclusterable, amsearcharray,
> >> amsearchnulls).
>
> >> Am I missing something or is this a significant oversight?
>
> Tom> It's absolutely not an oversight. We asked when 65c5fcd35 went in
> Tom> whether there was still any need for that information to be
> Tom> available at the SQL level, and nobody appeared to care.
>
> Perhaps you were asking the wrong people?

The capabilities strike me as useful to expose, they're pretty useful to
know. I believe we were right to hide the APIs/functions and don't see
any need to expose those to the SQL level.

> Tom> We could in theory expose a view to show the data --- but since a
> Tom> large part of the point of that change was to not need initdb for
> Tom> AM API changes, and to not be constrained to exactly
> Tom> SQL-compatible representations within that API, I'm disinclined to
> Tom> do so without a fairly compelling argument why it's needed.
>
> It could easily be exposed as a function interface of the form
> index_has_capability(oid,name) or indexam_has_capability(oid,name)
> without any initdb worries.

Hmm, that seems pretty reasonable.

> That would surely be better than the present condition of being
> completely unable to get this information from SQL.

Agreed.

Thanks!

Stephen