Re: Changes in amcanreturn() interface to support multicolumn indexes

Anastasia Lubennikova <lubennikovaav@gmail.com> writes:
> There is a question of support multicolumn index only scans for GIST.
> gistcanreturn() can return true if fetch is implemented for all indexed
> columns and false otherwise.
> But that's not very good case for multicolumn indexes.

> I think, it requires extend the interface to add separate columns checking.
> But I can't understand what kind of changes is required
> and how would it affect on previous amcanreturn interface.

Yeah, rather than just returning a bool, I suppose we need it to return
an indication of which index columns it's prepared to return values for.
(And then I guess the runtime API should return nulls for index columns
it can't handle.)  You could probably use either a bitmapset or an integer
List to return the column numbers.

I wouldn't be too worried about backwards compatibility in changing that
API.  If there are any people supporting outside-core index AMs, they've
got much bigger problems than this.
        regards, tom lane