Re: Fixing GIN for empty/null/full-scan cases

Robert Haas wrote:
> On Tue, Jan 4, 2011 at 4:49 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> > Robert Haas <robertmhaas@gmail.com> writes:
> >> On Tue, Jan 4, 2011 at 4:09 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> >>> * Existing GIN indexes are upwards compatible so far as on-disk storage
> >>> goes, but they will of course be missing entries for empty, null, or
> >>> null-containing items. ?Users who want to do searches that should find
> >>> such items will need to reindex after updating to 9.1.
> >
> >> This is the only part of this proposal that bothers me a little bit.
> >> It would be nice if the system could determine whether a GIN index is
> >> "upgraded from 9.0 or earlier and thus doesn't contain these entries"
> >> - and avoid trying to use the index for these sorts of queries in
> >> cases where it might return wrong answers.
> >
> > I don't think it's really worth the trouble. ?The GIN code has been
> > broken for these types of queries since day one, and yet we've had only
> > maybe half a dozen complaints about it. ?Moreover there's no practical
> > way to "avoid trying to use the index", since in many cases the fact
> > that a query requires a full-index scan isn't determinable at plan time.
> >
> > The best we could really do is throw an error at indexscan start, and
> > that doesn't seem all that helpful. ?But it probably wouldn't take much
> > code either, if you're satisfied with that answer. ?(I'm envisioning
> > adding a version ID to the GIN metapage and then checking that before
> > proceeding with a full-index scan.)
> 
> I'd be satisfied with that answer.  It at least makes it a lot more
> clear when you've got a problem.  If this were a more common scenario,
> I'd probably advocate for a better solution, but the one you propose
> seems adequate given the frequency of the problem as you describe it.

What does pg_upgrade need to do about this for 9.1?  Just tell people
they might get an GIN error someday?

--  Bruce Momjian  <bruce@momjian.us>        http://momjian.us EnterpriseDB
http://enterprisedb.com
 + It's impossible for everything to be true. +