Re: Index-only scan returns incorrect results when using a composite GIST index with a gist_trgm_ops column.

On Thu, Jan 18, 2018 at 8:48 AM, Kyotaro HORIGUCHI <
horiguchi(dot)kyotaro(at)lab(dot)ntt(dot)co(dot)jp> wrote:

> At Wed, 17 Jan 2018 22:26:15 +0300, Sergei Kornilov <sk(at)zsrv(dot)org>
> wrote in <412861516217175(at)web38o(dot)yandex(dot)ru>
> > Hello
> > I can reproduce on actual 9.6.6, 10.1 and fresh master build
> > (9c7d06d60680c7f00d931233873dee81fdb311c6 commit). I did not check
> > earlier versions
> >
> > set enable_indexonlyscan to off ;
> > postgres=# SELECT w FROM words WHERE w LIKE '%e%';
> > w
> > -------
> > Lorem
> > Index scan result is correct. Affected only index only scan,
> >
> > PS: i find GIST(w gist_trgm_ops, w); some strange idea, but result
> > is incorrect in any case.
>
> The cause is that gist_trgm_ops lacks "fetch" method but planner
> is failing to find that.
>
> https://www.postgresql.org/docs/10/static/gist-extensibility.html
> > The optional ninth method fetch is needed if the operator class
> > wishes to support index-only scans.
>
> Index only scan is not usable in the case since the first index
> column cannot be rechecked but check_index_only makes wrong
> decision by the second occurance of "w'. There may be a chance
> that recheck is not required but we cannot predict that until
> actually acquire a tuple during execution.
>

I didn't get this point. Same column is indexed twice using two
different opclasses. The first opclass doesn't support index-only scan,
while the second opclass does support it. So, if the first opclass needs
recheck then it can be rechecked using the value got from the second
opclass. Thus, I think we can potentially support index-only scan
in this case.

Another thing is that it's probably hard to do in our current
optimizer/executor/am
infrastructure. And assuming that use-case is quite narrow, and it looks
OK to just disable such feature as bug fix.

------
Alexander Korotkov
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company