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

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

At Wed, 17 Jan 2018 22:26:15 +0300, Sergei Kornilov <sk@zsrv.org>
wrote in <412861516217175@web38o.yandex.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.