Re: SQL/JSON path issues/questions

On Thu, Jul 18, 2019 at 5:08 PM Thom Brown <thom@linux.com> wrote:
> On Tue, 16 Jul 2019 at 19:44, Alexander Korotkov
> <a.korotkov@postgrespro.ru> wrote:
> >
> > On Tue, Jul 16, 2019 at 9:22 PM Thom Brown <thom@linux.com> wrote:
> > > Now I'm looking at the @? and @@ operators, and getting a bit
> > > confused.  This following query returns true, but I can't determine
> > > why:
> > >
> > > # SELECT '{"a":[1,2,3,4,5]}'::jsonb @? '$.b == "hello"'::jsonpath;
> > >  ?column?
> > > ----------
> > >  t
> > > (1 row)
> > >
> > > "b" is not a valid item, so there should be no match.  Perhaps it's my
> > > misunderstanding of how these operators are supposed to work, but the
> > > documentation is quite terse on the behaviour.
> >
> > So, the result of jsonpath evaluation is single value "false".
> >
> > # SELECT jsonb_path_query_array('{"a":[1,2,3,4,5]}'::jsonb, '$.b == "hello"');
> >  jsonb_path_query_array
> > ------------------------
> >  [false]
> > (1 row)
> >
> > @@ operator checks that result is "true".  This is why it returns "false".
> >
> > @? operator checks if result is not empty.  So, it's single "false"
> > value, not empty list.  This is why it returns "true".
> >
> > Perhaps, we need to clarify this in docs providing more explanation.
>
> Understood.  Thanks.
>
> Also, is there a reason why jsonb_path_query doesn't have an operator analog?

The point of existing operator analogues is index support.  We
introduced operators for searches we can accelerate using GIN indexes.

jsonb_path_query() doesn't return bool.  So, even if we have an
operator for that, it wouldn't get index support.

However, we can discuss introduction of operator analogues for other
functions as syntax sugar.

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