Re: jsonpath

On Tue, Jan 29, 2019 at 2:17 AM Nikita Glukhov <n.gluhov@postgrespro.ru> wrote:
> In the previous version of the patch jsonb_path_match() threw error when
> jsonpath did not return a singleton value, but in the last version in such cases
> NULL is returned.  This problem arises because we cannot guarantee at compile
> time that jsonpath expression used in jsonb_path_match() is a predicate.
> Predicates by standard can return only True, False, and Unknown (errors occurred
> during execution of their operands are transformed into Unknown values), so
> predicates cannot throw errors, and there are no problems with errors.

Attached patchset provides description of errors suppressed.  It also
clarifies how jsonb_path_match() works.

> GIN does not attempt to search non-predicate expressions, so there may be no
> problem even we throw "not a singleton" error.

Yes, I don't insist on that.  If majority of us wants to bring "not a
singleton" error back, I don't object to it.

> Here I want to remind that ability to use predicates in the root of jsonpath
> expression is an our extension to standard that was created specially for the
> operator @@.  By standard predicates are allowed only in filters.  Without this
> extension we are still able to rewrite @@ using @?:
> jsonb @@ 'predicate'        is equivalent to
> jsonb @? '$ ? (predicate)'
> but such @? expression is a bit slower to execute and a bit verbose to write.
>
> If we introduced special type 'jsonpath_predicate', then we could solve the
> problem by checking the type of jsonpath expression at compile-time.

For me it seems that separate datatype for this kind of problem is overkill.

> Another problem with error handling is that jsonb_path_query*() functions
> always throw SQL/JSON errors and there is no easy and effective way to emulate
> NULL ON ERROR behavior, which is used by default in SQL/JSON functions.  So I
> think it's worth trying to add some kind of flag 'throwErrors' to
> jsonb_path_query*() functions.

Good idea, but let's commit basic jsonpath implementation first.

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