Re: Bug in jsonb_path_exists (maybe _match) one-element scalar/variable jsonpath handling

On Fri, Dec 2, 2022 at 5:18 AM Alexander Korotkov <aekorotkov(at)gmail(dot)com>
wrote:

> On Thu, Nov 24, 2022 at 8:31 AM David G. Johnston
> <david(dot)g(dot)johnston(at)gmail(dot)com> wrote:
> > The following query produces an incorrect result. It should error (or
> at worse produce "false"), but it instead produces "true" (this applies to
> @? too)
> >
> > select jsonb_path_exists('{"foo": true}'::jsonb, '$bar', '{}', false);
>
> Variable case is definitely broken, but I don't think other cases are
> broken. If we're checking for existence and there is a constant, we
> can immediately return true because constant exists indeed. That
> logic doesn't work for variable, which could be non-existent.
>
> > select jsonb_path_exists('{"foo": true}'::jsonb, '"bar"', '{}', false);
> -- true (bar in double quotes)
>

I think my issue with the constant is that the function itself is said to
return whether or not the provided path matches the input json. It is
impossible to match the input json if there is no reference to the input
json in the jsonpath expression. As the existing wording promises: "Checks
whether the JSON path returns any item for the specified JSON value" - the
word item is rightly taken to mean that the path at minimum references the
root (i.e., mandatory $) - and that any true result from exists will, if
the expression is used for _match, produce the "item for the specified JSON
value" that was found.

So I'll stand by my conclusion that the behavior of constants is buggy -
though I suppose fixing the bug is probably most readily accomplished by
changing the definition of what behavior we are promising and fixing up the
documentation to express that change. In short, it is really an error to
not specify "$" in your expression - but if you don't you will simply get a
true outcome for the existence test - for backward compatibility reasons.

> > select jsonb_path_match('{"foo": true}'::jsonb, '"bar"', '{}', false);
> -- ERROR: single boolean result is expected
> > select jsonb_path_match('{"foo": true}'::jsonb, '$bar', '{"bar":"foo"}',
> false); -- same error as above, as expected
> >
> > I expect the missing variable specification to produce jperError and the
> rest of the block to produce jperNotFound. The "single boolean result
> expected" error seems incorrect though I'm not sure where that is coming
> from. But I'm also not considering, or am even aware of, what the standard
> we are guided by here says should actually happen.
>
> I think jsonb_path_match() behaves correctly, it expects jsonpatch
> expression to return single boolend and throws an error otherwise.
>

Yeah, I may have mis-interpreted the meaning of the error message.
Something like: "jsonpath expression must produce a single boolean result"
would be a bit more clear.

David J.