Re: Patch: Improve Boolean Predicate JSON Path Docs

"David E. Wheeler" <david@justatheory.com> writes:
> On Jan 20, 2024, at 12:34, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> Surely we're not helping anybody by leaving that behavior in place.
>> Making it do something useful, throwing an error, or returning NULL
>> all seem superior to this.  I observe that @@ returns NULL for the
>> path type it doesn't like, so maybe that's what to do here.

> I agree it would be far better for the behavior to be consistent, but frankly would like to see them raise an error.
Ideallythe hit would suggest the proper alternative operator or function to use, and maybe link to the docs that
describethe difference between SQL-standard JSONPath and "predicate check expressions”, and how they have separate
operatorsand functions. 

That ship's probably sailed.  However, I spent some time poking into
the odd behavior I showed for @?, and it seems to me that it's an
oversight in appendBoolResult.  That just automatically returns jperOk
in the !found short-circuit path for any boolean result, which is not
the behavior you'd get if the boolean value were actually returned
(cf. jsonb_path_match_internal).  I experimented with making it do
what seems like the right thing, and found that there is only one
regression test case that changes behavior:

 select jsonb '2' @? '$ == "2"';
  ?column?
 ----------
- t
+ f
 (1 row)


Now, JSON does not think that numeric 2 equals string "2", so
ISTM the expected output here is flat wrong.  It's certainly
inconsistent with @@:

regression=# select jsonb '2' @@ '$ == "2"';
 ?column?
----------

(1 row)

So I think we should consider a patch like the attached
(probably with some more test cases added).  I don't really
understand this code however, so maybe I missed something.

            regards, tom lane

diff --git a/src/backend/utils/adt/jsonpath_exec.c b/src/backend/utils/adt/jsonpath_exec.c
index ac16f5c85d..63c46cfab5 100644
--- a/src/backend/utils/adt/jsonpath_exec.c
+++ b/src/backend/utils/adt/jsonpath_exec.c
@@ -2065,7 +2065,14 @@ appendBoolResult(JsonPathExecContext *cxt, JsonPathItem *jsp,
     JsonbValue    jbv;

     if (!jspGetNext(jsp, &next) && !found)
-        return jperOk;            /* found singleton boolean value */
+    {
+        /*
+         * We have a predicate check expression, i.e. a path ending in a bare
+         * boolean operator, and we don't need to return the exact value(s)
+         * found.  Just report success or failure of the boolean.
+         */
+        return (res == jpbTrue) ? jperOk : jperNotFound;
+    }

     if (res == jpbUnknown)
     {
diff --git a/src/test/regress/expected/jsonb_jsonpath.out b/src/test/regress/expected/jsonb_jsonpath.out
index 6659bc9091..59e1b71051 100644
--- a/src/test/regress/expected/jsonb_jsonpath.out
+++ b/src/test/regress/expected/jsonb_jsonpath.out
@@ -1157,7 +1157,7 @@ select jsonb_path_query('2', '$ == "2"');
 select jsonb '2' @? '$ == "2"';
  ?column?
 ----------
- t
+ f
 (1 row)

 select jsonb '2' @@ '$ > 1';