Remove "invalid concatenation of jsonb objects" error case.

The jsonb || jsonb operator arbitrarily rejected certain combinations
of scalar and non-scalar inputs, while being willing to concatenate
other combinations.  This was of course quite undocumented.  Rather
than trying to document it, let's just remove the restriction,
creating a uniform rule that unless we are handling an object-to-object
concatenation, non-array inputs are converted to one-element arrays,
resulting in an array-to-array concatenation.  (This does not change
the behavior for any case that didn't throw an error before.)

Per complaint from Joel Jacobson.  Back-patch to all supported branches.

Discussion: https://postgr.es/m/163099.1608312033@sss.pgh.pa.us

doc/src/sgml/func.sgml

@@ -11455,10 +11455,13 @@ table2-mapping
 
   <note>
    <para>
-    The <literal>||</literal> operator concatenates the elements at the top level of
-    each of its operands. It does not operate recursively. For example, if
-    both operands are objects with a common key field name, the value of the
-    field in the result will just be the value from the right hand operand.
+    The <literal>||</literal> operator concatenates two JSON objects by
+    generating an object containing the union of their keys, taking the
+    second object's value when there are duplicate keys.  All other cases
+    produce a JSON array: first, any non-array input is converted into a
+    single-element array, and then the two arrays are concatenated.
+    It does not operate recursively; only the top-level array or object
+    structure is merged.
    </para>
   </note>
 

src/backend/utils/adt/jsonfuncs.c

@@ -4583,36 +4583,39 @@ IteratorConcat(JsonbIterator **it1, JsonbIterator **it2,
 				rk1,
 				rk2;
 
-	r1 = rk1 = JsonbIteratorNext(it1, &v1, false);
-	r2 = rk2 = JsonbIteratorNext(it2, &v2, false);
+	rk1 = JsonbIteratorNext(it1, &v1, false);
+	rk2 = JsonbIteratorNext(it2, &v2, false);
 
 	/*
-	 * Both elements are objects.
+	 * JsonbIteratorNext reports raw scalars as if they were single-element
+	 * arrays; hence we only need consider "object" and "array" cases here.
 	 */
 	if (rk1 == WJB_BEGIN_OBJECT && rk2 == WJB_BEGIN_OBJECT)
 	{
 		/*
-		 * Append the all tokens from v1 to res, except last WJB_END_OBJECT
+		 * Both inputs are objects.
+		 *
+		 * Append all the tokens from v1 to res, except last WJB_END_OBJECT
 		 * (because res will not be finished yet).
 		 */
-		pushJsonbValue(state, r1, NULL);
+		pushJsonbValue(state, rk1, NULL);
 		while ((r1 = JsonbIteratorNext(it1, &v1, true)) != WJB_END_OBJECT)
 			pushJsonbValue(state, r1, &v1);
 
 		/*
-		 * Append the all tokens from v2 to res, include last WJB_END_OBJECT
-		 * (the concatenation will be completed).
+		 * Append all the tokens from v2 to res, including last WJB_END_OBJECT
+		 * (the concatenation will be completed).  Any duplicate keys will
+		 * automatically override the value from the first object.
 		 */
 		while ((r2 = JsonbIteratorNext(it2, &v2, true)) != WJB_DONE)
 			res = pushJsonbValue(state, r2, r2 != WJB_END_OBJECT ? &v2 : NULL);
 	}
-
-	/*
-	 * Both elements are arrays (either can be scalar).
-	 */
 	else if (rk1 == WJB_BEGIN_ARRAY && rk2 == WJB_BEGIN_ARRAY)
 	{
-		pushJsonbValue(state, r1, NULL);
+		/*
+		 * Both inputs are arrays.
+		 */
+		pushJsonbValue(state, rk1, NULL);
 
 		while ((r1 = JsonbIteratorNext(it1, &v1, true)) != WJB_END_ARRAY)
 		{
@@ -4628,48 +4631,40 @@ IteratorConcat(JsonbIterator **it1, JsonbIterator **it2,
 
 		res = pushJsonbValue(state, WJB_END_ARRAY, NULL /* signal to sort */ );
 	}
-	/* have we got array || object or object || array? */
-	else if (((rk1 == WJB_BEGIN_ARRAY && !(*it1)->isScalar) && rk2 == WJB_BEGIN_OBJECT) ||
-			 (rk1 == WJB_BEGIN_OBJECT && (rk2 == WJB_BEGIN_ARRAY && !(*it2)->isScalar)))
+	else if (rk1 == WJB_BEGIN_OBJECT)
 	{
-
-		JsonbIterator **it_array = rk1 == WJB_BEGIN_ARRAY ? it1 : it2;
-		JsonbIterator **it_object = rk1 == WJB_BEGIN_OBJECT ? it1 : it2;
-
-		bool		prepend = (rk1 == WJB_BEGIN_OBJECT);
+		/*
+		 * We have object || array.
+		 */
+		Assert(rk2 == WJB_BEGIN_ARRAY);
 
 		pushJsonbValue(state, WJB_BEGIN_ARRAY, NULL);
 
-		if (prepend)
-		{
-			pushJsonbValue(state, WJB_BEGIN_OBJECT, NULL);
-			while ((r1 = JsonbIteratorNext(it_object, &v1, true)) != WJB_DONE)
-				pushJsonbValue(state, r1, r1 != WJB_END_OBJECT ? &v1 : NULL);
+		pushJsonbValue(state, WJB_BEGIN_OBJECT, NULL);
+		while ((r1 = JsonbIteratorNext(it1, &v1, true)) != WJB_DONE)
+			pushJsonbValue(state, r1, r1 != WJB_END_OBJECT ? &v1 : NULL);
 
-			while ((r2 = JsonbIteratorNext(it_array, &v2, true)) != WJB_DONE)
-				res = pushJsonbValue(state, r2, r2 != WJB_END_ARRAY ? &v2 : NULL);
-		}
-		else
-		{
-			while ((r1 = JsonbIteratorNext(it_array, &v1, true)) != WJB_END_ARRAY)
-				pushJsonbValue(state, r1, &v1);
-
-			pushJsonbValue(state, WJB_BEGIN_OBJECT, NULL);
-			while ((r2 = JsonbIteratorNext(it_object, &v2, true)) != WJB_DONE)
-				pushJsonbValue(state, r2, r2 != WJB_END_OBJECT ? &v2 : NULL);
-
-			res = pushJsonbValue(state, WJB_END_ARRAY, NULL);
-		}
+		while ((r2 = JsonbIteratorNext(it2, &v2, true)) != WJB_DONE)
+			res = pushJsonbValue(state, r2, r2 != WJB_END_ARRAY ? &v2 : NULL);
 	}
 	else
 	{
 		/*
-		 * This must be scalar || object or object || scalar, as that's all
-		 * that's left. Both of these make no sense, so error out.
+		 * We have array || object.
 		 */
-		ereport(ERROR,
-				(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
-				 errmsg("invalid concatenation of jsonb objects")));
+		Assert(rk1 == WJB_BEGIN_ARRAY);
+		Assert(rk2 == WJB_BEGIN_OBJECT);
+
+		pushJsonbValue(state, WJB_BEGIN_ARRAY, NULL);
+
+		while ((r1 = JsonbIteratorNext(it1, &v1, true)) != WJB_END_ARRAY)
+			pushJsonbValue(state, r1, &v1);
+
+		pushJsonbValue(state, WJB_BEGIN_OBJECT, NULL);
+		while ((r2 = JsonbIteratorNext(it2, &v2, true)) != WJB_DONE)
+			pushJsonbValue(state, r2, r2 != WJB_END_OBJECT ? &v2 : NULL);
+
+		res = pushJsonbValue(state, WJB_END_ARRAY, NULL);
 	}
 
 	return res;

src/test/regress/expected/jsonb.out

@@ -3654,9 +3654,41 @@ select '{"a":"b"}'::jsonb || '[]'::jsonb;
 (1 row)
 
 select '"a"'::jsonb || '{"a":1}';
-ERROR:  invalid concatenation of jsonb objects
+    ?column?     
+-----------------
+ ["a", {"a": 1}]
+(1 row)
+
 select '{"a":1}' || '"a"'::jsonb;
-ERROR:  invalid concatenation of jsonb objects
+    ?column?     
+-----------------
+ [{"a": 1}, "a"]
+(1 row)
+
+select '[3]'::jsonb || '{}'::jsonb;
+ ?column? 
+----------
+ [3, {}]
+(1 row)
+
+select '3'::jsonb || '[]'::jsonb;
+ ?column? 
+----------
+ [3]
+(1 row)
+
+select '3'::jsonb || '4'::jsonb;
+ ?column? 
+----------
+ [3, 4]
+(1 row)
+
+select '3'::jsonb || '{}'::jsonb;
+ ?column? 
+----------
+ [3, {}]
+(1 row)
+
 select '["a", "b"]'::jsonb || '{"c":1}';
        ?column?       
 ----------------------

src/test/regress/sql/jsonb.sql

@@ -973,6 +973,11 @@ select '{"a":"b"}'::jsonb || '[]'::jsonb;
 select '"a"'::jsonb || '{"a":1}';
 select '{"a":1}' || '"a"'::jsonb;
 
+select '[3]'::jsonb || '{}'::jsonb;
+select '3'::jsonb || '[]'::jsonb;
+select '3'::jsonb || '4'::jsonb;
+select '3'::jsonb || '{}'::jsonb;
+
 select '["a", "b"]'::jsonb || '{"c":1}';
 select '{"c": 1}'::jsonb || '["a", "b"]';