Fix JSON_OBJECTAGG uniquefying bug

Commit f4fb45d contained a bug in removing items with null values when
unique keys are required, where the leading items that are sorted
contained such values. Fix that and add a test for it.

Discussion: https://postgr.es/m/CAJA4AWQ_XbSmsNbW226UqNyRLJ+wb=iQkQMj77cQyoNkqtf=2Q@mail.gmail.com

Author: adunstan

src/backend/utils/adt/jsonb_util.c

@@ -1959,8 +1959,18 @@ uniqueifyJsonbObject(JsonbValue *object, bool unique_keys, bool skip_nulls)
 
 	if (hasNonUniq || skip_nulls)
 	{
-		JsonbPair  *ptr = object->val.object.pairs + 1,
-				   *res = object->val.object.pairs;
+		JsonbPair  *ptr, *res;
+
+		while (skip_nulls && object->val.object.nPairs > 0 &&
+			   object->val.object.pairs->value.type == jbvNull)
+		{
+			/* If skip_nulls is true, remove leading items with null */
+			object->val.object.pairs++;
+			object->val.object.nPairs--;
+		}
+
+		ptr = object->val.object.pairs + 1;
+		res = object->val.object.pairs;
 
 		while (ptr - object->val.object.pairs < object->val.object.nPairs)
 		{

src/test/regress/expected/sqljson.out

@@ -944,6 +944,13 @@ ERROR:  duplicate JSON object key value
 SELECT JSON_OBJECTAGG(k: v ABSENT ON NULL WITH UNIQUE KEYS RETURNING jsonb)
 FROM (VALUES (1, 1), (1, NULL), (2, 2)) foo(k, v);
 ERROR:  duplicate JSON object key value
+SELECT JSON_OBJECTAGG(k: v ABSENT ON NULL WITH UNIQUE KEYS RETURNING jsonb)
+FROM (VALUES (1, 1), (0, NULL),(4, null), (5, null),(6, null),(2, 2)) foo(k, v);
+  json_objectagg  
+------------------
+ {"1": 1, "2": 2}
+(1 row)
+
 -- Test JSON_OBJECT deparsing
 EXPLAIN (VERBOSE, COSTS OFF)
 SELECT JSON_OBJECT('foo' : '1' FORMAT JSON, 'bar' : 'baz' RETURNING json);

src/test/regress/sql/sqljson.sql

@@ -281,6 +281,9 @@ FROM (VALUES (1, 1), (1, NULL), (2, 2)) foo(k, v);
 SELECT JSON_OBJECTAGG(k: v ABSENT ON NULL WITH UNIQUE KEYS RETURNING jsonb)
 FROM (VALUES (1, 1), (1, NULL), (2, 2)) foo(k, v);
 
+SELECT JSON_OBJECTAGG(k: v ABSENT ON NULL WITH UNIQUE KEYS RETURNING jsonb)
+FROM (VALUES (1, 1), (0, NULL),(4, null), (5, null),(6, null),(2, 2)) foo(k, v);
+
 -- Test JSON_OBJECT deparsing
 EXPLAIN (VERBOSE, COSTS OFF)
 SELECT JSON_OBJECT('foo' : '1' FORMAT JSON, 'bar' : 'baz' RETURNING json);