MDEV-23029: JSON_OBJECTAGG returns NULL when used together with GROUP BY

Quick grouping is not supported for JSON_OBJECTAGG. The same for GROUP_CONCAT too
so make sure that Item::quick_group is set to FALSE. We need to make sure that in
the case of JSON_OBJECTAGG we don't create an index over grouping fields of
the temp table and update the result after each iteration.
Instead we should first sort the result in accordance to the
GROUP BY fields and then perform the grouping and
write the result to the temp table.

mysql-test/main/func_json.result

@@ -1328,5 +1328,19 @@ JSON_ARRAYAGG(col1)
 [{"color":"red", "size":1},{"color":"blue", "size":2}]
 drop table t1;
 #
+# MDEV-23029: JSON_OBJECTAGG returns NULL when used together with GROUP BY
+#
+CREATE TABLE t1 (e INT, a VARCHAR(255), v VARCHAR(255));
+INSERT INTO t1 VALUES (0, 'a1', '1') , (0, 'a2', '2') , (1, 'b1', '3');
+EXPLAIN SELECT B.e, JSON_OBJECTAGG(B.a, B.v) FROM t1 A, t1 B GROUP BY B.e;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	A	ALL	NULL	NULL	NULL	NULL	3	Using temporary; Using filesort
+1	SIMPLE	B	ALL	NULL	NULL	NULL	NULL	3	Using join buffer (flat, BNL join)
+SELECT B.e, JSON_OBJECTAGG(B.a, B.v) FROM t1 A, t1 B GROUP BY B.e;
+e	JSON_OBJECTAGG(B.a, B.v)
+0	{"a1":"1", "a1":"1", "a1":"1", "a2":"2", "a2":"2", "a2":"2"}
+1	{"b1":"3", "b1":"3", "b1":"3"}
+DROP TABLE t1;
+#
 # End of 10.5 tests
 #

mysql-test/main/func_json.test

@@ -826,6 +826,18 @@ insert into t1 values('{"color":"blue", "size":2}' );
 select JSON_ARRAYAGG(col1) from t1;
 drop table t1;
 
+--echo #
+--echo # MDEV-23029: JSON_OBJECTAGG returns NULL when used together with GROUP BY
+--echo #
+
+CREATE TABLE t1 (e INT, a VARCHAR(255), v VARCHAR(255));
+INSERT INTO t1 VALUES (0, 'a1', '1') , (0, 'a2', '2') , (1, 'b1', '3');
+
+EXPLAIN SELECT B.e, JSON_OBJECTAGG(B.a, B.v) FROM t1 A, t1 B GROUP BY B.e;
+SELECT B.e, JSON_OBJECTAGG(B.a, B.v) FROM t1 A, t1 B GROUP BY B.e;
+
+DROP TABLE t1;
+
 --echo #
 --echo # End of 10.5 tests
 --echo #

sql/item_jsonfunc.cc

@@ -3725,6 +3725,7 @@ Item_func_json_objectagg::
 Item_func_json_objectagg(THD *thd, Item_func_json_objectagg *item)
   :Item_sum(thd, item)
 {
+  quick_group= FALSE;
   result.set_charset(collation.collation);
   result.append("{");
 }

sql/item_jsonfunc.h

@@ -579,6 +579,7 @@ class Item_func_json_objectagg : public Item_sum
   Item_func_json_objectagg(THD *thd, Item *key, Item *value) :
     Item_sum(thd, key, value)
   {
+    quick_group= FALSE;
     result.append("{");
   }