[MDEV-23029] JSON_OBJECTAGG returns NULL when used together with GROUP BY - Jira

I might be missing somthing totally trivial, but the following example doesn't look correct to me:

(psergey: fixed formatting)

CREATE TABLE t (

    e INTEGER,

    a VARCHAR(255),

    v VARCHAR(255)

);

INSERT INTO t VALUES (0, 'a1', '1')

                   , (0, 'a2', '2')

                   , (1, 'b1', '3');

SELECT e, JSON_OBJECTAGG(a, v)

  FROM t

 GROUP BY e;

This is what I get:

MariaDB 5.5.5-10.5.4-MariaDB> SELECT e, JSON_OBJECTAGG(a, v)

    ->   FROM t

    ->  GROUP BY e;

+------+----------------------+

| e    | JSON_OBJECTAGG(a, v) |

+------+----------------------+

|    0 | NULL                 |

|    1 | NULL                 |

+------+----------------------+

2 rows in set (0.00 sec)

The second column is NULL, although it should contain the actual data (like MySQL does).

Removing the GROUP BY yields a correct result.

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