[MDEV-18284] JSON casting using JSON_COMPACT doesn't always work with values from subqueries - Jira

Given this test data

CREATE TABLE json_test (a JSON, b JSON);

INSERT INTO json_test VALUES ("[1,2,3]", '{"a":"foo"}');

I can observe the following behavior:

MariaDB [bugs]> SELECT JSON_OBJECT("a", JSON_COMPACT(a)) FROM (SELECT * FROM json_test) AS json_test_values;

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

| JSON_OBJECT("a", JSON_COMPACT(a)) |

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

| {"a": [1,2,3]}                    |

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

MariaDB [bugs]> SELECT JSON_OBJECT("a", JSON_COMPACT(a), "b", b) FROM (SELECT * FROM json_test) AS json_test_values;

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

| JSON_OBJECT("a", JSON_COMPACT(a), "b", b) |

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

| {"a": "[1,2,3]", "b": "{\"a\":\"foo\"}"}  |

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

MariaDB [bugs]> SELECT JSON_OBJECT("a", JSON_COMPACT(a), "b", JSON_COMPACT(b)) FROM (SELECT * FROM json_test) AS json_test_values;

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

| JSON_OBJECT("a", JSON_COMPACT(a), "b", JSON_COMPACT(b)) |

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

| {"a": [1,2,3], "b": {"a":"foo"}}                        |

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

The output of `JSON_COMPACT(a)` is sometimes treated as JSON and sometimes as a plain string

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