JSON_ARRAY_INTERSECT: nested call JSON_ARRAY_INTERSECT(JSON_ARRAY_INTERSECT(a,b), b) returns NULL instead of intersect result

Summary

Single-level `JSON_ARRAY_INTERSECT(c1, c2)` returns correct JSON arrays.
Nesting the function:

SELECT JSON_ARRAY_INTERSECT(JSON_ARRAY_INTERSECT(c1, c2), c2) FROM t1;

produces NULL for every row, although the inner intersect is non-empty.
The outer intersect with `c2` should return the same result as the single-level call (`[2,3]` and `[4]` for sample data).

Version

MariaDB 12.3.1-MariaDB-asan

Minimal reproducer

DROP TABLE IF EXISTS t1;

CREATE TABLE t1 (

c1 longtext CHARACTER SET utf8mb4 COLLATE utf8mb4_bin

DEFAULT NULL CHECK (json_valid(c1)),

c2 longtext CHARACTER SET utf8mb4 COLLATE utf8mb4_bin

DEFAULT NULL CHECK (json_valid(c2))

) ENGINE=InnoDB;

INSERT INTO t1 VALUES

('[1,2,3]', '[2,3,4]'),

('[2,3,4]', '[4,5,6]');

-- single-level

SELECT JSON_ARRAY_INTERSECT(c1, c2) AS r FROM t1;

-- nested (produces bug)

SELECT JSON_ARRAY_INTERSECT(JSON_ARRAY_INTERSECT(c1, c2), c2) AS r FROM t1;

Actual Result

-- single-level

r

[2, 3]

[4]

-- nested

r

NULL

NULL

Expected Result

Nested output should match single-level intersect:

[2, 3]

[4]

Type / Priority / Component

Notes

• No SQL error is produced; the wrong result is silent NULLs.
• Only the nested expression triggers the bug; single-level usage works as expected.
• Not related to filtering with `WHERE JSON_ARRAY_INTERSECT(...)`.

Discovery

• Tool: crdb_mutate n200 MariaDB batch (`--oracle --oracle-psi`)
• PSI: MDEV-31411 (`JSON_ARRAY_INTERSECT(c1,c2)`)
• Nested candidate confirmed manually.

Environment

• MariaDB tested: 12.3.1-MariaDB-asan
• Reproduced: 2026-06-06