COUNT(DISTINCT) returns 2 instead of 1 for a self-join with a constant equality predicate when big_tables=1

On MariaDB 11.8.9, enabling big_tables causes COUNT(DISTINCT t.a) to return 2 for a self-join filtered by t.a = 1. All qualifying rows have t.a = 1, so the distinct count should be 1.

Replacing the literal with an equivalent scalar COUNT() subquery returns the correct result. Disabling big_tables also restores the correct result.

The discrepancy was verified using the native mariadb command-line client in a fresh instance.

Reproducer:

Run the following statements in an empty database, using the same connection:

CREATE TABLE t (

a INT PRIMARY KEY

) ENGINE=InnoDB;

INSERT INTO t VALUES (1), (2);

SET SESSION big_tables = 1;

SELECT COUNT(DISTINCT t.a) AS result

FROM t, t AS t2

WHERE t.a = 1;

SELECT COUNT(DISTINCT t.a) AS result

FROM t, t AS t2

WHERE t.a = CAST(

(SELECT COUNT(*) FROM (SELECT 1) AS s)

AS SIGNED

);

SET SESSION big_tables = 0;

SELECT COUNT(DISTINCT t.a) AS result

FROM t, t AS t2

WHERE t.a = 1;

Actual result on 11.8.9-MariaDB:

Query 1, with big_tables=1 and a literal predicate:

result

2

Query 2, with big_tables=1 and an equivalent scalar-subquery predicate:

result

1

Query 3, after setting big_tables=0:

result

1

Expected result:

All three queries should return:

result

1

The self-join produces two qualifying rows, but t.a is 1 in both rows. COUNT(DISTINCT t.a) must therefore return 1.

The scalar subquery also returns exactly 1, so replacing the literal equality bound must not change the aggregate result.