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join_buffer_size > 2 MiB causes query to hang



	The following query takes 6 minutes to complete …

            SELECT DISTINCT user FROM mysql.db
                BINARY db IN (
                    SELECT DISTINCT db FROM mysql.db
                    WHERE BINARY user=?

… when I have a join_buffer_size over 2,048 KiB.

	The effect is pretty remarkable: if I set the buffer size to 2,048 KiB, the query finishes in about 0.1 seconds, but with the buffer size at 2,049 KiB it takes several minutes.

	The long query time doesn’t happen in MySQL 5.7, but we see it in MariaDB 10.1 and 10.2.

	Is this a known issue?

	Thank you!

-Felipe Gasper
Mississauga, Ontario