According to the documentation for "START TRANSACTION":

READ ONLY mode allows the storage engine to apply optimizations that cannot be used for transactions which write data. The only exception to this rule is that read only transactions can perform DDL statements on temporary tables.

Except it doesn't work:

SET TRANSACTION ISOLATION LEVEL REPEATABLE READ;

START TRANSACTION READ ONLY;

CREATE TEMPORARY TABLE t (id int unsigned NOT NULL AUTO_INCREMENT PRIMARY KEY,val int unsigned NOT NULL);

INSERT INTO t (val) VALUES (22),(96),(47),(5),(22);

SELECT * FROM t;

DROP TEMPORARY TABLE t;

COMMIT;

...Fails with "#1792 - Cannot execute statement in a READ ONLY transaction" on the CREATE TEMPORARY TABLE statement.