SELECT statements can return incorrect results in certain cases where the following are true:
<li> The query is a join,
<li> The query takes advantage of the OR optimization,
<li> At least one branch of the optimized OR expression in the WHERE clause involves a subquery.
<li> At least one branch of the optimized OR expression in the WHERE clause refers to a column of a table other than the table to which the OR optimization applies.
CREATE TABLE t1(a);
INSERT INTO t1 VALUES('A');
CREATE INDEX i1 ON t1(a);
CREATE TABLE t2(b);
INSERT INTO t2 VALUES('B');
CREATE TABLE t3(c);
INSERT INTO t3 VALUES('C');
SELECT * FROM t1, t2 WHERE
(a='A' AND b='X')
OR (a='A' AND EXISTS (SELECT * FROM t3 WHERE c='C'))
The bug causes the query above returns zero rows instead of the expected result - one row.
This bug was discovered in a real application and reported on the mailing list. Bisection reveals that it was introduced on 2009-08-13 by commit [86a06dd049]. It first appeared in release 3.6.18.