SELECT statements can return incorrect results in certain cases where the following are true:

<ul>
<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.
</ul>

For example:

<verbatim>
  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'))
</verbatim>

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.