• The query is a join,
• The query takes advantage of the OR optimization,
• At least one branch of the optimized OR expression in the WHERE clause involves a subquery.
• 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.

For example:

  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.