LP:1006164 - Multi-table DELETE that uses innodb + index_merge/intersect may fail to delete rows

Based on customer case, csc#250:

CREATE TABLE tbl (
id PRIMARY KEY,
key1 INT,
key2 INT,
KEY (key1),
KEY (key2) ) engine=innodb;
...

DELETE tbl FROM tbl WHERE key1=const1 AND key2=const2;

^^ this will delete fewer rows than it actually matches.

• Problem happens only with InnoDB (not with MyISAM)
• Setting "index_merge=off" fixes the problem.
• Using a non-multi-table-table DELETE statement fixes the problem.