7.2.6.2. The Index Merge Union Access Algorithm
The applicability criteria for this algorithm are similar to
those for the Index Merge method intersection algorithm. The
algorithm can be employed when the table's
WHERE clause was converted to several range
conditions on different keys combined with
OR, and each condition is one of the
following:
-
In this form, where the index has exactly
N parts (that is, all index
parts are covered):
key_part1=const1 AND key_part2=const2 ... AND key_partN=constN
Any range condition over a primary key of an
InnoDB or BDB table.
A condition for which the Index Merge method intersection
algorithm is applicable.
Examples:
SELECT * FROM t1 WHERE key1=1 OR key2=2 OR key3=3;
SELECT * FROM innodb_table WHERE (key1=1 AND key2=2) OR
(key3='foo' AND key4='bar') AND key5=5;
