Spec-Zone .ru
спецификации, руководства, описания, API

8.13.2.2. 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:

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;