MySQL Internals Manual  /  ...  /  OR Relations OR Relations

An ORed search has the form condition1 OR condition2, as in this example:

WHERE column1 = 'x' OR column2 = 'y'

Here the optimizer's decision is to use a sequential scan.

There is also an option to use index merge under such circumstances. See Section, “Index Merge Optimizer” and Index Merge Optimization, for more information.

The above warning does not apply if the same column is used in both conditions. For example:

WHERE column1 = 'x' OR column1 = 'y'

In such a case, the search is indexed because the expression is a range search. This subject will be revisited during the discussion of the IN predicate.