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MySQL 5.5 Reference Manual  /  ...  /  Searching on Two Keys

3.6.7 Searching on Two Keys

An OR using a single key is well optimized, as is the handling of AND.

The one tricky case is that of searching on two different keys combined with OR:

SELECT field1_index, field2_index FROM test_table
WHERE field1_index = '1' OR  field2_index = '1'

This case is optimized. See Section 8.2.1.3, “Index Merge Optimization”.

You can also solve the problem efficiently by using a UNION that combines the output of two separate SELECT statements. See Section 13.2.9.3, “UNION”.

Each SELECT searches only one key and can be optimized:

SELECT field1_index, field2_index
    FROM test_table WHERE field1_index = '1'
UNION
SELECT field1_index, field2_index
    FROM test_table WHERE field2_index = '1';