(mysql.info) searching-on-two-keys
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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 from MySQL 5.0.0. See
index-merge-optimization.
You can also solve the problem efficiently by using a `UNION' that
combines the output of two separate `SELECT' statements. See
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';
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