(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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