(mysql.info) index-merge-union

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 7.2.6.2 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:
 
    * In this form, where the index has exactly N parts (that is, all
      index parts are covered):
 
           KEY_PART1=CONST1 AND KEY_PART2=CONST2 ... AND KEY_PARTN=CONSTN
 
    * Any range condition over a primary key of an `InnoDB' or `BDB'
      table.
 
    * A condition for which the Index Merge method intersection
      algorithm is applicable.
 
 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;
 

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