(mysql.info) index-merge-intersection

(mysql.info) index-merge-optimization

 
 7.2.6.1 The Index Merge Intersection Access Algorithm
 .....................................................
 
 This access algorithm can be employed when a `WHERE' clause was
 converted to several range conditions on different keys combined with
 `AND', 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.
 
 Examples:
 
      SELECT * FROM INNODB_TABLE WHERE PRIMARY_KEY < 10 AND KEY_COL1=20;
 
      SELECT * FROM TBL_NAME
        WHERE (KEY1_PART1=1 AND KEY1_PART2=2) AND KEY2=2;
 
 The Index Merge intersection algorithm performs simultaneous scans on
 all used indexes and produces the intersection of row sequences that it
 receives from the merged index scans.
 
 If all columns used in the query are covered by the used indexes, full
 table rows are not retrieved (`EXPLAIN' output contains `Using index'
 in `Extra' field in this case). Here is an example of such a query:
 
      SELECT COUNT(*) FROM t1 WHERE key1=1 AND key2=1;
 
 If the used indexes don't cover all columns used in the query, full
 rows are retrieved only when the range conditions for all used keys are
 satisfied.
 
 If one of the merged conditions is a condition over a primary key of an
 `InnoDB' or `BDB' table, it is not used for row retrieval, but is used
 to filter out rows retrieved using other conditions.

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