Optimizing SELECT Statements •Index Merge Intersection Access Algorithm •Index Merge Union Access Algorithm •Index Merge Sort-Union Access Algorithm •Influencing Index Merge Optimization •An  N -part expression of this form, where the index has exactly  N  parts (that is, all index parts are covered): •Any range condition over the primary key of an  InnoDB  table. Index Merge Intersection Access Algorithm This access algorithm is applicable when a  WHERE  clause is converted to several range conditions on different keys combined with  AND , and each condition is one of the following: 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: If the used indexes do not 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 the primary key of an  InnoDB  table, it is not used for row retrieval, but is used to filter out rows retrieved using other conditions. The criteria for this algorithm are similar to those for the Index Merge intersection algorithm. The algorithm is applicable when the table’s  WHERE  clause is converted to several range conditions on different keys combined with  OR , and each condition is one of the following: key_part1  =  const1  AND  key_part2  =  const2  ... AND  key_partN  =  constN SELECT COUNT(*) FROM t1 WHERE key1 = 1 AND key2 = 1; key_part1  =  const1  AND  key_part2  =  const2  ... AND  key_partN  =  constN SELECT * FROM t1 SELECT * FROM  tbl_name WHERE  key1_part1  = 1 AND  key1_part2  = 2 AND  key2  = 2; SELECT * FROM  innodb_table WHERE  primary_key  < 10 AND  key_col1  = 20; Examples: Index Merge Union Access Algorithm •An  N -part expression of this form, where the index has exactly  N  parts (that is, all index parts are covered): •Any range condition over a primary key of an  InnoDB  table. •A condition for which the Index Merge intersection algorithm is applicable. Examples: 1652