如果你对一个列和一组有限的值进行比较, 优化器可能执行多次扫描并对结果进行合并连接.

举例:

    SELECT *

    FROM LODGING

    WHERE MANAGER IN (‘BILL GATES’,’KEN MULLER’);

    优化器可能将它转换成以下形式

    SELECT *

    FROM LODGING

    WHERE MANAGER = ‘BILL GATES’

    OR MANAGER = ’KEN MULLER’;

    当选择执行路径时, 优化器可能对每个条件采用LODGING$MANAGER上的索引范围扫描. 返回的ROWID用来访问LODGING表的记录 (通过TABLE ACCESS BY ROWID 的方式). 最后两组记录以连接(CONCATENATION)的形式被组合成一个单一的集合.

Explain Plan :

SELECT STATEMENT Optimizer=CHOOSE

   CONCATENATION

      TABLE ACCESS (BY INDEX ROWID) OF LODGING

         INDEX (RANGE SCAN ) OF LODGING$MANAGER (NON-UNIQUE)

     TABLE ACCESS (BY INDEX ROWID) OF LODGING

         INDEX (RANGE SCAN ) OF LODGING$MANAGER (NON-UNIQUE)