merge no_merge

select /*+ merge(a) */ a.ct, b.dname  from (select deptno, count(*) as ct from emp group by deptno) a, dept b  3   where b.deptno = a.deptno;已用时间:  00: 00: 00.01执行计划----------------------------------------------------------Plan hash value: 2708255165-----------------------------------------------------------------------------------------| Id  | Operation      | Name| Rows| Bytes | Cost (%CPU)| Time|-----------------------------------------------------------------------------------------|   0 | SELECT STATEMENT      | |     9 |   207 |     7  (29)| 00:00:01 ||   1 |  HASH GROUP BY      | |     9 |   207 |     7  (29)| 00:00:01 ||   2 |   MERGE JOIN      | |    14 |   322 |     6  (17)| 00:00:01 ||   3 |    TABLE ACCESS BY INDEX ROWID| DEPT|     4 |    80 |     2   (0)| 00:00:01 ||   4 |     INDEX FULL SCAN      | PK_DEPT |     4 ||     1   (0)| 00:00:01 ||*  5 |    SORT JOIN      | |    14 |    42 |     4  (25)| 00:00:01 ||   6 |     TABLE ACCESS FULL      | EMP|    14 |    42 |     3   (0)| 00:00:01 |-----------------------------------------------------------------------------------------Predicate Information (identified by operation id):---------------------------------------------------   5 - access("B"."DEPTNO"="DEPTNO")       filter("B"."DEPTNO"="DEPTNO")select /*+ no_merge(a) */ a.ct, b.dname  from (select deptno, count(*) as ct from emp group by deptno) a, dept b  3   where b.deptno = a.deptno;已用时间:  00: 00: 00.01执行计划----------------------------------------------------------Plan hash value: 2992795152----------------------------------------------------------------------------------------| Id  | Operation     | Name    | Rows  | Bytes | Cost (%CPU)| Time     |----------------------------------------------------------------------------------------|   0 | SELECT STATEMENT     |       |     3 |   117 |     7(29)| 00:00:01 ||   1 |  MERGE JOIN     |       |     3 |   117 |     7(29)| 00:00:01 ||   2 |   TABLE ACCESS BY INDEX ROWID| DEPT    |     4 |    52 |     2 (0)| 00:00:01 ||   3 |    INDEX FULL SCAN     | PK_DEPT |     4 |       |     1 (0)| 00:00:01 ||*  4 |   SORT JOIN     |       |     3 |    78 |     5(40)| 00:00:01 ||   5 |    VIEW      |       |     3 |    78 |     4(25)| 00:00:01 ||   6 |     HASH GROUP BY     |       |     3 |     9 |     4(25)| 00:00:01 ||   7 |      TABLE ACCESS FULL     | EMP     |    14 |    42 |     3 (0)| 00:00:01 |----------------------------------------------------------------------------------------Predicate Information (identified by operation id):---------------------------------------------------   4 - access("B"."DEPTNO"="A"."DEPTNO")       filter("B"."DEPTNO"="A"."DEPTNO")