Oracle三种链接方式的区别

1 nested loops join
--我们用设置statistics_level=all的方式来观察如下表连接语句的执行计划：

--T2表被访问100次(驱动表访问1次，被驱动表访问100次）
--这个set linesize 1000对dbms_xplan.display_cursor还是有影响的，如果没有设置，默认情况下的输出，将会少了很多列，如BUFFERS等
Set linesize 1000
alter session set statistics_level=all ;
SELECT /*+ leading(t1) use_nl(t2)*/ *
FROM t1, t2
WHERE t1.id = t2.t1_id;
--略去记录结果
select * from table(dbms_xplan.display_cursor(null,null,'allstats last'));
-------------------------------------------------------------------------------------
| Id  | Operation          | Name | Starts | E-Rows | A-Rows |   A-Time   | Buffers |
-------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |      |      1 |        |    100 |00:00:00.94 |     100K|
|   1 |  NESTED LOOPS      |      |      1 |    100 |    100 |00:00:00.94 |     100K|
|   2 |   TABLE ACCESS FULL| T1   |      1 |    100 |    100 |00:00:00.01 |      14 |
|*  3 |   TABLE ACCESS FULL| T2   |    100 |      1 |    100 |00:00:00.94 |     100K|
-------------------------------------------------------------------------------------
3 - filter("T1"."ID"="T2"."T1_ID")

---换个语句，这次T2表被访问2次(驱动表访问1次，被驱动表访问2次）
Set linesize 1000
alter session set statistics_level=all ;
SELECT /*+ leading(t1) use_nl(t2) */ *
FROM t1, t2
WHERE t1.id = t2.t1_id
AND t1.n in(17, 19);
select * from table(dbms_xplan.display_cursor(null,null,'allstats last'));
-------------------------------------------------------------------------------------
| Id  | Operation          | Name | Starts | E-Rows | A-Rows |   A-Time   | Buffers |
-------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |      |      1 |        |      2 |00:00:00.02 |    2019 |
|   1 |  NESTED LOOPS      |      |      1 |      2 |      2 |00:00:00.02 |    2019 |
|*  2 |   TABLE ACCESS FULL| T1   |      1 |      2 |      2 |00:00:00.01 |       8 |
|*  3 |   TABLE ACCESS FULL| T2   |      2 |      1 |      2 |00:00:00.02 |    2011 |
-------------------------------------------------------------------------------------
 2 - filter(("T1"."N"=17 OR "T1"."N"=19))
 3 - filter("T1"."ID"="T2"."T1_ID")

--继续换个语句，这次T2表被访问1次(驱动表访问1次，被驱动表访问1次）
Set linesize 1000
alter session set statistics_level=all ;
  SELECT /*+ leading(t1) use_nl(t2) */ *
FROM t1, t2
WHERE t1.id = t2.t1_id
AND t1.n = 19;
select * from table(dbms_xplan.display_cursor(null,null,'allstats last'));

SQL> select * from table(dbms_xplan.display_cursor(null,null,'allstats last'));
-------------------------------------------------------------------------------------
| Id  | Operation          | Name | Starts | E-Rows | A-Rows |   A-Time   | Buffers |
-------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |      |      1 |        |      1 |00:00:00.01 |    1014 |
|   1 |  NESTED LOOPS      |      |      1 |      1 |      1 |00:00:00.01 |    1014 |
|*  2 |   TABLE ACCESS FULL| T1   |      1 |      1 |      1 |00:00:00.01 |       8 |
|*  3 |   TABLE ACCESS FULL| T2   |      1 |      1 |      1 |00:00:00.01 |    1006 |
-------------------------------------------------------------------------------------
2 - filter("T1"."N"=19)
3 - filter("T1"."ID"="T2"."T1_ID")

---接下来，T2表居然被访问0次(驱动表访问1次，被驱动表访问0次）
SELECT /*+ leading(t1) use_nl(t2) */ *
FROM t1, t2
WHERE t1.id = t2.t1_id
AND t1.n = 999999999;
select * from table(dbms_xplan.display_cursor(null,null,'allstats last'));
-------------------------------------------------------------------------------------
| Id  | Operation          | Name | Starts | E-Rows | A-Rows |   A-Time   | Buffers |
-------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |      |      1 |        |      0 |00:00:00.01 |       7 |
|   1 |  NESTED LOOPS      |      |      1 |      1 |      0 |00:00:00.01 |       7 |
|*  2 |   TABLE ACCESS FULL| T1   |      1 |      1 |      0 |00:00:00.01 |       7 |
|*  3 |   TABLE ACCESS FULL| T2   |      0 |      1 |      0 |00:00:00.01 |       0 |
-------------------------------------------------------------------------------------
2 - filter("T1"."N"=999999999)
3 - filter("T1"."ID"="T2"."T1_ID")

---到最后，不只是T2表被访问0次，连T1表也访问0次
SELECT /*+ leading(t1) use_nl(t2) */ *
FROM t1, t2
WHERE t1.id = t2.t1_id
AND 1=2;
select * from table(dbms_xplan.display_cursor(null,null,'allstats last'));

----------------------------------------------------------------------------
| Id  | Operation           | Name | Starts | E-Rows | A-Rows |   A-Time   |
----------------------------------------------------------------------------
|   0 | SELECT STATEMENT    |      |      1 |        |      0 |00:00:00.01 |
|*  1 |  FILTER             |      |      1 |        |      0 |00:00:00.01 |
|   2 |   NESTED LOOPS      |      |      0 |    100 |      0 |00:00:00.01 |
|   3 |    TABLE ACCESS FULL| T1   |      0 |    100 |      0 |00:00:00.01 |
|*  4 |    TABLE ACCESS FULL| T2   |      0 |      1 |      0 |00:00:00.01 |
----------------------------------------------------------------------------
1 - filter(NULL IS NOT NULL)
4 - filter("T1"."ID"="T2"."T1_ID")

--分析T2表被访问次数不同的原因
---解释T2表为啥被访问100次
select count(*) from t1;
  COUNT(*)
----------
    100
---解释T2表为啥被访问2次
select count(*) from t1 where t1.n in (17,19);
  COUNT(*)
----------
     2
---解释T2表为啥被访问1次
select count(*) from t1 where t1.n = 19;
  COUNT(*)
----------
     1
---解释T2表为啥被访问0次
select count(*) from t1 where t1.n = 999999999;
  COUNT(*)
----------
     0

 

未完待续：
2 hash join
使用限制最多，不支持>、<、<>、like
3 merge sort join
支持>、<不支持<>、like