共计 9045 个字符,预计需要花费 23 分钟才能阅读完成。
这篇文章将为大家详细讲解有关如何理解 SQL 优化中连接谓词推入,文章内容质量较高,因此丸趣 TV 小编分享给大家做个参考,希望大家阅读完这篇文章后对相关知识有一定的了解。
SQL 优化之连接谓词推入:
环境准备:
create table emp1 as select * from emp;
create table emp2 as select * from emp;
create index idx_emp1 on emp1(empno);
create index idx_emp2 on emp2(empno);
create or replace view emp_view as select emp1.empno as empno1 from emp1;
create or replace view emp_view_union as select emp1.empno as empno1 from emp1 union all select emp2.empno as empno1 from emp2;
赋权,scott 用户可以开启 set autot
grant select on v_$sesstat to scott;
grant select on v_$statname to scott;
grant select on v_$mystat to scott;
sql 范例 1:
select /*+ no_merge(emp_view) */emp.empno from emp,emp_view where emp.empno=emp_view.empno1(+) and emp.ename= FROD
可以看到 emp 表和 emp_view 视图左外连接,视图是补充表。
查看执行计划:
SQL set autot traceonly
SQL set line 250
SQL select /*+ no_merge(emp_view) */emp.empno from emp,emp_view where emp.empno=emp_view.empno1(+) and emp.ename= FROD
no rows selected
Execution Plan
———————————————————-
Plan hash value: 101695337
————————————————————————————
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
————————————————————————————
| 0 | SELECT STATEMENT | | 1 | 12 | 4 (0)| 00:00:01 |
| 1 | NESTED LOOPS OUTER | | 1 | 12 | 4 (0)| 00:00:01 |
|* 2 | TABLE ACCESS FULL | EMP | 1 | 10 | 3 (0)| 00:00:01 |
| 3 | VIEW PUSHED PREDICATE | EMP_VIEW | 1 | 2 | 1 (0)| 00:00:01 |
|* 4 | INDEX RANGE SCAN | IDX_EMP1 | 1 | 13 | 1 (0)| 00:00:01 |
————————————————————————————
Predicate Information (identified by operation id):
—————————————————
2 – filter(EMP . ENAME = FROD)
4 – access(EMP1 . EMPNO = EMP . EMPNO)
Note
—–
– dynamic sampling used for this statement (level=2)
Statistics
———————————————————-
0 recursive calls
0 db block gets
7 consistent gets
0 physical reads
0 redo size
333 bytes sent via SQL*Net to client
508 bytes received via SQL*Net from client
1 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
0 rows processed
该执行计划比较好理解:步骤 2 与步骤 3 同级,但是步骤 2 没有子 ID,所以最先执行步骤 2.
步骤 2:该步骤有一个 filter 条件 filter(EMP . ENAME = FROD),全表扫描 emp 表,找出 ename=frod 的所有数据
步骤 4:索引范围扫描,目标条件满足 access(EMP1 . EMPNO = EMP . EMPNO),这里把视图和表左外连接的条件推入到了视图中。
步骤 3:VIEW PUSHED PREDICATE 说明没有做视图合并,把视图当做一个独立单元来执行,但是把外部条件推入到了视图内部
。如果没有做这次连接谓词推入,那么就不会在抓取视图内部数据的时候用到 emp1 表上的索引,那样的话就会全表扫描了。
步骤 1:然后两个结果集做循环嵌套外连接,得到结果。
下面验证一下,连接谓词未推入,抓取视图数据集的时候不会走 emp1 的索引,而是全表扫描 emp1 了。
select /*+ no_merge(emp_view) no_push_pred(emp_view) */emp.empno from emp,emp_view where emp.empno=emp_view.empno1(+) and emp.ename= FROD
Execution Plan
———————————————————-
Plan hash value: 3053348535
—————————————————————————————–
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
—————————————————————————————–
| 0 | SELECT STATEMENT | | 1 | 23 | 6 (17)| 00:00:01 |
| 1 | MERGE JOIN OUTER | | 1 | 23 | 6 (17)| 00:00:01 |
|* 2 | TABLE ACCESS BY INDEX ROWID| EMP | 1 | 10 | 2 (0)| 00:00:01 |
| 3 | INDEX FULL SCAN | PK_EMP | 14 | | 1 (0)| 00:00:01 |
|* 4 | SORT JOIN | | 14 | 182 | 4 (25)| 00:00:01 |
| 5 | VIEW | EMP_VIEW | 14 | 182 | 3 (0)| 00:00:01 |
| 6 | TABLE ACCESS FULL | EMP1 | 14 | 182 | 3 (0)| 00:00:01 |
—————————————————————————————–
Predicate Information (identified by operation id):
—————————————————
2 – filter(EMP . ENAME = FROD)
4 – access(EMP . EMPNO = EMP_VIEW . EMPNO1 (+))
filter(EMP . EMPNO = EMP_VIEW . EMPNO1 (+))
Note
—–
– dynamic sampling used for this statement (level=2)
Statistics
———————————————————-
11 recursive calls
0 db block gets
9 consistent gets
1 physical reads
0 redo size
333 bytes sent via SQL*Net to client
508 bytes received via SQL*Net from client
1 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
0 rows processed
这时可以看到对 emp 表进行索引全扫描,利用条件 EMP . ENAME = FROD 回表,得到数据集;视图并没有走 emp1 的索引,而是全表扫描,并将结果进行排序,然后与第一个结果集进行排序合并外连接。
范例 sql:
select emp.empno from emp,emp_view_union where emp.empno=emp_view_union.empno1 and emp.ename= FROD
Execution Plan
———————————————————-
Plan hash value: 2223410919
————————————————————————————-
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)|
————————————————————————————-
| 0 | SELECT STATEMENT | | 2 | 24 | 5 (0)|
| 1 | NESTED LOOPS | | 2 | 24 | 5 (0)|
|* 2 | TABLE ACCESS FULL | EMP | 1 | 10 | 3 (0)|
| 3 | VIEW | EMP_VIEW_UNION | 1 | 2 | 2 (0)|
| 4 | UNION ALL PUSHED PREDICATE | | | | |
|* 5 | INDEX RANGE SCAN | IDX_EMP1 | 1 | 13 | 1 (0)|
|* 6 | INDEX RANGE SCAN | IDX_EMP2 | 1 | 13 | 1 (0)|
————————————————————————————-
Predicate Information (identified by operation id):
—————————————————
2 – filter(EMP . ENAME = FROD)
5 – access(EMP1 . EMPNO = EMP . EMPNO)
6 – access(EMP2 . EMPNO = EMP . EMPNO)
Note
—–
– dynamic sampling used for this statement (level=2)
Statistics
———————————————————-
28 recursive calls
0 db block gets
39 consistent gets
0 physical reads
0 redo size
333 bytes sent via SQL*Net to client
508 bytes received via SQL*Net from client
1 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
0 rows processed
因为视图定义中有 union all,所以 EMP_VIEW_UNION 不能做视图合并,但是可以做连接谓词推入,所以看到步骤 5 和步骤 6 将连接条件推入到了视图内部,从而走了 emp1 和 emp2 表的索引。然后将结果集与全表扫描 emp 表得到的 ename=frod 的结果集做循环嵌套连接,得到最终结果。
同样地,如果阻止了连接谓词推入,那么视图内部结果集会按照全表扫描。
select /*+ no_push_pred(emp_view_union)*/emp.empno from emp,emp_view_union where emp.empno=emp_view_union.empno1 and emp.ename= FROD
Execution Plan
———————————————————-
Plan hash value: 894575737
————————————————————————————
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)|
————————————————————————————
| 0 | SELECT STATEMENT | | 2 | 46 | 9 (12)|
| 1 | MERGE JOIN | | 2 | 46 | 9 (12)|
|* 2 | TABLE ACCESS BY INDEX ROWID| EMP | 1 | 10 | 2 (0)|
| 3 | INDEX FULL SCAN | PK_EMP | 14 | | 1 (0)|
|* 4 | SORT JOIN | | 28 | 364 | 7 (15)|
| 5 | VIEW | EMP_VIEW_UNION | 28 | 364 | 6 (0)|
| 6 | UNION-ALL | | | | |
| 7 | TABLE ACCESS FULL | EMP1 | 14 | 182 | 3 (0)|
| 8 | TABLE ACCESS FULL | EMP2 | 14 | 182 | 3 (0)|
————————————————————————————
Predicate Information (identified by operation id):
—————————————————
2 – filter(EMP . ENAME = FROD)
4 – access(EMP . EMPNO = EMP_VIEW_UNION . EMPNO1)
filter(EMP . EMPNO = EMP_VIEW_UNION . EMPNO1)
Note
—–
– dynamic sampling used for this statement (level=2)
Statistics
———————————————————-
14 recursive calls
0 db block gets
14 consistent gets
0 physical reads
0 redo size
333 bytes sent via SQL*Net to client
508 bytes received via SQL*Net from client
1 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
0 rows processed
注意:能否做谓词推入,与视图能否合并,是否是内嵌视图没有关系,与目标视图的类型,与外部查询之间的连接类型以及连接方法有关。
如下是一个无法谓词推入的 sql:
原因:视图在外链接的右侧。
select /*+ no_merge(emp_view) use_nl(emp_view) push_pred(emp_view) */emp.empno from emp,emp_view where emp.empno=emp_view.empno1 and ename= FROD
Execution Plan
———————————————————-
Plan hash value: 3774177413
——————————————————————————–
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
——————————————————————————–
| 0 | SELECT STATEMENT | | 1 | 23 | 6 (0)| 00:00:01 |
| 1 | NESTED LOOPS | | 1 | 23 | 6 (0)| 00:00:01 |
|* 2 | TABLE ACCESS FULL | EMP | 1 | 10 | 3 (0)| 00:00:01 |
|* 3 | VIEW | EMP_VIEW | 1 | 13 | 3 (0)| 00:00:01 |
| 4 | TABLE ACCESS FULL| EMP1 | 14 | 182 | 3 (0)| 00:00:01 |
——————————————————————————–
Predicate Information (identified by operation id):
—————————————————
2 – filter(ENAME = FROD)
3 – filter(EMP . EMPNO = EMP_VIEW . EMPNO1)
Note
—–
– dynamic sampling used for this statement (level=2)
Statistics
———————————————————-
11 recursive calls
0 db block gets
14 consistent gets
0 physical reads
0 redo size
333 bytes sent via SQL*Net to client
508 bytes received via SQL*Net from client
1 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
0 rows processed
连接谓词推入条件:
视图定义语句中存在 union all/union/group by/distinct
视图与外部查询之间是外连接,半连接,反连接
以上只要满足一种条件就可以谓词推入,比如内连接,但是视图定义语句中有 union all。
如上面的范例 sql:select emp.empno from emp,emp_view_union where emp.empno=emp_view_union.empno1 and emp.ename= FROD
关于如何理解 SQL 优化中连接谓词推入就分享到这里了,希望以上内容可以对大家有一定的帮助,可以学到更多知识。如果觉得文章不错,可以把它分享出去让更多的人看到。