oracle 执行计划分步解读（未看）

oracle 执行计划分步解读

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参考案例一：

SQL> set autotrace traceonly

SQL> create table t(col varchar2(20),col2 number);

表已创建。

SQL>

SQL> insert into t select rownum,rownum+1 from dual connect by level<=1000000;

已创建1000000行。

执行计划

----------------------------------------------------------

Plan hash value: 1731520519

------------------------------------------------------------------------------

| Id | Operation | Name | Rows | Cost (%CPU)| Time |

------------------------------------------------------------------------------

| 0 | INSERT STATEMENT | | 1 | 2 (0)| 00:00:01 |

| 1 | COUNT | | | | |

| 2 | CONNECT BY WITHOUT FILTERING| | | | |

| 3 | FAST DUAL | | 1 | 2 (0)| 00:00:01 |

------------------------------------------------------------------------------

Note

-----

- 'PLAN_TABLE' is old version

统计信息

----------------------------------------------------------

892 recursive calls

24493 db block gets

4477 consistent gets

0 physical reads

22946168 redo size

667 bytes sent via SQL*Net to client

610 bytes received via SQL*Net from client

4 SQL*Net roundtrips to/from client

3 sorts (memory)

0 sorts (disk)

1000000 rows processed

SQL> create index idx_t on t(col);

索引已创建。

SQL> select * from t where col=2 and col2=3;

执行计划

----------------------------------------------------------

Plan hash value: 1601196873

--------------------------------------------------------------------------

| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |

--------------------------------------------------------------------------

| 0 | SELECT STATEMENT | | 103 | 2575 | 553 (8)| 00:00:07 |

|* 1 | TABLE ACCESS FULL| T | 103 | 2575 | 553 (8)| 00:00:07 |

--------------------------------------------------------------------------

Predicate Information (identified by operation id):

---------------------------------------------------

1 - filter("COL2"=3 AND TO_NUMBER("COL")=2)

Note

-----

- 'PLAN_TABLE' is old version

- dynamic sampling used for this statement

统计信息

----------------------------------------------------------

28 recursive calls

0 db block gets

2406 consistent gets

0 physical reads

0 redo size

459 bytes sent via SQL*Net to client

384 bytes received via SQL*Net from client

2 SQL*Net roundtrips to/from client

0 sorts (memory)

0 sorts (disk)

1 rows processed

SQL>

SQL>

SQL> select * from t where col='2' and col2=3;

执行计划

----------------------------------------------------------

Plan hash value: 1594971208

--------------------------------------------------------------------------------

-----

| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time

|

--------------------------------------------------------------------------------

-----

| 0 | SELECT STATEMENT | | 103 | 2575 | 8 (0)| 00:00

:01 |

|* 1 | TABLE ACCESS BY INDEX ROWID| T | 103 | 2575 | 8 (0)| 00:00

:01 |

|* 2 | INDEX RANGE SCAN | IDX_T | 26 | | 3 (0)| 00:00

:01 |

--------------------------------------------------------------------------------

-----

Predicate Information (identified by operation id):

---------------------------------------------------

1 - filter("COL2"=3)

2 - access("COL"='2')

Note

-----

- 'PLAN_TABLE' is old version

- dynamic sampling used for this statement

统计信息

----------------------------------------------------------

112 recursive calls

0 db block gets

92 consistent gets

2 physical reads

0 redo size

459 bytes sent via SQL*Net to client

384 bytes received via SQL*Net from client

2 SQL*Net roundtrips to/from client

3 sorts (memory)

0 sorts (disk)

1 rows processed

SQL>

分析：

执行sql ：select * from t where col=2 and col2=3; 未走索引，逻辑读2406，获取一行，2406/1=2406 ,不符合获取单行记录小于5个逻辑读的标准。由此可见，该sql 还存在很大的调优空间。从谓词部分的执行计划可以看出使用了to_number类型转换。

predicate information 信息中有两种取值，分别是filter和access,一般而言索引读和hash连接的时候，体现为access。此时我们看到的filter。

通过修改查询sql ，重新查看执行计划发现predicate information部分发生了变化，逻辑读和获取行数的比率也发生了变化。

参考案例二：

SQL> set autotrace traceonly;

SQL> select * from t where col=2 and col2=3;

执行计划

----------------------------------------------------------

Plan hash value: 1601196873

--------------------------------------------------------------------------

| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |

--------------------------------------------------------------------------

| 0 | SELECT STATEMENT | | 103 | 2575 | 553 (8)| 00:00:07 |

|* 1 | TABLE ACCESS FULL| T | 103 | 2575 | 553 (8)| 00:00:07 |

--------------------------------------------------------------------------

Predicate Information (identified by operation id):

---------------------------------------------------

1 - filter("COL2"=3 AND TO_NUMBER("COL")=2)

Note

-----

- 'PLAN_TABLE' is old version

- dynamic sampling used for this statement

统计信息

----------------------------------------------------------

139 recursive calls

0 db block gets

2425 consistent gets

3 physical reads

0 redo size

459 bytes sent via SQL*Net to client

385 bytes received via SQL*Net from client

2 SQL*Net roundtrips to/from client

5 sorts (memory)

0 sorts (disk)

1 rows processed

SQL> analyze table t compute statistics for table for all indexes for all indexed colu

表已分析。

SQL> select * from t where col=2 and col2=3;

执行计划

----------------------------------------------------------

Plan hash value: 1601196873

--------------------------------------------------------------------------

| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |

--------------------------------------------------------------------------

| 0 | SELECT STATEMENT | | 1 | 15 | 552 (8)| 00:00:07 |

|* 1 | TABLE ACCESS FULL| T | 1 | 15 | 552 (8)| 00:00:07 |

--------------------------------------------------------------------------

Predicate Information (identified by operation id):

---------------------------------------------------

1 - filter("COL2"=3 AND TO_NUMBER("COL")=2)

Note

-----

- 'PLAN_TABLE' is old version

统计信息

----------------------------------------------------------

32 recursive calls

0 db block gets

2337 consistent gets

0 physical reads

0 redo size

459 bytes sent via SQL*Net to client

385 bytes received via SQL*Net from client

2 SQL*Net roundtrips to/from client

0 sorts (memory)

0 sorts (disk)

1 rows processed

分析：

在执行计划中看到“ - dynamic sampling used for this statement” 说明进行了动态采样。但我们提前收集表的信息是，可以看到能够减少一些额外的开销。