分类： Oracle
2011-12-22 15:50:08
SQL> execute dbms_stats.gather_table_stats('SCOTT','T1');
PL/SQL 过程已成功完成。
SQL> SET AUTOT TRACE EXP
SQL> SELECT COUNT(*) FROM T1;
执行计划
--------------------------------------------------
| Id | Operation | Name | Rows | Cost (%CPU)| Time |
-------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 124 (4)| 00:00:02 |
| 1 | SORT AGGREGATE | | 1 | | |
| 2 | TABLE ACCESS FULL| T1 | 116K| 124 (4)| 00:00:02 |
-----------------------------------------------------
代价为124，运行的计划为全表扫描。
SQL> DELETE T1 WHERE DEPTNO=10;
已删除24576行。
SQL> COMMIT;
提交完成。
SQL> execute dbms_stats.gather_table_stats('SCOTT','T1');
PL/SQL 过程已成功完成。
SQL> SELECT COUNT(*) FROM T1;
执行计划
-----------------------------------------------------
| Id | Operation | Name | Rows | Cost (%CPU)| Time |
-------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 123 (3)| 00:00:02 |
| 1 | SORT AGGREGATE | | 1 | | |
| 2 | TABLE ACCESS FULL| T1 | 90286 | 123 (3)| 00:00:02 |
-----------------------------------------------------
SQL> --1.降低高水位
SQL> alter table t1 move tablespace users;
表已更改。
SQL> execute dbms_stats.gather_table_stats('SCOTT','T1');
PL/SQL 过程已成功完成。
SQL> SELECT COUNT(*) FROM T1;
执行计划
-----------------------------------------------------
| Id | Operation | Name | Rows |Cost (%CPU)| Time |
-------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 102 (3)| 00:00:02 |
| 1 | SORT AGGREGATE | | 1 | | |
| 2 | TABLE ACCESS FULL| T1 | 90667 | 102 (3)| 00:00:02 |
-----------------------------------------------------
代价为102，降低了
SQL> --2.修改pctfree
SQL> alter table t1 pctfree 0;
表已更改。
SQL> alter table t1 move tablespace users;
表已更改。
SQL> execute dbms_stats.gather_table_stats('SCOTT','T1');
PL/SQL 过程已成功完成。
SQL> SELECT COUNT(*) FROM T1;
执行计划
----------------------------------------------------------
Plan hash value: 3724264953
-------------------------------------------------------------------
| Id | Operation | Name | Rows | Cost (%CPU)| Time |
-------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 92 (4)| 00:00:02 |
| 1 | SORT AGGREGATE | | 1 | | |
| 2 | TABLE ACCESS FULL| T1 | 91791 | 92 (4)| 00:00:02 |
-------------------------------------------------------------------
代价为92，降低了10%
SQL> --3.参数db_file_multiblock_read_count=64
SQL> --4.建立b*tree类型的索引
SQL> create index i1 on t1(empno);
索引已创建。
SQL> execute dbms_stats.gather_index_stats('SCOTT','I1');
PL/SQL 过程已成功完成。
SQL> SELECT COUNT(*) FROM T1;
执行计划
----------------------------------------------------------
Plan hash value: 3724264953
-------------------------------------------------------------------
| Id | Operation | Name | Rows | Cost (%CPU)| Time |
-------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 92 (4)| 00:00:02 |
| 1 | SORT AGGREGATE | | 1 | | |
| 2 | TABLE ACCESS FULL| T1 | 91791 | 92 (4)| 00:00:02 |
-------------------------------------------------------------------
为什么没有使用我们建立的索引，因为null不进入普通的索引！
SQL> alter table t1 modify(empno not null);
表已更改。
SQL> SELECT COUNT(*) FROM T1;
执行计划
----------------------------------------------------------
Plan hash value: 129980005
----------------------------------------------------------------------
| Id | Operation | Name | Rows | Cost (%CPU)| Time |
----------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 36 (6)| 00:00:01 |
| 1 | SORT AGGREGATE | | 1 | | |
| 2 | INDEX FAST FULL SCAN| I1 | 91791 | 36 (6)| 00:00:01 |
----------------------------------------------------------------------
我们的索引起到了很大的作用！
SQL> --5.使用并行查询的特性
强制全表扫描，屏蔽索引
SQL> select /*+ full(t1) parallel(t1 2) */ COUNT(*) FROM T1;
执行计划
----------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Cost (%CPU)| Time | TQ |IN-OUT| PQ Distrib |
----------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 51 (4)| 00:00:01 | | | |
| 1 | SORT AGGREGATE | | 1 | | | | | |
| 2 | PX COORDINATOR | | | | | | | |
| 3 | PX SEND QC (RANDOM) | :TQ10000 | 1 | | | Q1,00 | P->S | QC(RAND) |
| 4 | SORT AGGREGATE | | 1 | | | Q1,00 | PCWP | |
| 5 | PX BLOCK ITERATOR | | 91791 | 51 (4)| 00:00:01 | Q1,00 | PCWC| |
| 6 | TABLE ACCESS FULL| T1 | 91791 | 51 (4)| 00:00:01 | Q1,00 | PCWP | |
-----------------------------------------------------------------------------------------------
并行度越高，代价越低
SQL> alter table t1 parallel 4;
表已更改。
也可以通过使用表的属性来定义并行度，但是影响比较大，不如语句级别限制并行！
SQL> select count(*) from t1;
执行计划
-----------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Cost (%CPU)| Time | TQ |IN-OUT| PQDistrib |
---------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 25 (0)| 00:00:01 | | | |
| 1 | SORT AGGREGATE | | 1 | | | | | |
| 2 | PX COORDINATOR | | | | | | | |
| 3 | PX SEND QC (RANDOM) | :TQ10000 | 1 | | | Q1,00 | P->S | QC(RAND) |
| 4 | SORT AGGREGATE | | 1 | | | Q1,00 | PCWP | |
| 5 | PX BLOCK ITERATOR | | 91791 | 25 (0)| 00:00:01 | Q1,00 | PCWC | |
| 6 | TABLE ACCESS FULL| T1 | 91791 | 25 (0)| 00:00:01 | Q1,00 | PCWP | |
---------------------------------------------------------------------------------------------
代价为25，代价比两个的又少一半！
SQL> --6.建立位图索引来避免全表扫描
SQL> create bitmap index i2 on t1(deptno);
索引已创建。
SQL> execute dbms_stats.gather_index_stats('SCOTT','I2');
PL/SQL 过程已成功完成。
SQL> select count(*) from t1;
执行计划
----------------------------------------------------------
Plan hash value: 3738977131
------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Cost (%CPU)| Time |
------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 4 (0)| 00:00:01 |
| 1 | SORT AGGREGATE | | 1 | | |
| 2 | BITMAP CONVERSION COUNT | | 91791 | 4 (0)| 00:00:01 |
| 3 | BITMAP INDEX FAST FULL SCAN| I2 | | | |
------------------------------------------------------------------------------
SQL> alter index i2 parallel 4;
索引已更改。
SQL> select count(*) from t1;
执行计划
----------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Cost (%CPU)| Time | TQ |IN-OUT| PQ Distrib |
----------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 2 (0)| 00:00:01 | | | |
| 1 | SORT AGGREGATE | | 1 | | | | | |
| 2 | PX COORDINATOR | | | | | | | |
| 3 | PX SEND QC (RANDOM) | :TQ10000 | 1 | | | Q1,00 | P->S | QC (RAND) |
| 4 | SORT AGGREGATE | | 1 | | | Q1,00 | PCWP | |
| 5 | PX BLOCK ITERATOR | | 91791 | 2 (0)| 00:00:01 | Q1,00 |PCWC | |
| 6 | BITMAP CONVERSION COUNT | | 91791 | 2 (0)| 00:00:01 | Q1,00 |PCWP | |
| 7 | BITMAP INDEX FAST FULL SCAN| I2 | | | | Q1,00 | PCWP | |
--------------------------------------------------------------------------------------------
代价为2，原来为124，优化无止境呀！
只有你把握原理，一切尽在掌握！
