oracle 全扫描

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如何选择全扫描操作

先创建数据：

C:\>sqlplus "/as sysdba"SQL*Plus: Release 10.2.0.1.0 - Production on 星期日 3月 30 20:32:22 2014Copyright (c) 1982, 2005, Oracle.  All rights reserved.连接到:Oracle Database 10g Enterprise Edition Release 10.2.0.1.0 - Production With the Partitioning, OLAP and Data Mining optionsSQL> create table t1 as select trunc((rownum-1)/100) id, rpad(rownum, 100) t_pad from dba_source where rownum <= 10000;表已创建。SQL> create index t1_idx1 on t1(id);索引已创建。SQL> create table t2 as select mod(rownum, 100) id, rpad(rownum, 100) t_pad from dba_source where rownum<=10000;表已创建。SQL> create index t2_idx1 on t2(id);索引已创建。SQL> exec dbms_stats.gather_table_stats(user,'t2', method_opt=>'FOR ALL COLUMNS SIZE 1', cascade=>TRUE);PL/SQL 过程已成功完成。SQL>

查看t1的执行计划：

SQL> set autotrace traceonlySQL> alter system flush buffer_cache;系统已更改。SQL> alter system flush shared_pool;系统已更改。SQL> select * from t1 where id=1;已选择100行。执行计划----------------------------------------------------------Plan hash value: 2623418078---------------------------------------------------------------------------------------| Id  | Operation                   | Name    | Rows  | Bytes | Cost (%CPU)| Time     |---------------------------------------------------------------------------------------|   0 | SELECT STATEMENT            |         |   100 | 11500 |     3   (0)| 00:00:01 ||   1 |  TABLE ACCESS BY INDEX ROWID| T1      |   100 | 11500 |     3   (0)| 00:00:01 ||*  2 |   INDEX RANGE SCAN          | T1_IDX1 |   100 |       |     1   (0)| 00:00:01 |---------------------------------------------------------------------------------------Predicate Information (identified by operation id):---------------------------------------------------   2 - access("ID"=1)Note-----   - dynamic sampling used for this statement统计信息----------------------------------------------------------        307  recursive calls          0  db block gets        123  consistent gets        114  physical reads          0  redo size      12167  bytes sent via SQL*Net to client        451  bytes received via SQL*Net from client          8  SQL*Net roundtrips to/from client          3  sorts (memory)          0  sorts (disk)        100  rows processed

查看t2的执行计划：

SQL> alter system flush buffer_cache;系统已更改。SQL> alter system flush shared_pool;系统已更改。SQL> select * from t2 where id=1;已选择100行。执行计划----------------------------------------------------------Plan hash value: 1513984157--------------------------------------------------------------------------| Id  | Operation         | Name | Rows  | Bytes | Cost (%CPU)| Time     |--------------------------------------------------------------------------|   0 | SELECT STATEMENT  |      |   100 | 10300 |    36   (3)| 00:00:01 ||*  1 |  TABLE ACCESS FULL| T2   |   100 | 10300 |    36   (3)| 00:00:01 |--------------------------------------------------------------------------Predicate Information (identified by operation id):---------------------------------------------------   1 - filter("ID"=1)统计信息----------------------------------------------------------        203  recursive calls          0  db block gets        192  consistent gets        158  physical reads          0  redo size      11845  bytes sent via SQL*Net to client        451  bytes received via SQL*Net from client          8  SQL*Net roundtrips to/from client          3  sorts (memory)          0  sorts (disk)        100  rows processedSQL>

全扫描与舍弃

SQL> select table_name, num_rows, blocks from user_tables where table_name = 'T2';TABLE_NAME                       NUM_ROWS     BLOCKS------------------------------ ---------- ----------T2                                  10000        152SQL>

这里是152个数据块。

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