oracle 全扫描
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 2014 Copyright (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 options SQL> 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 traceonly
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 t1 where id=1;
已选择100行。
执行计划
----------------------------------------------------------
Plan hash value: 2623418078
--------------------------------------------------------------------------------
-------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Tim
e |
--------------------------------------------------------------------------------
-------
| 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 processed
SQL>
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 152 SQL>
