从Oracle8i开始Oracle提供采样表扫描特性。
Oracle访问数据的基本方法有:
1.全表扫描
2.采样表扫描
全表扫描(Full table Scan)
全表扫描返回表中所有的记录。
执行全表扫描，Oracle读表中的所有记录，考查每一行是否满足WHERE条件。Oracle顺序的读分配给该表的每一个数据块，这样全表扫描能够受益于多块读.
每个数据块Oracle只读一次.
采样表扫描(sample table scan)
采样表扫描返回表中随机采样数据。
这种访问方式需要在FROM语句中包含SAMPLE选项或者SAMPLE BLOCK选项.
SAMPLE选项:
当按行采样来执行一个采样表扫描时，Oracle从表中读取特定百分比的记录，并判断是否满足WHERE子句以返回结果。
SAMPLE BLOCK选项:
使用此选项时，Oracle读取特定百分比的BLOCK，考查结果集是否满足WHERE条件以返回满足条件的纪录.
Sample_Percent:
Sample_Percent是一个数字，定义结果集中包含记录占总记录数量的百分比。
Sample值应该在[0.000001,99.999999]之间。
1.使用SAMPLE选项
SQL> select * from employee SAMPLE(30);
EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO
---------- ---------- --------- ---------- --------- ---------- ---------- ----------
7369 SMITH CLERK 7902 17-DEC-80 800 20
7788 SCOTT ANALYST 7566 19-APR-87 3000 20
7839 KING PRESIDENT 17-NOV-81 5000 10
Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=CHOOSE (Cost=2 Card=25 Bytes=2175)
1 0 TABLE ACCESS (SAMPLE) OF 'EMPLOYEE' (Cost=2 Card=25 Bytes=2175)
Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
5 consistent gets
0 physical reads
0 redo size
880 bytes sent via SQL*Net to client
503 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
3 rows processed
SQL> select * from employee SAMPLE(20);
EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO
---------- ---------- --------- ---------- --------- ---------- ---------- ----------
7654 MARTIN SALESMAN 7698 28-SEP-81 1250 1400 30
7844 TURNER SALESMAN 7698 08-SEP-81 1500 0 30
Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=CHOOSE (Cost=2 Card=16 Bytes=1392)
1 0 TABLE ACCESS (SAMPLE) OF 'EMPLOYEE' (Cost=2 Card=16 Bytes=1392)
Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
5 consistent gets
0 physical reads
0 redo size
839 bytes sent via SQL*Net to client
503 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
2 rows processed
|
2.使用SAMPLE BLOCK选项
SQL> SELECT * FROM employee SAMPLE BLOCK (50);
EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO
---------- ---------- --------- ---------- --------- ---------- ---------- ----------
7369 SMITH CLERK 7902 17-DEC-80 800 20
7499 ALLEN SALESMAN 7698 20-FEB-81 1600 300 30
7521 WARD SALESMAN 7698 22-FEB-81 1250 500 30
7566 JONES MANAGER 7839 02-APR-81 2975 20
7654 MARTIN SALESMAN 7698 28-SEP-81 1250 1400 30
7698 BLAKE MANAGER 7839 01-MAY-81 2850 30
7782 CLARK MANAGER 7839 09-JUN-81 2450 10
7788 SCOTT ANALYST 7566 19-APR-87 3000 20
7839 KING PRESIDENT 17-NOV-81 5000 10
7844 TURNER SALESMAN 7698 08-SEP-81 1500 0 30
10 rows selected.
Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=CHOOSE (Cost=2 Card=41 Bytes=3567)
1 0 TABLE ACCESS (SAMPLE) OF 'EMPLOYEE' (Cost=2 Card=41 Bytes=3567)
Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
4 consistent gets
0 physical reads
0 redo size
1162 bytes sent via SQL*Net to client
503 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
10 rows processed
SQL>
|
3.采样前n条记录的查询
也可以使用dbms_random包实现
SQL> select * from (
2 select * from employee
3 order by dbms_random.value )
4 where rownum <= 4;
EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO
---------- ---------- --------- ---------- --------- ---------- ---------- ----------
7654 MARTIN SALESMAN 7698 28-SEP-81 1250 1400 30
7839 KING PRESIDENT 17-NOV-81 5000 10
7369 SMITH CLERK 7902 17-DEC-80 800 20
7788 SCOTT ANALYST 7566 19-APR-87 3000 20
Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=CHOOSE
1 0 COUNT (STOPKEY)
2 1 VIEW
3 2 SORT (ORDER BY STOPKEY)
4 3 TABLE ACCESS (FULL) OF 'EMPLOYEE'
Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
3 consistent gets
0 physical reads
0 redo size
927 bytes sent via SQL*Net to client
503 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
1 sorts (memory)
0 sorts (disk)
4 rows processed
|
对比一下SAMPLE选项
SQL> SELECT * FROM employee SAMPLE (40);
EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO
---------- ---------- --------- ---------- --------- ---------- ---------- ----------
7499 ALLEN SALESMAN 7698 20-FEB-81 1600 300 30
7521 WARD SALESMAN 7698 22-FEB-81 1250 500 30
7698 BLAKE MANAGER 7839 01-MAY-81 2850 30
7839 KING PRESIDENT 17-NOV-81 5000 10
7844 TURNER SALESMAN 7698 08-SEP-81 1500 0 30
Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=CHOOSE (Cost=2 Card=33 Bytes=2871)
1 0 TABLE ACCESS (SAMPLE) OF 'EMPLOYEE' (Cost=2 Card=33 Bytes=2871)
Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
5 consistent gets
0 physical reads
0 redo size
961 bytes sent via SQL*Net to client
503 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
5 rows processed
SQL>
|
主要注意以下几点:
1.sample只对单表生效，不能用于表连接和远程表
2.sample会使SQL自动使用CBO
