Oracle 表访问方式

SQL> set autotrace traceonly;
1.INDEX SKIP SCAN  
CREATE TABLE test
 AS
SELECT DECODE(MOD(ROWNUM,2), 0, '1', '2' ) a,
ROWNUM-1 b,
    ROWNUM-2 c,
 ROWNUM-3 d,
 ROWNUM-4 e
 FROM all_objects
 /

表已创建。

SQL> set autotrace off
SQL> select distinct a from test;

A
--
1
2

--A列只有两个唯一值

SQL> CREATE INDEX test_idx ON test(a,b,c)
2 /

索引已创建。


SQL> ANALYZE TABLE test COMPUTE STATISTICS
 FOR TABLE
 FOR ALL INDEXES
 FOR ALL INDEXED COLUMNS
 /

表已分析。

SQL> set autotrace traceonly explain
SQL> SELECT * FROM test WHERE b = 99
2 /
SQL>  SELECT * FROM test WHERE b = 99;


执行计划
----------------------------------------------------------
Plan hash value: 2705879578

----------------------------------------------------------------------------------------
| Id  | Operation                   | Name     | Rows  | Bytes | Cost (%CPU)| Time     |
----------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT            |          |     1 |    23 |     4   (0)| 00:00:01 |
|   1 |  TABLE ACCESS BY INDEX ROWID| TEST     |     1 |    23 |     4   (0)| 00:00:01 |
|*  2 |   INDEX SKIP SCAN           | TEST_IDX |     1 |       |     3   (0)| 00:00:01 |
----------------------------------------------------------------------------------------

2.INDEX RANGE SCAN
SQL> select empno from emp where empno > 4567;

已选择14行。


执行计划
----------------------------------------------------------
Plan hash value: 1567865628

---------------------------------------------------------------------------
| Id  | Operation        | Name   | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------
|   0 | SELECT STATEMENT |        |    14 |    56 |     1   (0)| 00:00:01 |
|*  1 |  INDEX RANGE SCAN| PK_EMP |    14 |    56 |     1   (0)| 00:00:01 |
---------------------------------------------------------------------------


3.INDEX UNIQUE SCAN

SQL> select empno from emp where empno = 7521;


执行计划
----------------------------------------------------------
Plan hash value: 56244932

----------------------------------------------------------------------------
| Id  | Operation         | Name   | Rows  | Bytes | Cost (%CPU)| Time     |
----------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |        |     1 |     4 |     0   (0)| 00:00:01 |
|*  1 |  INDEX UNIQUE SCAN| PK_EMP |     1 |     4 |     0   (0)| 00:00:01 |
----------------------------------------------------------------------------



4.TABLE ACCESS BY USER ROWID
SQL> select * from emp where rowid = 'AAAIwdAAEAAAAAiAAA';


执行计划
----------------------------------------------------------
Plan hash value: 1116584662

-----------------------------------------------------------------------------------
| Id  | Operation                  | Name | Rows  | Bytes | Cost (%CPU)| Time     |
-----------------------------------------------------------------------------------
|   0 | SELECT STATEMENT           |      |     1 |    37 |     1   (0)| 00:00:01 |
|   1 |  TABLE ACCESS BY USER ROWID| EMP  |     1 |    37 |     1   (0)| 00:00:01 |
-----------------------------------------------------------------------------------



5.TABLE ACCESS FULL
SQL> select * from emp;

已选择14行。


执行计划
----------------------------------------------------------
Plan hash value: 3956160932

--------------------------------------------------------------------------
| Id  | Operation         | Name | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |      |    14 |   518 |     3   (0)| 00:00:01 |
|   1 |  TABLE ACCESS FULL| EMP  |    14 |   518 |     3   (0)| 00:00:01 |
--------------------------------------------------------------------------



6.INDEX FULL SCAN
SQL> select empno from emp;

已选择14行。


执行计划
----------------------------------------------------------
Plan hash value: 179099197

---------------------------------------------------------------------------
| Id  | Operation        | Name   | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------
|   0 | SELECT STATEMENT |        |    14 |    56 |     1   (0)| 00:00:01 |
|   1 |  INDEX FULL SCAN | PK_EMP |    14 |    56 |     1   (0)| 00:00:01 |
---------------------------------------------------------------------------



7.INDEX FAST FULL SCAN
SQL> select count(empno) from emp_1;


执行计划
----------------------------------------------------------
Plan hash value: 2411118849

-----------------------------------------------------------------------------------
| Id  | Operation             | Name      | Rows  | Bytes | Cost (%CPU)| Time     |
-----------------------------------------------------------------------------------
|   0 | SELECT STATEMENT      |           |     1 |     3 |    23   (0)| 00:00:01 |
|   1 |  SORT AGGREGATE       |           |     1 |     3 |            |          |
|   2 |   INDEX FAST FULL SCAN| IDX_EMP_1 | 28672 | 86016 |    23   (0)| 00:00:01 |
-----------------------------------------------------------------------------------