Oracle 表的访问方式(2)-----索引扫描

索引扫描（Index scan）

　　我们先通过index查找到数据对应的rowid值(对于非唯一索引可能返回多个rowid值)，然后根据rowid直接从表中得到具体的数据，这种查找方式称为索引扫描或索引查找(index lookup)。一个rowid唯一的表示一行数据，该行对应的数据块是通过一次i/o得到的，在此情况下该次i/o只会读取一个数据库块。在索引中，除了存储每个索引的值外，索引还存储具有此值的行对应的ROWID值。索引扫描可以由2步组成： (1) 扫描索引得到对应的rowid值。 (2) 通过找到的rowid从表中读出具体的数据。

根据索引的类型与where限制条件的不同，有5种类型的索引扫描:

1）索引唯一扫描(index unique scan)

2）索引范围扫描(index range scan)

3）索引全扫描(index full scan)

4）索引快速扫描(index fast full scan)

5）索引跳跃扫描（INDEX SKIP SCAN）

索引唯一扫描(INDEX UNIQUE SCAN)

　　通过唯一索引查找一个数值经常返回单个ROWID

唯一索引由单独列组成:

 1 --收集统计信息
 2 SQL> exec dbms_stats.gather_table_stats(‘SCOTT‘,‘EMP‘);
 3
 4 PL/SQL procedure successfully completed.
 5
 6 Commit complete.
 7 SQL>
 8
 9
10 --获取创建索引语句
11 SQL> SELECT DBMS_METADATA.GET_DDL(‘INDEX‘,u.index_name)
12   2  FROM USER_INDEXES u
13   3  WHERE u.TABLE_NAME=‘EMP‘;
14
15 DBMS_METADATA.GET_DDL(‘INDEX‘,U.INDEX_NAME)
16 --------------------------------------------------------------------------------
17   CREATE UNIQUE INDEX "SCOTT"."PK_EMP" ON "SCOTT"."EMP" ("EMPNO")
18   PCTFREE 10
19
20 SQL>
21
22     1.索引名称 PK_EMP
23     2.索引包含列 EMPNO
24     3.索引为唯一索引
25
26     --执行计划走唯一索引的语句
27     SQL> SELECT * FROM SCOTT.EMP WHERE EMPNO=‘7369‘;
28
29     Execution Plan
30     ----------------------------------------------------------
31     Plan hash value: 2949544139
32
33     --------------------------------------------------------------------------------------
34     | Id  | Operation                   | Name   | Rows  | Bytes | Cost (%CPU)| Time     |
35     --------------------------------------------------------------------------------------
36     |   0 | SELECT STATEMENT            |        |     1 |    38 |     1   (0)| 00:00:01 |
37     |   1 |  TABLE ACCESS BY INDEX ROWID| EMP    |     1 |    38 |     1   (0)| 00:00:01 |
38     |*  2 |   INDEX UNIQUE SCAN         | PK_EMP |     1 |       |     0   (0)| 00:00:01 |
39     --------------------------------------------------------------------------------------
40
41     Predicate Information (identified by operation id):
42     ---------------------------------------------------
43
44        2 - access("EMPNO"=7369)
45
46
47
48     SQL> SELECT * FROM SCOTT.EMP WHERE EMPNO IN (‘7499‘,‘7521‘);
49
50
51     Execution Plan
52     ----------------------------------------------------------
53     Plan hash value: 2355049923
54
55     ---------------------------------------------------------------------------------------
56     | Id  | Operation                    | Name   | Rows  | Bytes | Cost (%CPU)| Time     |
57     ---------------------------------------------------------------------------------------
58     |   0 | SELECT STATEMENT             |        |     2 |    76 |     2   (0)| 00:00:01 |
59     |   1 |  INLIST ITERATOR             |        |       |       |            |          |
60     |   2 |   TABLE ACCESS BY INDEX ROWID| EMP    |     2 |    76 |     2   (0)| 00:00:01 |
61     |*  3 |    INDEX UNIQUE SCAN         | PK_EMP |     2 |       |     1   (0)| 00:00:01 |
62     ---------------------------------------------------------------------------------------
63
64     Predicate Information (identified by operation id):
65     ---------------------------------------------------
66
67        3 - access("EMPNO"=7499 OR "EMPNO"=7521)
68
69     SQL> SELECT * FROM SCOTT.EMP WHERE EMPNO=‘7499‘ OR EMPNO=‘7521‘;
70
71     Execution Plan
72     ----------------------------------------------------------
73     Plan hash value: 2355049923
74
75     ---------------------------------------------------------------------------------------
76     | Id  | Operation                    | Name   | Rows  | Bytes | Cost (%CPU)| Time     |
77     ---------------------------------------------------------------------------------------
78     |   0 | SELECT STATEMENT             |        |     2 |    76 |     2   (0)| 00:00:01 |
79     |   1 |  INLIST ITERATOR             |        |       |       |            |          |
80     |   2 |   TABLE ACCESS BY INDEX ROWID| EMP    |     2 |    76 |     2   (0)| 00:00:01 |
81     |*  3 |    INDEX UNIQUE SCAN         | PK_EMP |     2 |       |     1   (0)| 00:00:01 |
82     ---------------------------------------------------------------------------------------
83
84     Predicate Information (identified by operation id):
85     ---------------------------------------------------
86
87        3 - access("EMPNO"=7499 OR "EMPNO"=7521)
88     SQL>

SELECT * FROM SCOTT.EMP WHERE EMPNO=‘7369‘;
 SELECT * FROM SCOTT.EMP WHERE EMPNO IN (‘7499‘,‘7521‘);
 SELECT * FROM SCOTT.EMP WHERE EMPNO=‘7499‘ OR EMPNO=‘7521‘
 总结，索引在where条件中，且谓词条件可以确定唯一值时，走唯一索引。思考下2，3语句的查询过程

唯一索引由多个列组成(即组合索引)

 1     --创建一个唯一索引（优质索引）
 2     create unique index scott.idx_test on scott.emp(ename, deptno); --ename为引导列，表中ename列值具有唯一性
 3
 4   --谓词条件中的列顺序与索引的列顺序完全一致，走唯一索引
 5     SQL> select * from scott.emp where  ename = ‘ALLEN‘ and deptno = 20 ;
 6
 7     no rows selected
 8
 9
10     Execution Plan
11     ----------------------------------------------------------
12     Plan hash value: 4010583877
13
14     ----------------------------------------------------------------------------------------
15     | Id  | Operation                   | Name     | Rows  | Bytes | Cost (%CPU)| Time     |
16     ----------------------------------------------------------------------------------------
17     |   0 | SELECT STATEMENT            |          |     1 |    38 |     1   (0)| 00:00:01 |
18     |   1 |  TABLE ACCESS BY INDEX ROWID| EMP      |     1 |    38 |     1   (0)| 00:00:01 |
19     |*  2 |   INDEX UNIQUE SCAN         | IDX_TEST |     1 |       |     0   (0)| 00:00:01 |
20     ----------------------------------------------------------------------------------------
21
22     Predicate Information (identified by operation id):
23     ---------------------------------------------------
24
25        2 - access("ENAME"=‘ALLEN‘ AND "DEPTNO"=20)
26
27
28   --谓词条件中的列顺序与唯索引的列顺序不一致    ，走唯一索引
29     SQL> select * from scott.emp where deptno = 20 and ename = ‘ALLEN‘;
30
31     no rows selected
32
33
34     Execution Plan
35     ----------------------------------------------------------
36     Plan hash value: 4010583877
37
38     ----------------------------------------------------------------------------------------
39     | Id  | Operation                   | Name     | Rows  | Bytes | Cost (%CPU)| Time     |
40     ----------------------------------------------------------------------------------------
41     |   0 | SELECT STATEMENT            |          |     1 |    38 |     1   (0)| 00:00:01 |
42     |   1 |  TABLE ACCESS BY INDEX ROWID| EMP      |     1 |    38 |     1   (0)| 00:00:01 |
43     |*  2 |   INDEX UNIQUE SCAN         | IDX_TEST |     1 |       |     0   (0)| 00:00:01 |
44     ----------------------------------------------------------------------------------------
45
46     Predicate Information (identified by operation id):
47     ---------------------------------------------------
48
49        2 - access("ENAME"=‘ALLEN‘ AND "DEPTNO"=20)
50
51     --只有引导列在谓词条件中
52     SQL> select * from scott.emp where ename = ‘ALLEN‘; --即使是唯一数据 也不走唯一索引
53
54
55     Execution Plan
56     ----------------------------------------------------------
57     Plan hash value: 2317538385
58
59     ----------------------------------------------------------------------------------------
60     | Id  | Operation                   | Name     | Rows  | Bytes | Cost (%CPU)| Time     |
61     ----------------------------------------------------------------------------------------
62     |   0 | SELECT STATEMENT            |          |     1 |    38 |     2   (0)| 00:00:01 |
63     |   1 |  TABLE ACCESS BY INDEX ROWID| EMP      |     1 |    38 |     2   (0)| 00:00:01 |
64     |*  2 |   INDEX RANGE SCAN          | IDX_TEST |     1 |       |     1   (0)| 00:00:01 |
65     ----------------------------------------------------------------------------------------
66
67     Predicate Information (identified by operation id):
68     ---------------------------------------------------
69
70        2 - access("ENAME"=‘ALLEN‘)
71
72
73     --引导列不在谓词条件中
74     SQL> select * from scott.emp where deptno = 20;
75
76
77     Execution Plan
78     ----------------------------------------------------------
79     Plan hash value: 3956160932
80
81     --------------------------------------------------------------------------
82     | Id  | Operation         | Name | Rows  | Bytes | Cost (%CPU)| Time     |
83     --------------------------------------------------------------------------
84     |   0 | SELECT STATEMENT  |      |     5 |   190 |     3   (0)| 00:00:01 |
85     |*  1 |  TABLE ACCESS FULL| EMP  |     5 |   190 |     3   (0)| 00:00:01 |
86     --------------------------------------------------------------------------
87
88     Predicate Information (identified by operation id):
89     ---------------------------------------------------
90
91        1 - filter("DEPTNO"=20)
92
93     SQL>     

 1     --创建一个唯一索引（劣质索引）
 2     create unique index idx_test on scott.emp(deptno,ename); --deptno为引导列，表中deptno列值不具有唯一性
 3
 4     分别对别如下sql的执行计划：
 5     --谓词条件中的列顺序与索引的列顺序完全一致，，走唯一索引
 6     SQL> select * from scott.emp where deptno = 20 and ename = ‘ALLEN‘;
 7
 8     no rows selected
 9
10     Execution Plan
11     ----------------------------------------------------------
12     Plan hash value: 1531058326
13
14     ------------------------------------------------------------------------------------------
15     | Id  | Operation                   | Name       | Rows  | Bytes | Cost (%CPU)| Time     |
16     ------------------------------------------------------------------------------------------
17     |   0 | SELECT STATEMENT            |            |     1 |    38 |     1   (0)| 00:00:01 |
18     |   1 |  TABLE ACCESS BY INDEX ROWID| EMP        |     1 |    38 |     1   (0)| 00:00:01 |
19     |*  2 |   INDEX UNIQUE SCAN         | IDX_TEST01 |     1 |       |     0   (0)| 00:00:01 |
20     ------------------------------------------------------------------------------------------
21
22     Predicate Information (identified by operation id):
23     ---------------------------------------------------
24
25        2 - access("DEPTNO"=20 AND "ENAME"=‘ALLEN‘)
26
27     SQL>
28
29   --谓词条件中的列顺序与唯索引的列顺序不一致    ，走唯一索引
30     SQL> select * from scott.emp where  ename = ‘ALLEN‘ and deptno = 20 ;
31
32     no rows selected
33
34     Execution Plan
35     ----------------------------------------------------------
36     Plan hash value: 1531058326
37
38     ------------------------------------------------------------------------------------------
39     | Id  | Operation                   | Name       | Rows  | Bytes | Cost (%CPU)| Time     |
40     ------------------------------------------------------------------------------------------
41     |   0 | SELECT STATEMENT            |            |     1 |    38 |     1   (0)| 00:00:01 |
42     |   1 |  TABLE ACCESS BY INDEX ROWID| EMP        |     1 |    38 |     1   (0)| 00:00:01 |
43     |*  2 |   INDEX UNIQUE SCAN         | IDX_TEST01 |     1 |       |     0   (0)| 00:00:01 |
44     ------------------------------------------------------------------------------------------
45
46     Predicate Information (identified by operation id):
47     ---------------------------------------------------
48
49        2 - access("DEPTNO"=20 AND "ENAME"=‘ALLEN‘)
50
51     SQL>
52
53     --只有引导列在谓词条件中
54     SQL> select * from scott.emp where deptno = 20;
55
56     Execution Plan
57     ----------------------------------------------------------
58     Plan hash value: 560737562
59
60     ------------------------------------------------------------------------------------------
61     | Id  | Operation                   | Name       | Rows  | Bytes | Cost (%CPU)| Time     |
62     ------------------------------------------------------------------------------------------
63     |   0 | SELECT STATEMENT            |            |     5 |   190 |     2   (0)| 00:00:01 |
64     |   1 |  TABLE ACCESS BY INDEX ROWID| EMP        |     5 |   190 |     2   (0)| 00:00:01 |
65     |*  2 |   INDEX RANGE SCAN          | IDX_TEST01 |     5 |       |     1   (0)| 00:00:01 |
66     ------------------------------------------------------------------------------------------
67
68     Predicate Information (identified by operation id):
69     ---------------------------------------------------
70
71        2 - access("DEPTNO"=20)
72     SQL>
73
74     --引导列不在谓词条件
75     SQL> select * from scott.emp where ename = ‘ALLEN‘;
76
77     Execution Plan
78     ----------------------------------------------------------
79     Plan hash value: 3956160932
80
81     --------------------------------------------------------------------------
82     | Id  | Operation         | Name | Rows  | Bytes | Cost (%CPU)| Time     |
83     --------------------------------------------------------------------------
84     |   0 | SELECT STATEMENT  |      |     1 |    38 |     3   (0)| 00:00:01 |
85     |*  1 |  TABLE ACCESS FULL| EMP  |     1 |    38 |     3   (0)| 00:00:01 |
86     --------------------------------------------------------------------------
87
88     Predicate Information (identified by operation id):
89     ---------------------------------------------------
90
91        1 - filter("ENAME"=‘ALLEN‘)
92
93     SQL>     

总结：使用组合索引时，遵守以下原则：

1.引导列标识性要强;

2.索引列尽量全部出现在谓词条件中

3.引导列尽量出现在谓词条件中

索引范围扫描(INDEX RANGE SCAN)

　　使用一个索引存取多行数据，在唯一索引上使用索引范围扫描的典型情况下是在谓词(where限制条件)中使用了范围操作符 (如>、<、<>、>=、<=、between)。在非唯一索引上，谓词"="也可能返回多行数据，所以在非唯一索引上都使用索引范围扫描。

　　使用index rang scan的3种情况：  1.在唯一索引列上使用了range操作符(> < <> >= <= between)  2.在组合索引上，只使用部分列进行查询，导致查询出多行  3.对非唯一索引列上进行的任何查询。

索引全扫描(index full scan)

　　与全表扫描对应，也有相应的全索引扫描。在某些情况下，可能进行全索引扫描而不是范围扫描，需要注意的是全索引扫描只在CBO模式下才有效。 CBO根据统计数值得知进行全索引扫描比进行全表扫描更有效时，才进行全索引扫描，而且此时查询出的数据都必须从索引中可以直接得到。
 
  一般通过索引进行排序时,会用到(index full scan)

索引快速扫描(index fast full scan)

　　扫描索引中的所有的数据块，与 index full scan很类似，但是一个显著的区别就是它不对查询出的数据进行排序，即数据不是以排序顺序被返回。在这种存取方法中，可以使用多块读功能，也可以使用并行读入，以便获得最大吞吐量与缩短执行时间。

索引跳跃扫描（INDEX SKIP SCAN）

　　Skip Scans are initiated by probing the index for distinct values of the prefix column. Each of these distinct values is then used as a starting point for a regular index search. The result is several separate searches of a single index that, when combined, eliminate the affect of the prefix column.

　　skip scan会探测出索引前导列的唯一值个数，每个唯一值都会作为常规扫描的入口，在此基础上做一次查找，最后合并 这些查询。例如：表employees (sex, employee_id, address) ，有一个组合索引(sex, employee_id). 在索引跳跃的情况 下，我们可以逻辑上把他们看成两个索引，一个是（男，employee_id)，一个是(女，employee_id). select * from employees where employee_id=1;发出这个查询后，oracle先进入sex为男的入口，查找employee_id=1的条目。 再进入sex为女的入口，查找employee_id=1的条目。最后合并两个结果集

参考blog：http://www.itpub.net/thread-1372696-1-1.html

　　　　　http://blog.csdn.net/dba_waterbin/article/details/8550405