内容简介:

1.普通B-TREE 索引;

2.唯一B-TREE 索引;

3.复合索引;

ORACLE 默认的索引类型为B-TREE 索引,表中的行标识符(ROWID)和行相关的列值被存储在一个平衡树的树状结构的索引块中;使用B-TREE索引有以下几个原因:

▢ 提高SQL语句的性能;

▢ 强制执行主键和唯一键约束的唯一性;

▢ 减少通过主键和外键约束关联的父表和子表间潜在的锁定问题 ;

1.普通B-TREE 索引

在一张未建立任何索引的500万行人员信息表中根据人员ID查询人员信息

select id,name,gender,homeaddr from th01 where id=998698;

--------------------------------------------------------------------------

| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |

--------------------------------------------------------------------------

| 0 | SELECT STATEMENT | | 1 | 38 | 16715 (1)| 00:03:21 |

|* 1 | TABLE ACCESS FULL| TH01 | 1 | 38 | 16715 (1)| 00:03:21 |

--------------------------------------------------------------------------

Predicate Information (identified by operation id):

---------------------------------------------------

1 - filter("ID"=998698)

Statistics

----------------------------------------------------------

61222 consistent gets

61208 physical reads

通过观察执行计划,CBO优化器执行了全表扫描,一致读取61222个块,61208个物理读,基于性能的考虑和表结构的分析,为其B-TREE索引:

SQL> CREATE INDEX IND_TH01_ID ON TH01(ID) TABLESPACE TBS02;

Index created.

Elapsed: 00:00:33.03

SQL> execute dbms_stats.gather_table_stats('sywu','th01',cascade=>true);

PL/SQL procedure successfully completed.

Elapsed: 00:00:04.17

SQL> @/oracle/getind

TABLE_NAME     INDEX_NAME    COLUMN_NAME    SIZE_GB    INDEX_TY   STATUS    LOGGING    DEGREE    NUM_ROWS    DISTINCT_KEYS

------------------------------ ------------------------------ ------------------------------ ---------- -------- -------- --------- ---------- -------- ---------- ------------- ---

TH01                    IND_TH01_ID          ID                    .091796875       NORMAL VALID             YES               1            DISABLED        5000000   5000000

显然对于高基数的列创建B-TREE索引是明智之选,对表进行分析后再次查询:

SQL>select id,name,gender,homeaddr from th01 where id=998698;

Elapsed: 00:00:00.00

-------------------------------------------------------------------------------------------

| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |

-------------------------------------------------------------------------------------------

| 0 | SELECT STATEMENT | | 1 | 38 | 4 (0)| 00:00:01 |

| 1 | TABLE ACCESS BY INDEX ROWID| TH01 | 1 | 38 | 4 (0)| 00:00:01 |

|* 2 | INDEX RANGE SCAN | IND_TH01_ID | 1 | | 3 (0)| 00:00:01 |

------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):

---------------------------------------------------

2 - access("ID"=998698)

Statistics

----------------------------------------------------------

5 consistent gets

0 physical reads

基于成本的考虑,CBO优化器选择了通过索引的方式读取数据,一致读取5个块,有效减少了额外的物理读;做个基于索引列的统计查询:

SQL> select count(id) from th01;

Elapsed: 00:00:00.15

| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |

-------------------------------------------------------------------------------------

| 0 | SELECT STATEMENT | | 1 | 6 | 3170 (1)| 00:00:39 |

| 1 | SORT AGGREGATE | | 1 | 6 | | |

| 2 | INDEX FAST FULL SCAN| IND_TH01_ID | 5000K| 28M| 3170 (1)| 00:00:39 |

-------------------------------------------------------------------------------------

Statistics

----------------------------------------------------------

11810 consistent gets

11794 physical reads

CBO 优化器选择了全索引扫描,依旧消耗额外的资源;但当统计列发生改变时:

SQL> select count(*) from th01;

Elapsed: 00:00:00.14

-------------------------------------------------------------------

| Id | Operation | Name | Rows | Cost (%CPU)| Time |

-------------------------------------------------------------------

| 0 | SELECT STATEMENT | | 1 | 16707 (1)| 00:03:21 |

| 1 | SORT AGGREGATE | | 1 | | |

| 2 | TABLE ACCESS FULL| TH01 | 5000K| 16707 (1)| 00:03:21 |

Statistics

----------------------------------------------------------

61221 consistent gets

61208 physical reads

此时CBO 优化器选择了全表扫描,并消耗更多的资源;

2.唯一B-TREE
索引:

在为表创建(主键、唯一约束)时,ORACLE
会默认创建一个B-TREE索引,这样既保证了数据的唯一性也提高了数据的检索效率:

SQL>
alter table th01 add constraints cs_th01_uq unique(idcard);

Table
altered.

Elapsed:
00:00:56.11

TABLE_NAME     
INDEX_NAME     COLUMN_NAME     SIZE_GB    INDEX_TY      STATUS     LOGGING       
DEGREE    COMPRESS    NUM_ROWS    DISTINCT_KEYS

------------------------------
------------------------------ ------------------------------
---------- -------- -------- --------- ---------- -------- ----------
------------- ---

TH01            
CS_TH01_UQ                  IDCARD                .15625               NORMAL   VALID              YES                  1             
DISABLED    4969898           4969898

以IDCARD查询人员信息:

------------------------------------------------------------------------------------------

|
0 | SELECT STATEMENT | | 1 | 35 | 3 (0)| 00:00:01
|

|
1 | TABLE ACCESS BY INDEX ROWID| TH01 | 1 | 35 | 3
(0)| 00:00:01 |

|*
2 | INDEX UNIQUE SCAN |
CS_TH01_UQ | 1 | | 2 (0)|
00:00:01 |

------------------------------------------------------------------------------------------

Predicate
Information (identified by operation id):

---------------------------------------------------

2
- access("IDCARD"='562456864646565545')

Statistics

----------------------------------------------------------

3 consistent gets

0 physical reads

做为默认创建的索引,它依旧能高效的工作,但ORACLE是不允许将其作为独立的索引删除的,只能通过删除约束的方式删除;对于主键,它的情况要复杂些,因为还要考虑外键的约束;基于这种方式创建的索引,当约束被删除时还要重新创建索引,显然在一张大表上花费的代价和时间是昂贵的;
so,采用如下的方式合理的建立约束和索引:

SQL>
alter table th01 add constraints CS_TH01_UQ unique(idcard)

2*
using index tablespace tbs03 ;

Table
altered.

Elapsed:
00:00:59.27

倘若有一天业务发生了改变,唯一约束已经不是必须的,但索引是必须的,那只需要删除约束保留索引:

SQL>
alter table th01 drop constraints CS_TH01_UQ keep index;

Table
altered.

Elapsed:
00:00:00.01

再次通过IDCARD
查询人员信息:

SQL>
select id,name,idcard from th01 where idcard='56234256878945';

------------------------------------------------------------------------------------------

|
Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |

------------------------------------------------------------------------------------------

|
0 | SELECT STATEMENT | | 1 | 35 | 3 (0)| 00:00:01
|

|
1 | TABLE ACCESS BY INDEX ROWID| TH01 | 1 | 35 | 3
(0)| 00:00:01 |

|*
2 | INDEX UNIQUE SCAN |
CS_TH01_UQ | 1 | | 2 (0)|
00:00:01 |

------------------------------------------------------------------------------------------

Predicate
Information (identified by operation id):

---------------------------------------------------

2
- access("IDCARD"='56234256878945')

Statistics

----------------------------------------------------------

1 recursive calls

0 db block gets

3 consistent gets

只创建唯一索引:

SQL>
create unique index ind_th02 on th02(idcard) tablespace tbs03;

唯一索引与唯一约束相比,唯一索引只创建索引而不添加约束,它保证索引列数值唯一性,允许有空值;

3.复合索引:

可以在多个列上创建索引,其结果称为复合索引或组合索引:

SQL>
create index ind_th01_union on th01(id,name,idcard) tablespace tbs03;

当查询的WHERE子句引用了索引的所有列或者只是前导列,CBO会使用复合索引

SQL>select
id,name,idcard from th01 where idcard='9876534655635666' and
id=68956254 and name='张三';

-----------------------------------------------------------------------------------

|
Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |

-----------------------------------------------------------------------------------

|
0 | SELECT STATEMENT | | 1 | 35 | 3 (0)| 00:00:01 |

|*
1 | INDEX RANGE SCAN|
IND_TH01_UNION | 1 | 35 | 3 (0)| 00:00:01 |

-----------------------------------------------------------------------------------

Predicate
Information (identified by operation id):

---------------------------------------------------

1
- access("ID"=68956254 AND "NAME"='张三'
AND

"IDCARD"='9876534655635666')

Statistics

----------------------------------------------------------

1 recursive calls

0 db block gets

3 consistent gets

2 physical reads

通过查询结果,对于之前创建的单列索引(IND_TH01_ID、CS_TH01_UQ),优化器已不再使用;对于复合索引(
IND_TH01_UNION) 来说,
id | id, name| id,name,idcard
三个组合都被认为是前导列,假如我只是在WHERE
子句中引用了第一个主导列ID,那么优化器依旧会选择复合索引(IND_TH01_UNION
)忽略单列索引(
IND_TH01_ID)

SQL>
select * from th01 where id=698698;

----------------------------------------------------------------------------------------------

|
Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time
|

----------------------------------------------------------------------------------------------

|
0 | SELECT STATEMENT | | 1 | 77 | 4 (0)|
00:00:01 |

|
1 | TABLE ACCESS BY INDEX ROWID| TH01 | 1 | 77 | 4
(0)| 00:00:01 |

|*
2 | INDEX RANGE SCAN
| IND_TH01_UNION
| 1 | | 3 (0)| 00:00:01 |

----------------------------------------------------------------------------------------------

Predicate
Information (identified by operation id):

---------------------------------------------------

2
- access("ID"=698698)

Statistics

----------------------------------------------------------

0 db block gets

5 consistent gets

假如where
子句中的条件不符合复合索引前导列的要求,那么优化器会忽略复合索引(
IND_TH01_UNION)选择单列索引(CS_TH01_UQ):

SQL>
select * from th01 where idcard='5623546566564665';

------------------------------------------------------------------------------------------

|
Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |

------------------------------------------------------------------------------------------

|
0 | SELECT STATEMENT | | 1 | 77 | 3 (0)|
00:00:01 |

|
1 | TABLE ACCESS BY INDEX ROWID| TH01 | 1 | 77 | 3
(0)| 00:00:01 |

|*
2 | INDEX UNIQUE SCAN
| CS_TH01_UQ
| 1 | | 2 (0)| 00:00:01 |

------------------------------------------------------------------------------------------

Predicate
Information (identified by operation id):

---------------------------------------------------

2
- access("IDCARD"='5623546566564665')

Statistics

----------------------------------------------------------

1 recursive calls

0 db block gets

3 consistent gets

1 physical reads

对于此时的查询条件
where idcard='5623546566564665'
已经不再符合(
id | id, name| id,name,idcard)
复合索引前导列的条件,优化器选择单列索引(
CS_TH01_UQ);又假如,我的WHERE
子句条件符合复合索引前导列要求但不是全部满足:

SQL>
select * from th01 where id=698698 and name='张三';

----------------------------------------------------------------------------------------------

|
Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time
|

----------------------------------------------------------------------------------------------

|
0 | SELECT STATEMENT | | 1 | 77 | 4 (0)|
00:00:01 |

|
1 | TABLE ACCESS BY INDEX ROWID| TH01 | 1 | 77 | 4
(0)| 00:00:01 |

|*
2 | INDEX RANGE SCAN
| IND_TH01_UNION
| 1 | | 3 (0)| 00:00:01 |

----------------------------------------------------------------------------------------------

Predicate
Information (identified by operation id):

---------------------------------------------------

2
- access("ID"=698698 AND "NAME"='张三')

Statistics

----------------------------------------------------------

3 consistent gets

0 physical reads

通过分析(where
id=698698 and name='张三'
)符合复合索引前导列要求,优化器选择复合索引(
IND_TH01_UNION) 忽略单列索引(
IND_TH01_ID);

创建复合索引时,排序是个很大的问题,ORACLE
建议将最频繁访问的列放在索引中最靠前的位置,应避免使用低基数的列作为复合索引的前导列.

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