Oracle教程之Oracle管理索引（三）--Oracle建立索引
1、建立b-tree索引
（1）相关概念
根块（一个）：索引顶级块，它包含指向下一级节点（分支块或叶块）的信息。
分支块：它包含指向下一级的节点（分支块或叶块）的信息。
叶块：它包含索引入口数据，索引入口包含索引列值或受限ROWID
（2）建立索引
如果在where子句中要经常引用某列或某几列，应该给予这些列值建立B-*树索引
10:23:58 SQL> create index ind_ename on scott.emp(ename) pctfree 30
10:24:32 2 tablespace indexes;
Index created.
（3）使用索引
10:24:41 SQL> set autotrace on explain
10:26:54 SQL> select * from scott.emp where ename='SCOTT';
EMPNO ENAME JOB HIREDATE SAL COMM DEPTNO
---------- ---------- --------- ------------------- ---------- ---------- ----------
7788 SCOTT ANALYST 1987-04-19 00:00:00 3000 20
Execution Plan
----------------------------------------------------------
Plan hash value: 48385638
-----------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-----------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 29 | 2 (0)| 00:00:01 |
| 1 | TABLE ACCESS BY INDEX ROWID| EMP | 1 | 29 | 2 (0)| 00:00:01 |
|* 2 | INDEX RANGE SCAN | IND_ENAME | 1 | | 1 (0)| 00:00:01 |
-----------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - access("ENAME"='SCOTT')
2、建立位图索引
位图索引以为位值标识索引数据，它主要用于在DSS(Decision Support System)系统中执行数据统计、数据汇总等操作。
B*-树索引建立在重复值很少的列上，而位图索引建立在重复值很多，不同值相对固定的列上。
使用位图索引可以节省大量的磁盘空间，为B*-树索引所用空间1/20~1/10.
建立位图索引时，Oracle会基于每个不同值建立一个位图。
（1）建立位图索引
SQL> create bitmap index test_sex_bitind on test(sex) tablespace indexes;
Index created.
（2）分析索引结构
SQL> analyze index test_sex_bitind validate structure;
Index analyzed.
SQL> select index_name,index_type,tablespace_name,blevel,leaf_blocks,num_rows from user_indexes where index_name='TEST_SEX_BITIND';
INDEX_NAME INDEX_TYPE TABLESPACE_NAME BLEVEL LEAF_BLOCKS NUM_ROWS
--------------- --------------- ---------------- ------- ----------- ----------
TEST_SEX_BITIND BITMAP INDEXES 0 1 2
――在重复值高的列上适合建立bitmap的索引
（3）使用位图索引
SQL> select /*+ index (test TEST_SEX_BITIND)*/ name,sex from test where sex='F';
10000 rows selected.
Execution Plan
----------------------------------------------------------
Plan hash value: 2624764158
------------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 10000 | 97K| 85 (0)| 00:00:02 |
| 1 | TABLE ACCESS BY INDEX ROWID | TEST | 10000 | 97K| 85 (0)| 00:00:02 |
| 2 | BITMAP CONVERSION TO ROWIDS| | | | | |
|* 3 | BITMAP INDEX SINGLE VALUE | TEST_SEX_BITIND | | | | |
------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
3 - access("SEX"='F')
Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
0 consistent gets
0 physical reads
0 redo size
0 bytes sent via SQL*Net to client
0 bytes received via SQL*Net from client
0 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
10000 rows processed
SQL> drop index TEST_SEX_BITIND;
Index dropped.
3、建立函数索引
函数索引是基于函数或表达式所建立的索引
（1）建立函数索引
SQL> conn scott/tiger
Connected.
SQL> create index emp_ename_funind on emp(lower(ename)) tablespace indexes;
Index created.
（2）使用函数索引
SQL> set autotrace on;
SQL> select * from emp where lower(ename)='scott';
EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO
---------- ------------------------------ --------------------------- ---------- ------------------- ---------- ---------- ----------
7788 SCOTT ANALYST 7566 1987-04-19 00:00:00 3000 20
Execution Plan
----------------------------------------------------------
Plan hash value: 519807088
------------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 87 | 2 (0)| 00:00:01 |
| 1 | TABLE ACCESS BY INDEX ROWID| EMP | 1 | 87 | 2 (0)| 00:00:01 |
|* 2 | INDEX RANGE SCAN | EMP_ENAME_FUNIND | 1 | | 1 (0)| 00:00:01 |
------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - access(LOWER("ENAME")='scott')
Note
-----
- dynamic sampling used for this statement
Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
0 consistent gets
0 physical reads
0 redo size
0 bytes sent via SQL*Net to client
0 bytes received via SQL*Net from client
0 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
4、建立反向索引
反向索引时索引列值按相反的顺序存放的索引。
在顺序递增的列上建立普通B*-树索引时，如果表的数据量非常大，将导致索引数据分布布局（偏向某个方向）。为了避免出现这种情况， 应在顺序递增的列上建立反向索引。
用于建立索引的列值是连续的或通过 序列生成时，避免索引存放到集中的leaf block上,避免生成热块。
（1）建立反向索引
03:08:46 SQL> create index r_empno_ind on test(empno) reverse;
Index created.
对于emp表里empno列来说，因为客户ID号顺序递增，所以为了均衡索引数据分布，应在该列上建立反向索引。
（2）重建索引
03:09:46 SQL> alter index r_empno_ind rebuild reverse;
Index altered.
（3）使用反向索引
03:12:41 SQL> set autotrace on explain
03:12:43 SQL> select ename from test
03:13:07 2 where empno=7788;
ENAME
----------
SCOTT
Execution Plan
----------------------------------------------------------
Plan hash value: 514902531
-------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 20 | 2 (0)| 00:00:01 |
| 1 | TABLE ACCESS BY INDEX ROWID| TEST | 1 | 20 | 2 (0)| 00:00:01 |
|* 2 | INDEX RANGE SCAN | R_EMPNO_IND | 1 | | 1 (0)| 00:00:01 |
-------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - access("EMPNO"=7788)
Note
-----
- dynamic sampling used for this statement
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