索引的应用：
索引的分类：
逻辑分类：
单列索引 & 复合列索引
create index i_emp_ename on emp (ename);
create index i_emp_3 on emp (empno,ename,sal);
非唯一键索引 & 唯一键索引
create unique index i_emp_ename on emp (ename);
基于函数的索引
create index i_emp_ename on emp (lower(ename));
应用程序域索引:
oracle查看表是否有索引：
select index_name from user_indexes where table_name=‘EMP‘;
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物理分类:
非分区索引 & 分区索引
B-Tree索引：
常规B-Tree ：
反向键B-Tree：create index i_emp_ename on emp (ename) reverse;
Bitmap索引:适合使用在低基数列（重复值多的列：性别）
create bitmap index i_ob1_id on ob1 (object_id);
SQL> create index i_e01_empno on e01 (empno);
SQL> create bitmap index i_e01_empno on e01 (empno);
SQL> select blocks/128 from user_segments where segment_name=‘I_E01_EMPNO‘;
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
影响索引使用的因素：
1.空值影响B-Tree索引的使用：B-Tree索引不记录空值
create index i_emp_ename on emp (ename);
SQL> analyze index i_emp_ename validate structure;
SQL> select NAME,HEIGHT,BLOCKS,BR_BLKS,BR_ROWS,LF_BLKS,LF_ROWS from index_stats;
alter table e01 modify (empno number not null);
合并索引叶集块碎片：不会降低索引的二元高度
SQL> alter index I_EMP_ENAME coalesce;
如果需要降低索引的二元高度使用重建索引
SQL> alter index I_EMP_ENAME rebuild; --> 使用现有索引的关键字重建新索引
SQL> alter index I_EMP_ENAME rebuild online;
报如下错误，需要online解决。比如：rebuild online
ERROR at line 1:
ORA-00054: resource busy and acquire with NOWAIT specified or timeout expired
SQL> create index I_EMP_ENAME on emp (ename) online;
2.隐式数据类型
create table t01 (code_id varchar2(20));
insert into t01 values (1);
create index i_t01_code on t01 (code_id);
select * from t01 where code_id=1;
select * from t01 where code_id=‘1‘;
3.初始化参数影响索引使用:索引权重
optimizer_index_cost_adj=[1~10000]%
alter session set optimizer_index_cost_adj=50;
全表的成本 PK 索引访问的成本*optimizer_index_cost_adj
4.索引的聚簇因子：顺序访问索引键值而在表的数据块上面跳转的次数；反映的是通过索引顺序取值对表的数据块访问的次数
select * from e02 where empno<46572; --> .203037807
select * from e01 where empno<376; --> .00163923
empno<2000
select dbms_rowid.rowid_block_number(rowid),count(*) from e01 where empno<2000 group by dbms_rowid.rowid_block_number(rowid);
select blocks from user_segments where segment_name=‘E01‘;
select blocks from user_tables where table_name=‘E01‘;
select dbms_rowid.rowid_block_number(rowid),count(*) from e02 where empno<2000 group by dbms_rowid.rowid_block_number(rowid);
select blocks from user_segments where segment_name=‘E02‘;
select blocks from user_tables where table_name=‘E02‘;
SQL> select index_name,clustering_factor from user_indexes where index_name in (‘I_E01_EMPNO‘,‘I_E02_EMPNO‘);
INDEX_NAME CLUSTERING_FACTOR
------------------------------ -----------------
I_E01_EMPNO 229204
I_E02_EMPNO 1384
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
alter table ob1 modify (object_id number not null)
索引的扫描算法和效率：
索引的唯一键扫描:INDEX UNIQUE SCAN
select * from emp where empno=7839;
索引的快速全扫描:INDEX FAST FULL SCAN,不读取根与分枝块，只读取叶子，而且使用批量读取，结果无序
select object_id from ob1;
索引的全扫描：INDEX FULL SCAN,结果有序
select object_id from ob1 order by 1;
索引的范围扫描:INDEX RANGE SCAN
select * from ob1 where object_id<100;
索引的跳跃扫描:复合列索引，前导列在where子句中不能出现，索引列要拥有列级别的统计信息
analyze table ob1 compute statistics for all indexed columns;
create index i_ob1 on ob1 (owner,object_type,object_name);
select owner,object_type,object_name from ob1 where object_type=‘WINDOWS‘;
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打开sql自动跟踪
SET AUTOT[RACE] {OFF | ON | TRACE[ONLY]} [EXP[LAIN]] [STAT[ISTICS]]
SET AUTOT ON
SET AUTOT TRACE
SET AUTOT TRACE EXP
SET AUTOT TRACE STAT
关闭sql自动跟踪
SET AUTOT OFF
select /*+index(dept pk_dept)*/ * from scott.dept;
select distinct FILE#,BLOCK# from v$bh where objd=dba_objects.data_object_id;
select object_name,data_object_id from dba_objects where object_name in (‘DEPT‘,‘PK_DEPT‘);
select count(*) from v$bh where objd=87106;
select FILE#,BLOCK# from v$bh where objd=87106;
select distinct FILE#,BLOCK# from v$bh where objd=87106;