索引与表一样，也属于段（segment）的一种。里面存放了用户的数据，跟表一样需要占用磁盘空间。索引是一种允许直接访问数据表中某一数据行的树型结构，为了提高查询效率而引入，是一个独立于表的对象，可以存放在与表不同的表空间中。索引记录中存有索引关键字和指向表中数据的指针（地址）。对索引进行的I/O操作比对表进行操作要少很多。索引一旦被建立就将被Oracle系统自动维护，查询语句中不用指定使用哪个索引.
从物理上说，索引通常可以分为：分区和非分区索引、常规B树索引、位图（bitmap）索引、翻转（reverse）索引等。其中，B树索引属于最常见的索引。
一、 ROWID的概念
存储了row在数据文件中的具体位置：64位 编码的数据，A-Z, a-z, 0-9, +, 和 /，
row在数据块中的存储方式
SELECT ROWID, last_name FROM hr.employees WHERE department_id = 20;
比 如：OOOOOOFFFBBBBBBRRR
OOOOOO：data object number, 对应dba_objects.data_object_id
FFF：file#, 对应v$datafile.file#
BBBBBB：block#
RRR：row#
Dbms_rowid包
SELECT dbms_rowid.rowid_block_number('AAAGFqAABAAAIWEAAA') from dual;
具 体到特定的物理文件
二、 索引的概念
1、 类似书的目录结构
2、 Oracle 的“索引”对象，与表关联的可选对象，提高SQL查询语句的速度
3、 索引直接指向包含所查询值的行的位置，减少磁盘I/O
4、 与所索引的表是相互独立的物理结构
5、 Oracle 自动使用并维护索引，插入、删除、更新表后，自动更新索引
6、 语法：CREATE INDEX index ON table (column[, column]...);
7、 B-tree结构(非bitmap)：
[一]了解索引的工作原理：
表：emp
目标：查询Frank的工资salary
建立索 引：create index emp_name_idx on emp(name);
索引的三大特性
1、索引的高度较低
sqlplus ljb/ljb
drop table t1 purge;
drop table t2 purge;
drop table t3 purge;
drop table t4 purge;
drop table t5 purge;
drop table t6 purge;
drop table t7 purge;
create table t1 as select rownum as id ,rownum+1 as id2,rpad('*',1000,'*') as contents from dual connect by level<=1;
create table t2 as select rownum as id ,rownum+1 as id2,rpad('*',1000,'*') as contents from dual connect by level<=10;
create table t3 as select rownum as id ,rownum+1 as id2,rpad('*',1000,'*') as contents from dual connect by level<=100;
create table t4 as select rownum as id ,rownum+1 as id2,rpad('*',1000,'*') as contents from dual connect by level<=1000;
create table t5 as select rownum as id ,rownum+1 as id2,rpad('*',1000,'*') as contents from dual connect by level<=10000;
create table t6 as select rownum as id ,rownum+1 as id2,rpad('*',1000,'*') as contents from dual connect by level<=100000;
create table t7 as select rownum as id ,rownum+1 as id2,rpad('*',1000,'*') as contents from dual connect by level<=1000000;
create index idx_id_t1 on t1(id);
create index idx_id_t2 on t2(id);
create index idx_id_t3 on t3(id);
create index idx_id_t4 on t4(id);
create index idx_id_t5 on t5(id);
create index idx_id_t6 on t6(id);
create index idx_id_t7 on t7(id);
set linesize 1000
set autotrace off
select index_name,
blevel,
leaf_blocks,
num_rows,
distinct_keys,
clustering_factor
from user_ind_statistics
where table_name in( 'T1','T2','T3','T4','T5','T6','T7');
INDEX_NAME BLEVEL LEAF_BLOCKS NUM_ROWS DISTINCT_KEYS CLUSTERING_FACTOR
------------------ ----------- ---------- ------------- -----------------
IDX_ID_T1 0 1 1 1 1
IDX_ID_T2 0 1 10 10 2
IDX_ID_T3 0 1 100 100 15
IDX_ID_T4 1 3 1000 1000 143
IDX_ID_T5 1 21 10000 10000 1429
IDX_ID_T6 1 222 100000 100000 14286
IDX_ID_T7 2 2226 1000000 1000000 142858
sqlplus ljb/ljb
drop table t1 purge;
drop table t2 purge;
drop table t3 purge;
drop table t4 purge;
drop table t5 purge;
drop table t6 purge;
drop table t7 purge;
create table t1 as select rownum as id ,rownum+1 as id2,rpad('*',1000,'*') as contents from dual connect by level<=1;
create table t2 as select rownum as id ,rownum+1 as id2,rpad('*',1000,'*') as contents from dual connect by level<=10;
create table t3 as select rownum as id ,rownum+1 as id2,rpad('*',1000,'*') as contents from dual connect by level<=100;
create table t4 as select rownum as id ,rownum+1 as id2,rpad('*',1000,'*') as contents from dual connect by level<=1000;
create table t5 as select rownum as id ,rownum+1 as id2,rpad('*',1000,'*') as contents from dual connect by level<=10000;
create table t6 as select rownum as id ,rownum+1 as id2,rpad('*',1000,'*') as contents from dual connect by level<=100000;
create table t7 as select rownum as id ,rownum+1 as id2,rpad('*',1000,'*') as contents from dual connect by level<=1000000;
create index idx_id_t1 on t1(id);
create index idx_id_t2 on t2(id);
create index idx_id_t3 on t3(id);
create index idx_id_t4 on t4(id);
create index idx_id_t5 on t5(id);
create index idx_id_t6 on t6(id);
create index idx_id_t7 on t7(id);
set linesize 1000
select index_name,
blevel,
leaf_blocks,
num_rows,
distinct_keys,
clustering_factor
from user_ind_statistics
where table_name in( 'T1','T2','T3','T4','T5','T6','T7');
INDEX_NAME BLEVEL LEAF_BLOCKS NUM_ROWS DISTINCT_KEYS CLUSTERING_FACTOR
------------------ ----------- ---------- ------------- -----------------
IDX_ID_T1 0 1 1 1 1
IDX_ID_T2 0 1 10 10 2
IDX_ID_T3 0 1 100 100 15
IDX_ID_T4 1 3 1000 1000 143
IDX_ID_T5 1 21 10000 10000 1429
IDX_ID_T6 1 222 100000 100000 14286
IDX_ID_T7 2 2226 1000000 1000000 142858
set autotrace traceonly statistics
set linesize 1000
--以下注意观察逻辑读的次数，另外注意尽量每条语句执行2遍以上，观察第2遍的结果。
select * from t1 where id=1;
统计信息
-----------------------------------------
0 recursive calls
0 db block gets
2 consistent gets
select /*+full(t1)*/ * from t1 where id=1;
统计信息
-------------------------------
0 recursive calls
0 db block gets
3 consistent gets
select * from t2 where id=1;
统计信息
-----------------------------
0 recursive calls
0 db block gets
3 consistent gets
select /*+full(t2)*/ * from t2 where id=1;
统计信息
-----------------------------
0 recursive calls
0 db block gets
5 consistent gets
select * from t3 where id=1;
统计信息
-----------------------------
0 recursive calls
0 db block gets
3 consistent gets
select /*+full(t3)*/ * from t3 where id=1;
统计信息
----------------------------
0 recursive calls
0 db block gets
19 consistent gets
select * from t4 where id=1;
统计信息
-----------------------------
0 recursive calls
0 db block gets
4 consistent gets
select /*+full(t4)*/ * from t4 where id=1;
统计信息
----------------------------
0 recursive calls
0 db block gets
148 consistent gets
select * from t5 where id=1;
统计信息
------------------------------
0 recursive calls
0 db block gets
4 consistent gets
select /*+full(t5)*/ * from t5 where id=1;
统计信息
-----------------------------
0 recursive calls
0 db block gets
1435 consistent gets
select * from t6 where id=1;
统计信息
-----------------------------
0 recursive calls
0 db block gets
4 consistent gets
select /*+full(t6)*/ * from t6 where id=1;
统计信息
-----------------------------
0 recursive calls
0 db block gets
14298 consistent gets
select * from t7 where id=1;
统计信息
-----------------------------
0 recursive calls
0 db block gets
5 consistent gets
select /*+full(t7)*/ * from t7 where id=1;
统计信息
-----------------------------
0 recursive calls
0 db block gets
142866 consistent gets
/*
规律:
从t1到t7(表记录依次增大10倍，从1到1000000),索引读的逻辑读是 2,3,3,4,4,4,5
从t1到t7(表记录依次增大10倍，从1到1000000)全表扫描的逻辑读是 3,5,19,148,1435,14298,142866
*/
2、索引存储列值
--要领：只要索引能回答问题，索引就可以当成一个"瘦表"，访问路径就会减少。另外切记不存储空值
drop table t purge;
create table t as select * from dba_objects;
update t set object_id=rownum;
commit;
create index idx1_object_id on t(object_id);
set autotrace on
select count(*) from t;
执行计划
-------------------------------------------------------------------
| Id | Operation | Name | Rows | Cost (%CPU)| Time |
-------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 292 (1)| 00:00:04 |
| 1 | SORT AGGREGATE | | 1 | | |
| 2 | TABLE ACCESS FULL| T | 69485 | 292 (1)| 00:00:04 |
-------------------------------------------------------------------
统计信息
----------------------------------------------------------
0 recursive calls
0 db block gets
1048 consistent gets
--为啥用不到索引，因为索引不能存储空值，所以加上一个is not null,再试验看看
select count(*) from t where object_id is not null;
执行计划
----------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
----------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 13 | 50 (2)| 00:00:01 |
| 1 | SORT AGGREGATE | | 1 | 13 | | |
|* 2 | INDEX FAST FULL SCAN| IDX1_OBJECT_ID | 69485 | 882K| 50 (2)| 00:00:01 |
----------------------------------------------------------------------------------------
统计信息
----------------------------------------------------------
0 recursive calls
0 db block gets
170 consistent gets
--也可以不加is not null，直接把列的属性设置为not null，也成，继续试验如下：
alter table t modify OBJECT_ID not null;
select count(*) from t;
执行计划
--------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Cost (%CPU)| Time |
--------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 49 (0)| 00:00:01 |
| 1 | SORT AGGREGATE | | 1 | | |
| 2 | INDEX FAST FULL SCAN| IDX1_OBJECT_ID | 69485 | 49 (0)| 00:00:01 |
--------------------------------------------------------------------------------
统计信息
----------------------------------------------------------
0 recursive calls
0 db block gets
170 consistent gets
0 physical reads
0 redo size
425 bytes sent via SQL*Net to client
416 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
--如果是主键就无需定义列是否允许为空了。
drop table t purge;
create table t as select * from dba_objects;
update t set object_id=rownum;
alter table t add constraint pk1_object_id primary key (OBJECT_ID);
set autotrace on
select count(*) from t;
执行计划
-------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Cost (%CPU)| Time |
-------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 46 (0)| 00:00:01 |
| 1 | SORT AGGREGATE | | 1 | | |
| 2 | INDEX FAST FULL SCAN| PK1_OBJECT_ID | 69485 | 46 (0)| 00:00:01 |
-------------------------------------------------------------------------------
统计信息
----------------------------------------------------------
0 recursive calls
0 db block gets
160 consistent gets
SUM/AVG的优化
drop table t purge;
create table t as select * from dba_objects;
create index idx1_object_id on t(object_id);
set autotrace on
set linesize 1000
set timing on
select sum(object_id) from t;
执行计划
----------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
----------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 13 | 49 (0)| 00:00:01 |
| 1 | SORT AGGREGATE | | 1 | 13 | | |
| 2 | INDEX FAST FULL SCAN| IDX1_OBJECT_ID | 92407 | 1173K| 49 (0)| 00:00:01 |
----------------------------------------------------------------------------------------
统计信息
----------------------------------------------------------
0 recursive calls
0 db block gets
170 consistent gets
0 physical reads
0 redo size
432 bytes sent via SQL*Net to client
415 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
--比较一下假如不走索引的代价，体会一下这个索引的重要性
select /*+full(t)*/ sum(object_id) from t;
SUM(OBJECT_ID)
--------------
2732093100
执行计划
---------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 13 | 292 (1)| 00:00:04 |
| 1 | SORT AGGREGATE | | 1 | 13 | | |
| 2 | TABLE ACCESS FULL| T | 92407 | 1173K| 292 (1)| 00:00:04 |
---------------------------------------------------------------------------
统计信息
----------------------------------------------------------
0 recursive calls
0 db block gets
1047 consistent gets
0 physical reads
0 redo size
432 bytes sent via SQL*Net to client
415 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
--起来类似的比如AVG，和SUM是一样的，如下：
select avg(object_id) from t;
AVG(OBJECT_ID)
--------------
37365.5338
执行计划
----------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
----------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 13 | 49 (0)| 00:00:01 |
| 1 | SORT AGGREGATE | | 1 | 13 | | |
| 2 | INDEX FAST FULL SCAN| IDX1_OBJECT_ID | 92407 | 1173K| 49 (0)| 00:00:01 |
----------------------------------------------------------------------------------------
统计信息
----------------------------------------------------------
0 recursive calls
0 db block gets
170 consistent gets
0 physical reads
0 redo size
448 bytes sent via SQL*Net to client
415 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
--不知大家注意到没，这里的试验已经告诉我们了，OBJECT_ID列是否为空，也不影响SUM/AVG等聚合的结果。
3、索引本身有序
--colocated表根据x列有一定的物理顺序
drop table colocated purge;
create table colocated ( x int, y varchar2(80) );
begin
for i in 1 .. 100000
loop
insert into colocated(x,y)
values (i, rpad(dbms_random.random,75,'*') );
end loop;
end;
/
alter table colocated
add constraint colocated_pk
primary key(x);
begin
dbms_stats.gather_table_stats( user, 'COLOCATED', cascade=>true );
end;
/
--disorganized 表数据根据x列完全无序
drop table disorganized purge;
create table disorganized
as
select x,y
from colocated
order by y;
alter table disorganized
add constraint disorganized_pk
primary key (x);
begin
dbms_stats.gather_table_stats( user, 'DISORGANIZED', cascade=>true );
end;
/
set autotrace off
alter session set statistics_level=all;
set linesize 1000
---两者性能差异显著
select /*+ index( colocated colocated_pk ) */ * from colocated where x between 20000 and 40000;
SELECT * FROM table(dbms_xplan.display_cursor(NULL,NULL,'runstats_last'));
------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers |
------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 20001 |00:00:00.05 | 2900 |
| 1 | TABLE ACCESS BY INDEX ROWID| COLOCATED | 1 | 20002 | 20001 |00:00:00.05 | 2900 |
|* 2 | INDEX RANGE SCAN | COLOCATED_PK | 1 | 20002 | 20001 |00:00:00.03 | 1375 |
------------------------------------------------------------------------------------------------------
select /*+ index( disorganized disorganized_pk ) */* from disorganized where x between 20000 and 40000;
SELECT * FROM table(dbms_xplan.display_cursor(NULL,NULL,'runstats_last'));
---------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers |
---------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 20001 |00:00:00.09 | 21360 |
| 1 | TABLE ACCESS BY INDEX ROWID| DISORGANIZED | 1 | 20002 | 20001 |00:00:00.09 | 21360 |
|* 2 | INDEX RANGE SCAN | DISORGANIZED_PK | 1 | 20002 | 20001 |00:00:00.03 | 1375 |
---------------------------------------------------------------------------------------------------------
---看聚合因子，就明白真正的原因了。
select a.index_name,
b.num_rows,
b.blocks,
a.clustering_factor
from user_indexes a, user_tables b
where index_name in ('COLOCATED_PK', 'DISORGANIZED_PK' )
and a.table_name = b.table_name;
INDEX_NAME NUM_ROWS BLOCKS CLUSTERING_FACTOR
------------------------------ ---------- ---------- -----------------
COLOCATED_PK 100000 1252 1190
DISORGANIZED_PK 100000 1219 99899
--索引回表读（TABLE ACCESS BY INDEX ROWID）的例子
drop table t purge;
create table t as select * from dba_objects;
create index idx1_object_id on t(object_id);
--试验1
set autotrace traceonly
set linesize 1000
set timing on
select * from t where object_id<=5;
执行计划
----------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
----------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 4 | 828 | 3 (0)| 00:00:01 |
| 1 | TABLE ACCESS BY INDEX ROWID| T | 4 | 828 | 3 (0)| 00:00:01 |
|* 2 | INDEX RANGE SCAN | IDX1_OBJECT_ID | 4 | | 2 (0)| 00:00:01 |
----------------------------------------------------------------------------------------------
统计信息
----------------------------------------------------------
0 recursive calls
0 db block gets
5 consistent gets
0 physical reads
0 redo size
1666 bytes sent via SQL*Net to client
415 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
4 rows processed
--比较消除TABLE ACCESS BY INDEX ROWID回表后的性能，将select * from改为select object_id from
set autotrace traceonly
set linesize 1000
set timing on
select object_id from t where object_id<=5;
执行计划
-----------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-----------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 4 | 52 | 2 (0)| 00:00:01 |
|* 1 | INDEX RANGE SCAN| IDX1_OBJECT_ID | 4 | 52 | 2 (0)| 00:00:01 |
-----------------------------------------------------------------------------------
统计信息
----------------------------------------------------------
0 recursive calls
0 db block gets
3 consistent gets
0 physical reads
0 redo size
478 bytes sent via SQL*Net to client
415 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
4 rows processed
--试验2：通过构造联合索引，再观察一个消除TABLE ACCESS BY INDEX ROWID的例子
set autotrace traceonly
set linesize 1000
select object_id,object_name from t where object_id<=5;
执行计划
----------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
----------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 4 | 316 | 3 (0)| 00:00:01 |
| 1 | TABLE ACCESS BY INDEX ROWID| T | 4 | 316 | 3 (0)| 00:00:01 |
|* 2 | INDEX RANGE SCAN | IDX1_OBJECT_ID | 4 | | 2 (0)| 00:00:01 |
----------------------------------------------------------------------------------------------
统计信息
----------------------------------------------------------
0 recursive calls
0 db block gets
5 consistent gets
0 physical reads
0 redo size
567 bytes sent via SQL*Net to client
415 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
4 rows processed
--准备工作，对t表建联合索引
create index idx_un_objid_objname on t(object_id,object_name);
--该联合索引建完后，产生功效了！消除了TABLE ACCESS BY INDEX ROWID
select object_id,object_name from t where object_id<=5;
执行计划
-----------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-----------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 12 | 948 | 2 (0)| 00:00:01 |
|* 1 | INDEX RANGE SCAN| IDX_UN_OBJID_OBJNAME | 12 | 948 | 2 (0)| 00:00:01 |
-----------------------------------------------------------------------------------------
统计信息
----------------------------------------------------------
0 recursive calls
0 db block gets
3 consistent gets
0 physical reads
0 redo size
567 bytes sent via SQL*Net to client
415 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
4 rows processed
--：索引无法消除UNION 排序，一般来说在使用UNION时要确定必要性，在数据不会重复时只需UNION ALL即可。
--DISTINCT测试前的准备
drop table t purge;
create table t as select * from dba_objects;
update t set object_id=rownum;
alter table T modify OBJECT_ID not null;
update t set object_id=2;
update t set object_id=3 where rownum<=25000;
commit;
/*
在oracle10g的R2环境之后，DISTINCT由于其 HASH UNIQUE的算法导致其不会产生排序，其调整的
ALTER SESSION SET "_gby_hash_aggregation_enabled" = FALSE
*/
set linesize 1000
set autotrace traceonly
select distinct object_id from t ;
执行计划
-----------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes |TempSpc| Cost (%CPU)| Time |
-----------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 88780 | 1127K| | 717 (1)| 00:00:09 |
| 1 | HASH UNIQUE | | 88780 | 1127K| 1752K| 717 (1)| 00:00:09 |
| 2 | TABLE ACCESS FULL| T | 88780 | 1127K| | 292 (1)| 00:00:04 |
-----------------------------------------------------------------------------------
统计信息
----------------------------------------------------------
0 recursive calls
0 db block gets
1047 consistent gets
0 physical reads
0 redo size
462 bytes sent via SQL*Net to client
416 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
2 rows processed
/*不过虽然没有排序，通过观察TempSpc可知distinct消耗PGA内存进行HASH UNIQUE运算，
接下来看看建了索引后的情况，TempSpc关键字立即消失，COST也立即下降许多，具体如下*/
--为T表的object_id列建索引
create index idx_t_object_id on t(object_id);
set linesize 1000
set autotrace traceonly
select /*+index(t)*/ distinct object_id from t ;
执行计划
--------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 88780 | 1127K| 582 (1)| 00:00:07 |
| 1 | SORT UNIQUE NOSORT| | 88780 | 1127K| 582 (1)| 00:00:07 |
| 2 | INDEX FULL SCAN | IDX_T_OBJECT_ID | 88780 | 1127K| 158 (1)| 00:00:02 |
--------------------------------------------------------------------------------------
统计信息
----------------------------------------------------------
0 recursive calls
0 db block gets
145 consistent gets
0 physical reads
0 redo size
462 bytes sent via SQL*Net to client
416 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
2 rows processed
--索引与排序
drop table t purge;
create table t as select * from dba_objects ;
set autotrace traceonly
--oracle还算智能，不会傻到这里都去排序，做了查询转换，忽略了这个排序
select count(*) from t order by object_id;
---以下语句说明排序
set autotrace traceonly
set linesize 1000
drop table t purge;
create table t as select * from dba_objects;
--以下语句没有索引又有order by ，必然产生排序
select * from t where object_id>2 order by object_id;
执行计划
-----------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes |TempSpc| Cost (%CPU)| Time |
-----------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 92407 | 18M| | 4454 (1)| 00:00:54 |
| 1 | SORT ORDER BY | | 92407 | 18M| 21M| 4454 (1)| 00:00:54 |
|* 2 | TABLE ACCESS FULL| T | 92407 | 18M| | 294 (2)| 00:00:04 |
-----------------------------------------------------------------------------------
统计信息
----------------------------------------------------------
0 recursive calls
0 db block gets
1047 consistent gets
0 physical reads
0 redo size
3513923 bytes sent via SQL*Net to client
54029 bytes received via SQL*Net from client
4876 SQL*Net roundtrips to/from client
1 sorts (memory)
0 sorts (disk)
73117 rows processed
---新增索引后，Oracle就有可能利用索引本身就有序的特点，利用索引来避免排序，如下：
create index idx_t_object_id on t(object_id);
set autotrace traceonly
select * from t where object_id>2 order by object_id;
执行计划
-----------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-----------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 92407 | 18M| 1302 (1)| 00:00:16 |
| 1 | TABLE ACCESS BY INDEX ROWID| T | 92407 | 18M| 1302 (1)| 00:00:16 |
|* 2 | INDEX RANGE SCAN | IDX_T_OBJECT_ID | 92407 | | 177 (1)| 00:00:03 |
-----------------------------------------------------------------------------------------------
统计信息
----------------------------------------------------------
0 recursive calls
0 db block gets
10952 consistent gets
0 physical reads
0 redo size
8115221 bytes sent via SQL*Net to client
54029 bytes received via SQL*Net from client
4876 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
73117 rows processed
--如下情况Oracle肯定毫不犹豫的选择用索引,因为回表取消了 !
select object_id from t where object_id>2 order by object_id;
执行计划
------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 92407 | 1173K| 177 (1)| 00:00:03 |
|* 1 | INDEX RANGE SCAN| IDX_T_OBJECT_ID | 92407 | 1173K| 177 (1)| 00:00:03 |
------------------------------------------------------------------------------------
统计信息
----------------------------------------------------------
0 recursive calls
0 db block gets
5027 consistent gets
0 physical reads
0 redo size
1062289 bytes sent via SQL*Net to client
54029 bytes received via SQL*Net from client
4876 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
73117 rows processed
--另外，如果是如下语句，Oracle打死也不用索引了。
select object_id from t where object_id>2;
--MAX/MIN 的索引优化
drop table t purge;
create table t as select * from dba_objects;
update t set object_id=rownum;
alter table t add constraint pk_object_id primary key (OBJECT_ID);
set autotrace on
set linesize 1000
select max(object_id) from t;
执行计划
-------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 13 | 2 (0)| 00:00:01 |
| 1 | SORT AGGREGATE | | 1 | 13 | | |
| 2 | INDEX FULL SCAN (MIN/MAX)| PK_OBJECT_ID | 1 | 13 | 2 (0)| 00:00:01 |
-------------------------------------------------------------------------------------------
统计信息
----------------------------------------------------------
0 recursive calls
0 db block gets
2 consistent gets
0 physical reads
0 redo size
431 bytes sent via SQL*Net to client
415 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
--最小值老师的试验就无需展现执行计划结果了，必然和最大值的执行计划一样！
select min(object_id) from t;
--如果没用到索引的情况是如下,请看看执行计划有何不同，请看看代价和逻辑读的差异！
select /*+full(t)*/ max(object_id) from t;
执行计划
---------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 13 | 292 (1)| 00:00:04 |
| 1 | SORT AGGREGATE | | 1 | 13 | | |
| 2 | TABLE ACCESS FULL| T | 92407 | 1173K| 292 (1)| 00:00:04 |
---------------------------------------------------------------------------
统计信息
----------------------------------------------------------
0 recursive calls
0 db block gets
1047 consistent gets
0 physical reads
0 redo size
431 bytes sent via SQL*Net to client
415 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
---另外，可以做如下试验观察在有索引的情况下，随这记录数增加，性能差异是否明显？
set autotrace off
drop table t_max purge;
create table t_max as select * from dba_objects;
insert into t_max select * from t_max;
insert into t_max select * from t_max;
insert into t_max select * from t_max;
insert into t_max select * from t_max;
insert into t_max select * from t_max;
select count(*) from t_max;
create index idx_t_max_obj on t_max(object_id);
set autotrace on
select max(object_id) from t_max;
执行计划
--------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 13 | 3 (0)| 00:00:01 |
| 1 | SORT AGGREGATE | | 1 | 13 | | |
| 2 | INDEX FULL SCAN (MIN/MAX)| IDX_T_MAX_OBJ | 1 | 13 | 3 (0)| 00:00:01 |
--------------------------------------------------------------------------------------------
统计信息
----------------------------------------------------------
0 recursive calls
0 db block gets
3 consistent gets
0 physical reads
0 redo size
431 bytes sent via SQL*Net to client
415 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
/*
object_id如果允许为空，加个索引后，会走INDEX FULL SCAN (MIN/MAX)高效算法吗，
当然会了！取最大最小还怕啥空值？
*/
drop table t purge;
create table t as select * from dba_objects ;
create index idx_object_id on t(object_id);
set autotrace on
set linesize 1000
select max(object_id) from t;
组合索引经要素!
/* 1.适用在单独查询返回记录很多，组合查询后忽然返回记录很少的情况:
比如where 学历=硕士以上 返回不少的记录
比如where 职业=收银员 同样返回不少的记录
于是无论哪个条件查询做索引，都不合适。
可是，如果学历为硕士以上，同时职业又是收银员的，返回的就少之又少了。
于是联合索引就可以这么开始建了。
*/
/* 2.组合查询的组合顺序，要考虑单独的前缀查询情况（否则单独前缀查询的索引不能生效或者只能用到跳跃索引）
比如你在建id,object_type的联合索引时，要看考虑是单独where id=xxx查询的多，还是单独where object_type查询的多。
这里细节就暂时略去了，在案例的部分中还有描述
*/
--3.仅等值无范围查询时，组合索引顺序不影响性能（比如where col1=xxx and col2=xxx，无论COL1+COL2组合还是COL2+COL1组合）
drop table t purge;
create table t as select * from dba_objects;
insert into t select * from t;
insert into t select * from t;
insert into t select * from t;
update t set object_id=rownum ;
commit;
create index idx_id_type on t(object_id,object_type);
create index idx_type_id on t(object_type,object_id);
set autotrace off
alter session set statistics_level=all ;
set linesize 366
select /*+index(t,idx_id_type)*/ * from t where object_id=20 and object_type='TABLE';
select * from table(dbms_xplan.display_cursor(null,null,'allstats last'));
-----------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers |
-----------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 1 |00:00:00.01 | 5 |
| 1 | TABLE ACCESS BY INDEX ROWID| T | 1 | 57 | 1 |00:00:00.01 | 5 |
|* 2 | INDEX RANGE SCAN | IDX_ID_TYPE | 1 | 9 | 1 |00:00:00.01 | 4 |
-----------------------------------------------------------------------------------------------------
select /*+index(t,idx_type_id)*/ * from t where object_id=20 and object_type='TABLE';
select * from table(dbms_xplan.display_cursor(null,null,'allstats last'));
Plan hash value: 3420768628
-----------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers |
-----------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 1 |00:00:00.01 | 5 |
| 1 | TABLE ACCESS BY INDEX ROWID| T | 1 | 57 | 1 |00:00:00.01 | 5 |
|* 2 | INDEX RANGE SCAN | IDX_TYPE_ID | 1 | 9 | 1 |00:00:00.01 | 4 |
-----------------------------------------------------------------------------------------------------
--4.组合索引最佳顺序一般是将列等值查询的列置前。（测试组合索引在条件是不等的情况下的情况，条件经常是不等的，要放在后面，让等值的在前面）
select /*+index(t,idx_id_type)*/ * from t where object_id>=20 and object_id<2000 and object_type='TABLE';
select * from table(dbms_xplan.display_cursor(null,null,'allstats last'));
-----------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers |
-----------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 469 |00:00:00.01 | 86 |
| 1 | TABLE ACCESS BY INDEX ROWID| T | 1 | 14 | 469 |00:00:00.01 | 86 |
|* 2 | INDEX RANGE SCAN | IDX_ID_TYPE | 1 | 1 | 469 |00:00:00.01 | 40 |
-----------------------------------------------------------------------------------------------------
select /*+index(t,idx_type_id)*/ * from t where object_id>=20 and object_id<2000 and object_type='TABLE';
-----------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers |
-----------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 469 |00:00:00.01 | 81 |
| 1 | TABLE ACCESS BY INDEX ROWID| T | 1 | 469 | 469 |00:00:00.01 | 81 |
|* 2 | INDEX RANGE SCAN | IDX_TYPE_ID | 1 | 469 | 469 |00:00:00.01 | 35 |
-----------------------------------------------------------------------------------------------------
--5.注意组合索引与组合条件中关于IN 的优化(将会在案例部分描述，展现结果就不在这里贴出了）
--案例1
UPDATE t SET OBJECT_ID=20 WHERE ROWNUM<=26000;
UPDATE t SET OBJECT_ID=21 WHERE OBJECT_ID<>20;
COMMIT;
set linesize 1000
set pagesize 1
alter session set statistics_level=all ;
select /*+index(t,idx1_object_id)*/ * from t where object_TYPE='TABLE' AND OBJECT_ID >= 20 AND OBJECT_ID<= 21;
--6.案例2
--依然是关于IN的优化 (col1,col2,col3的索引情况，如果没有为COL2赋予查询条件时，COL3只能起到检验作用)
drop table t purge;
create table t as select * from dba_objects;
UPDATE t SET OBJECT_ID=20 WHERE ROWNUM<=26000;
UPDATE t SET OBJECT_ID=21 WHERE OBJECT_ID<>20;
Update t set object_id=22 where rownum<=10000;
COMMIT;
create index idx_union on t(object_type,object_id,owner);
set autotrace off
alter session set statistics_level=all ;
set linesize 1000
select * from t where object_type='VIEW' and OWNER='LJB';
select * from table(dbms_xplan.display_cursor(null,null,'allstats last'));
--这里就略去了展现结果，在案例中有描述。
select /*+INDEX(T,idx_union)*/ * from t T where object_type='VIEW' and OBJECT_ID IN (20,21,22) AND OWNER='LJB';
select * from table(dbms_xplan.display_cursor(null,null,'allstats last'));