oracle 索引扫描类型的分类与构造

Posted 2020-11-08 钱若离花落

1. INDEX RANGE SCAN
--请记住这个INDEX RANGE SCAN扫描方式
drop table t purge;
create table t as select * from dba_objects;
update t set object_id=rownum;
commit;
create  index idx_object_id on t(object_id);
set autotrace traceonly
set linesize 1000
exec dbms_stats.gather_table_stats(ownname => ‘LJB‘,tabname => ‘T‘,estimate_percent => 10,method_opt=> ‘for all indexed
columns‘,cascade=>TRUE) ;
select * from t where object_id=8;

2. INDEX UNIQUE SCAN
--请注意这个INDEX UNIQUE SCAN扫描方式,在唯一索引情况下使用。
drop table t purge;
create table t as select * from dba_objects;
update t set object_id=rownum;
commit;
create unique index idx_object_id on t(object_id);
set autotrace traceonly
set linesize 1000
select * from t where object_id=8;

3. TABLE ACCESS BY USER ROWID
--请注意这个TABLE ACCESS BY USER ROWID扫描方式,直接根据rowid来访问，最快的访问方式！
drop table t purge;
create table t as select * from dba_objects;
update t set object_id=rownum;
commit;
--注意，这里连索引都没建!
--create  index idx_object_id on t(object_id);
set autotrace off
select rowid from t where object_id=8;
ROWID
-----
AAAZxiAAGAAAB07AAH
set autotrace traceonly
set linesize 1000
select * from t where object_id=8 and rowid=‘AAAZxiAAGAAAB07AAH‘;

4. INDEX FULL SCAN
--请记住这个INDEX FULL SCAN扫描方式，并体会与INDEX FAST FULL SCAN的区别
drop table t purge;
create table t as select * from dba_objects;
update t set object_id=rownum;
commit;
alter table T modify object_id not null;
create  index idx_object_id on t(object_id);
set autotrace traceonly
set linesize 1000
select * from t  order by object_id;

5. INDEX FAST FULL SCAN
---请记住这个INDEX FAST FULL SCAN扫描方式，并体会与INDEX FULL SCAN的区别
drop table t purge;
create table t as select * from dba_objects ;
update t set object_id=rownum;
commit;
alter table T modify object_id not null;
create  index idx_object_id on t(object_id);
set autotrace traceonly
set linesize 1000
select count(*) from t;

6. INDEX FULL SCAN (MINMAX)
--请注意这个INDEX FULL SCAN (MIN/MAX)扫描方式
drop table t purge;
create table t as select * from dba_objects;
update t set object_id=rownum;
commit;
create  index idx_object_id on t(object_id);
set autotrace traceonly
set linesize 1000
select max(object_id) from t;

7. INDEX SKIP SCAN
--请记住这个INDEX SKIP SCAN扫描方式
drop table t purge;
create table t as select * from dba_objects;
update t set object_type=‘TABLE‘ ;
commit;
update t set object_type=‘VIEW‘ where rownum<=30000;
commit;
create  index idx_type_id on t(object_type,object_id);
exec dbms_stats.gather_table_stats(ownname => ‘LJB‘,tabname => ‘T‘,estimate_percent => 10,method_opt=> ‘for all indexed
columns‘,cascade=>TRUE) ;
set autotrace traceonly
set linesize 1000
select * from t where object_id=8;

8. TABLE ACCESS BY INDEX ROWID
--好好地体会前后两个试验，记住这个TABLE ACCESS BY INDEX ROWID
drop table t purge;
create table t as select * from dba_objects;
update t set object_id=rownum;
commit;
create  index idx_object_id on t(object_id);
set autotrace traceonly explain
set linesize 1000
select object_id from t where object_id=2 and object_type=‘TABLE‘;
--在接下来的试验中，你会看到，哇塞，TABLE ACCESS BY INDEX ROWID消失了。
create  index idx_id_type on t(object_id,object_type);
select object_id from t where object_id=2 and object_type=‘TABLE‘;