count查詢條數效能試驗

--最慢速度(無索引）
drop table t purge;
create table t as  select * from dba_objects;
alter table T modify OBJECT_NAME not null;
select count(*) from t;
set autotrace traceonly
set linesize 1000
set timing on 
select COUNT(*) FROM T; 
/


--快了一點（有普通索引）
drop table t purge;
create table t as  select * from dba_objects;
alter table T modify OBJECT_NAME not null;
create  index idx_object_name on t(object_name);
set autotrace traceonly
set timing on 
select count(*) from t;
/


--又快一點（有了一個合適的點陣圖索引）
drop table t purge;
create table t as  select * from dba_objects;
 Update t  Set object_name='abc'; 
 Update t Set object_name='evf' Where rownum<=20000;
create bitmap index idx_object_name on t(object_name);
set autotrace traceonly
set timing on
select count(*) from t;
/
注：如果記錄數不重複或者說重複度很低，ORACLE會選擇全表掃描，如果用
來強制，可以發現效能很低下。
alter session set statistics_level=all ;
set linesize 1000
set pagesize 1
select /*+index(t,idx_object_name)*/ count(*) from test t;
select * from table(dbms_xplan.display_cursor(null,null,'allstats last'));



--再快一點（物化檢視，注意使用的場景）
drop materialized view MV_COUNT_T;
drop table t purge;
create table t as  select * from dba_objects;
 Update t  Set object_name='abc'; 
 Update t Set object_name='evf' Where rownum<=20000;

create  materialized view  mv_count_t
                    build immediate
                    refresh on commit
                    enable query rewrite
                    as
                    select count(*) FROM T;

set autotrace traceonly
set linesize 1000
select COUNT(*) FROM T; 
/

--又再快一點（快取結果集，也是要注意使用的場景）
drop table t purge;
create table t as  select * from dba_objects;
select count(*) from t;
set linesize 1000
set autotrace traceonly
select /*+ result_cache */ count(*) from t;
/


--速度之王來咯！（原來需求才是王道）
select count(*) from t where rownum=1;