[20180626]函数与标量子查询14.txt
[20180626]函数与标量子查询14.txt
--//前面看http://www.cnblogs.com/kerrycode/p/9099507.html链接,里面提到:
通俗来将，当使用标量子查询的时候，ORACLE会将子查询结果缓存在哈希表中， 如果后续的记录出现同样的值，优化器通过缓存在哈希
表中的值，判断重复值不用重复调用函数，直接使用上次计算结果即可。从而减少调用函数次数，从而达到优化性能的效果。另外在
ORACLE 10和11中， 哈希表只包含了255个Buckets，也就是说它能存储255个不同值，如果超过这个范围，就会出现散列冲突，那些出现
散列冲突的值就会重复调用函数，即便如此，依然能达到大幅改善性能的效果。
--//前几天测试11.2.0.4 for linux下,哈希表不止255个Buckets.
--//另外也测试再10g下到底有512个Buckets.11.2.0.4是1024.
--//昨天看链接http://www.cnblogs.com/kerrycode/p/9223093.html =>ORACLE中Scalar subquery Caching的hash table大小测试浅析
--//里面建立一个函数,很容易确定调用函数的次数.重复测试,另外前面的测试有点慢,看看我现在的测试是否快一点.^_^ .
1.环境:
SCOTT@book> @ &r/ver1
PORT_STRING VERSION BANNER
------------------------------ -------------- --------------------------------------------------------------------------------
x86_64/Linux 2.4.xx 11.2.0.4.0 Oracle Database 11g Enterprise Edition Release 11.2.0.4.0 - 64bit Production
create or replace function f( x in varchar2 ) return number
as
begin
dbms_application_info.set_client_info(userenv('client_info')+1 );
return length(x);
end;
/
2.建立测试脚本:
create table t as select rownum id1,mod(rownum-1,10000)+1 id2 from dual connect by level<=20000;
--//分析表略.
create table t1 ( a number ,b number);
--//字段a 表示调用函数次数.
--//建立脚本cy.txt
exec dbms_application_info.set_client_info(0);
set term off
exec :x := &&1;
select count(distinct f_id2) from (select id2,(select f(id2) from dual) as f_id2 from t where id2 <= :x );
set term on
insert into t1 values (userenv('client_info') ,:x) ;
commit ;
--//建立shell脚本cy.sh:
#! /bin/bash
sqlplus -s -l scott/book <<EOF >> hz.txt
variable x number;
$(seq 10000 | xargs -I{} echo @cy.txt {})
quit
EOF
3.另外的测试脚本:
--//自己也写PL/SQL脚本.仅仅为了学习...
declare
x number;
begin
for i in 1..10000 loop
dbms_application_info.set_client_info(0);
select count(distinct f_id2) into x from (select id2,(select f(id2) from dual) as f_id2 from t where id2 <= i );
insert into t1 values (userenv('client_info') ,i) ;
commit ;
end loop;
end;
/
--//在家里的12c for windows测试看看:
SCOTT@test01p> @ ver1
PORT_STRING VERSION BANNER CON_ID
------------------------------ -------------- -------------------------------------------------------------------------------- ----------
IBMPC/WIN_NT64-9.1.0 12.1.0.1.0 Oracle Database 12c Enterprise Edition Release 12.1.0.1.0 - 64bit Production 0
select max(id2) from (
SELECT id2, r, rp
FROM ( SELECT b id2, a r, LAG (a) OVER (ORDER BY b) rp
FROM t1
ORDER BY b)
WHERE r - rp = 1 order by id2);
MAX(ID2)
----------
9234
--//说明:我前面的测试id2 记录的数量加倍,而这次记录的是变量:X,这样不要在除以2.
SELECT id2, r, rp
FROM ( SELECT b id2, a r, LAG (a) OVER (ORDER BY b) rp
FROM t1
ORDER BY b)
WHERE r - rp >= 2 and id2<=9234 ;
...
8210 rows selected.
--// 9234-8210 = 1024 ,与前面测试一样.
4.验证测试是否正确:
--//执行以上脚本.cy.sh,等....
select max(id2) from (
SELECT id2, r, rp
FROM ( SELECT b id2, a r, LAG (a) OVER (ORDER BY b) rp
FROM t1
ORDER BY b)
WHERE r - rp = 1 order by id2);
MAX(ID2)
------------
9234
--//9234还会进入buckupset,后面的数字带入都是出现hash 冲突的情况.
SELECT id2, r, rp
FROM ( SELECT b id2, a r, LAG (a) OVER (ORDER BY b) rp
FROM t1
ORDER BY b)
WHERE r - rp >= 2 and id2<=9234 ;
...
8210 rows selected.
--//输出太长,一共8210个值,略,这个结果就是在1-9234之间,出现hash冲突的值.
select count(*) from
(SELECT id2, r, rp
FROM ( SELECT b id2, a r, LAG (a) OVER (ORDER BY b) rp
FROM t1
ORDER BY b)
WHERE r - rp >= 2 and id2<=9234 );
COUNT(*)
------------
8210
--//9234-8210 = 1024 ,可以看出11.2.0.4标量子查询的哈希表大小是1024个buckets.
--//后面的测试不再做了.
