Thread: SubQuery Performance

Hi All,

I have a poor performance SQL as following. The table has about 200M
records, each employee have average 100 records. The query lasts about
3 hours. All I want is to update the flag for highest version of each
client's record. Any suggestion is welcome!

Thanks,

Mike


====SQL===========
update empTbl A
set flag=1
where
rec_ver =
( select max(rec_ver)
from empTbl
where empNo = A.empNo)



===Table empTbl=====

empTbl
{
int empNo;
int flag;
char[256] empDesc;
int rec_ver;
}

In response to mike :
> Hi All,
>
> I have a poor performance SQL as following. The table has about 200M
> records, each employee have average 100 records. The query lasts about
> 3 hours. All I want is to update the flag for highest version of each
> client's record. Any suggestion is welcome!
>
> Thanks,
>
> Mike
>
>
> ====SQL===========
> update empTbl A
> set flag=1
> where
> rec_ver =
> ( select max(rec_ver)
> from empTbl
> where empNo = A.empNo)
>
>
>
> ===Table empTbl=====
>
> empTbl
> {
> int empNo;
> int flag;
> char[256] empDesc;
> int rec_ver;
> }

Try this:

update empTbl A set flag=1 from (select empno, max(rec_ver) as rec_ver from empTbl group by empno) foo where
(a.empno,a.rec_ver)= (foo.empno, foo.rec_ver); 

You should create an index on empTbl(empNo,rec_ver).

Please show us the EXPLAIN ANALYSE <query> for both selects.


Regards, Andreas
--
Andreas Kretschmer
Kontakt:  Heynitz: 035242/47150,   D1: 0160/7141639 (mehr: -> Header)
GnuPG: 0x31720C99, 1006 CCB4 A326 1D42 6431  2EB0 389D 1DC2 3172 0C99