Re: AW: [HACKERS] inserts/updates problem under stressing !

Andreas,

I rewrote my function but got a problem how to know if update fails:

CREATE FUNCTION "acc_hits" (int4) RETURNS datetime AS ' 
Declare   keyval      Alias For $1;   cnt         int4;   curtime     datetime;
Begin   curtime := ''now'';   Update hits set count = count + 1,last_access = curtime where msg_id = keyval;   if Not
Foundthen     ??????????       -- first_access inserted on default, last_access is NULL       Insert Into hits
(msg_id,count)values (keyval, 1);        End If;   return curtime;   
 
End;
' LANGUAGE 'plpgsql';


regards,    Oleg






On Mon, 26 Jul 1999, Zeugswetter Andreas IZ5 wrote:

> Date: Mon, 26 Jul 1999 10:31:33 +0200
> From: Zeugswetter Andreas IZ5 <Andreas.Zeugswetter@telecom.at>
> To: 'Oleg Bartunov' <oleg@sai.msu.su>
> Subject: AW: [HACKERS] inserts/updates problem under stressing !
> 
> 
> > CREATE FUNCTION "acc_hits" (int4) RETURNS int4 AS '
> > Declare
> >     keyval      Alias For $1;
> >     cnt         int4;
> >     curtime     datetime;
> > Begin
> >     curtime := ''now'';
> >     Select count into cnt from hits where msg_id = keyval;
> >     if Not Found then
> >         cnt := 1;
> >         -- first_access inserted on default, last_access is NULL
> >         Insert Into hits (msg_id,count) values (keyval, cnt);
> >     else
> >         cnt := cnt + 1;
> >         Update hits set count = cnt,last_access = curtime where msg_id =
> > keyval;
> >     End If;
> >     return cnt;
> > End;
> > ' LANGUAGE 'plpgsql';
> > 
> > 
> Ok, this proc is not concurrent capable. This is because in the time between
> the select and the update some other connection can update count.
> 
> 1. Change the update to:
> > Update hits set count = count+1, last_access = curtime where msg_id =
> > keyval;
> > 
> 2. the insert is also not concurrent capable, since there could be two
> simultaneous
> first accesses.
> 
> It looks like there will be more updates than inserts, so I would change the
> above to 
> 1. try update
> 2. if num rows affected = 0 do the insert 
> 
> I don't know how to get the rows affected, but this should be possible.
> 
> Andreas
> 
> 

_____________________________________________________________
Oleg Bartunov, sci.researcher, hostmaster of AstroNet,
Sternberg Astronomical Institute, Moscow University (Russia)
Internet: oleg@sai.msu.su, http://www.sai.msu.su/~megera/
phone: +007(095)939-16-83, +007(095)939-23-83