Re: [SQL] inserts/updates problem under stressing !

Oleg Bartunov <oleg@sai.msu.su> writes:
> I did some benchmarks of my Web site and notice I lost some hits
> which I accumulate in postgres (6.5.1) database on Linux 2.0.36 system

> 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';

I wonder whether this doesn't have a problem with concurrent access:

1. Transaction A does 'Select count into cnt', gets (say) 200.
2. Transaction B does 'Select count into cnt', gets 200.
3. Transaction A writes 201 into hits record.
4. Transaction B writes 201 into hits record.

and variants thereof.  (Even if A has already written 201, I don't think
B will see it until A has committed...)

I am not too clear on MVCC yet, but I think you need "SELECT FOR UPDATE"
or possibly an explicit lock on the hits table in order to avoid this
problem.  Vadim, any comments?
        regards, tom lane