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

Oleg Bartunov <oleg(at)sai(dot)msu(dot)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