On Mon, 26 Jul 1999, Vadim Mikheev wrote:
> Date: Mon, 26 Jul 1999 10:43:00 +0800
> From: Vadim Mikheev <vadim@krs.ru>
> To: Tom Lane <tgl@sss.pgh.pa.us>
> Cc: Oleg Bartunov <oleg@sai.msu.su>, pgsql-hackers@postgreSQL.org,
> pgsql-sql@postgreSQL.org
> Subject: Re: [SQL] inserts/updates problem under stressing !
>
> (Sorry for incomplete prev message).
>
> Tom Lane wrote:
> >
> > 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...)
>
> You're right, Tom.
>
> > 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?
>
> SELECT FOR UPDATE will not help: if there was no record for
> particular key then nothing will be locked and two records with
> the same key will be inserted.
>
> Oleg, use LOCK IN SHARE ROW EXCLUSIVE MODE.
Thanks Vadim. Just tried this, but still I see a difference between
count hits (accumulated) from db and access_log. In my test these numbers are:
95 and 109. So I lost 14 hits ! And no errors !
In my handler I have now:
my $sth = $dbh->do("LOCK TABLE hits IN SHARE ROW EXCLUSIVE MODE");
my $sth = $dbh->do("SELECT acc_hits($1)") || die $dbh->errstr;
am I right ?
I created hits table as:
create table hits ( msg_id int4 not null primary key, count int4 not null, first_access datetime default now(),
last_accessdatetime
);
and in error_log sometimes I see
ERROR: Cannot insert a duplicate key into a unique index
How this could be possible if I use
LOCK TABLE hits IN SHARE ROW EXCLUSIVE MODE ?
Oleg
PS.
I remind my functions is:
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';
>
> Vadim
>
_____________________________________________________________
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
