Re: Database Insertion commitment

On 7/9/07, Viatcheslav Kalinin <vka(at)ipcb(dot)net> wrote:
>
> Jasbinder Singh Bali wrote:
> > My scenario is something like this. I'll try to make it modular and
> > simple.
> >
> > Start Function A (Written in plperlu with no subtransactions)
> > Insert 1 on tbl_abc; (fires trigger A)
> > Insert 2 on tbl_abc; (fires trigger A)
> > End Function A
> >
> > Start Trigger A
> > check the value of col_abc in tbl_abc
> > Start Activity A if col_abc in tbl_abc doesn't is not duplicated.
> > End Trigger A
> >
> > Now, if Insert 1 inserts col_abc = 'xyz' in tbl_abc
> > and Insert 2 inserts the same value of col_abc ='xyz' the its not able
> > to see the value of insert 1
> > and erroneously starts Activity A that it should not actually.
> >
> > Do you think I am missing something vital here?
> > I'm kind of stuck and confused because fundamentally Insert 2 should
> > be able to see the value of Insert 1 as there is no subtransaction
> > involved.
> >
> > Thanks,
> > ~Jas
> >
> > On 7/9/07, *Viatcheslav Kalinin* <vka(at)ipcb(dot)net <mailto:vka(at)ipcb(dot)net>>
> > wrote:
> >
> > Jasbinder Singh Bali wrote:
> > > Hi,
> > >
> > > If I have a series of Insert statements within a loop in a
> > function on
> > > the same table.
> > > Would an Insert be able to see the values of previous insert in
> > that
> > > table ?
> > > I just wanted to know, when would the records be committed, as
> > in, is
> > > it after the whole function is done for with its execution or
> > > right after one single insert.
> > >
> > > Right now what I'm observing is that all the inserts are committed
> > > after the whole function is executed and one insert doesn't see
> the
> > > value of its previous insert.
> > > In this scenario, how can an insert see the value of its previous
> > > insert even though the whole transaction that lies within the
> > function
> > > is not complete.
> > >
> > > Thanks,
> > > ~Jas
> > Functions are run in a single separate transaction (unless then have
> > BEGIN ... EXCEPTION ... END block inside them which implies
> > subtransaction) thus inside a function all statements can see
> > results of
> > the previous ones just like if you ran them one by one. All
> > changes the
> > function does are committed at the end of the transaction, whether
> > they
> > are visible or not from the outside of that transaction depends on
> > the
> > transaction isolation level. There are only two distinct levels of
> > isolation in Postgresql: READ COMMITTED and SERIALIZABLE, hence
> > uncommitted data can never be seen before the transaction which
> > changed
> > them is over, the second one makes transaction fully independent
> > just as
> > the name states.
> >
> >
>
> Hmm, afaik triggers are run within the same transaction so it shouldn't
> really matter if the trigger is involved. I've made some tests too
> (written in plpgsql, I hope you are fine with it):
>
> CREATE TABLE test (x varchar);
>
> CREATE OR REPLACE FUNCTION "public"."test_trg" () RETURNS trigger AS
> $body$
> BEGIN
> perform 1 from test1 where x = new.x;
> if not found then
> raise info 'not found';
> else
> raise info 'found';
> end if;
>
> return new;
> END;
> $body$
> LANGUAGE 'plpgsql' VOLATILE CALLED ON NULL INPUT SECURITY INVOKER;
>
> CREATE TRIGGER "trigger1" BEFORE INSERT
> ON "public"."test" FOR EACH ROW
> EXECUTE PROCEDURE "public"."test_trg"();
>
> CREATE OR REPLACE FUNCTION "public"."test" () RETURNS "pg_catalog"."void"
> AS
> $body$
> begin
> insert into test values ('xxx');
> insert into test values ('xxx');
> end;
> $body$
> LANGUAGE 'plpgsql' VOLATILE CALLED ON NULL INPUT SECURITY DEFINER;
> ----------------
>
> select test();
>
> >INFO: not found
> >CONTEXT: SQL statement "INSERT INTO test values ('xxx')"
> >PL/pgSQL function "test" line 5 at SQL statement
> >INFO: found
> >CONTEXT: SQL statement "INSERT INTO test values ('xxx')"
> >PL/pgSQL function "test" line 6 at SQL statement
>
>
> As you can see it has found inserted value on the second insert. Could
> it be that you misused after trigger instead of before?

The only difference between you test case my a sample test case that I would
provide is the Perform part in the trigger function.

In my program Perform part is about opening a socket connection with a Unix
Tools server that runs traceroute and populates records in table test.

Before this Perform part, I would check if table test has one row whose col1
value is 'xyz' say.

If No, then Perform
else
don't perform.

One reason I see that new insert does't see the values of old insert is
because as soon as socket connection is established, my trigger returns and
1st insert is complete even though I'm not sure whether the unix tools
server has already inserted values in table test or not. There might be a
time lag based on how fast traceroute returns, though right now its very
fast.

Any comments?

Thanks,
Jas