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#1 (**permalink**) 04-10-2008, 08:52 AM

Hi everybody!

Recently, I've discovered an interesting feature (or a bug?) of PostgreSql (checked 7.4.x and 8.0.x):

One may define such tables:

create table ref(
id serial primary key,
name text);

create table dat(
id serial,
ref_id int references ref(id),
comment text);

Let us fill the ref table:

insert into ref(name) values('feature');
insert into ref(name) values('bug');

The test case:
For example we have 2 concurrent transactions (tr1 and tr2):

tr1: begin;
tr2: begin;
tr1: insert into dat(ref_id, comment) values (1, 'all ok');
tr2: insert into dat(ref_id, comment) values (2, 'all ok');
tr1: insert into dat(ref_id, comment) values (2, 'lockup');
tr2: insert into dat(ref_id, comment) values (1, 'deadlock');

.... and we recieve a deadlock!
Easy to understand why: each insert statement generates query like:
SELECT 1 FROM ONLY "public"."ref" x WHERE "id" = $1 FOR UPDATE OF x

So, is this behaviour of postgresql is a bug or feature?

Thanks,
Eugene.

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#2 (**permalink**) 04-10-2008, 08:53 AM

evgeny gridasov <eugrid@fpm.kubsu.ru> writes:
> ... and we recieve a deadlock!
> Easy to understand why: each insert statement generates query like:
> SELECT 1 FROM ONLY "public"."ref" x WHERE "id" = $1 FOR UPDATE OF x
> So, is this behaviour of postgresql is a bug or feature?

An extremely well-known bug ... which is fixed in CVS tip.

regards, tom lane

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INSERT deadlocks (bug or feature?)