[GENERAL] serialization failure why?

[Filipe Pina]

I have these 2 tables:

CREATE TABLE "stuff" ("number" integer NOT NULL PRIMARY KEY, "title" 
varchar(40) NOT NULL);
CREATE TABLE "stuff_ext" ("stuff_ptr_id" integer NOT NULL PRIMARY KEY, 
"extra" integer NOT NULL);
ALTER TABLE "stuff_ext" ADD CONSTRAINT 
"stuff_ext_stuff_ptr_id_5a4ee8edae53404b" FOREIGN KEY ("stuff_ptr_id") 
REFERENCES "stuff" ("number") DEFERRABLE INITIALLY DEFERRED;
CREATE SEQUENCE stuff_seq;
And then the function:

CREATE OR REPLACE FUNCTION create_stuff(number integer, title text)
   RETURNS integer AS $$
DECLARE
   a1 stuff;
   a2 stuff_ext;
BEGIN
   IF number IS NULL THEN
       number := nextval('stuff_seq');
   END IF;

   a1.number := number;
   a1.title := title;

   a2.stuff_ptr_id := a1.number;

   INSERT INTO stuff VALUES (a1.*);
   INSERT INTO stuff_ext VALUES (a2.*);

   RETURN number;
END
$$
LANGUAGE plpgsql;


The DB is configured for SERIALIZABLE transaction mode.

Now, if I can the function without passing number, such as:

select create_stuff(NULL,'title');

in 10 forked processes in a loop with a few iterations in each, I get 
quite a few SERIALIZATON FAILURE (sqlstate 40001).

If I comment out the "INSERT INTO stuff_ext" line, I don't get any.

How is the second insert causing serialize dependencies...?

The specific error messages vary between

ERROR: could not serialize access due to read/write dependencies among 
transactions
DETAIL: Reason code: Canceled on identification as a pivot, during 
commit attempt.
HINT: The transaction might succeed if retried.

and

ERROR: could not serialize access due to read/write dependencies among 
transactions
DETAIL: Reason code: Canceled on commit attempt with conflict in from 
prepared pivot.
HINT: The transaction might succeed if retried.

Thanks!