PostgreSQL foreign key violation inside transaction

Question

I have a function with several queries inside, like this:

CREATE OR REPLACE FUNCTION public.myfunction(winningid integer, losingid integer)
 RETURNS void
 LANGUAGE plpgsql
AS $function$
begin

update partecipants set id_be=winningId where id_be=losingId;
...
many other updates and deletes regarding other tables
...

delete from business_entity where id_be=losingId;

end;
$function$
;

There is a foreign key between partecipants and business_entity:

ALTER TABLE partecipants ADD CONSTRAINT partecipants_fk FOREIGN KEY (id_be) REFERENCES business_entity(id_be)

Sometimes (like 1 in 1000 times) this function hangs with error:

error: update or delete on table "business_entity" violates foreign key constraint "partecipants_fk" on table "partecipants"
detail:
   'Key (id_be)=(315017) is still referenced from table "partecipants".

If I run the same function a second time, ends without error.

How is this possible?

As a background information, other processes are using (often also with locks) the tables involved in the function while is running.

Answer 1

Concurrent transactions could add or modify rows in partecipants between your UPDATE and your DELETE so that the latter fails.

If you want to avoid that, do both in a single statement:

WITH dummy AS (
   update partecipants set id_be=winningId where id_be=losingId
)
delete from business_entity where id_be=losingId;