Re: trying to analyze deadlock

Mariel Cherkassky wrote:
> Hi all,
> I'm trying to analyze a deadlock that I have in one of our environments.
> The deadlock message :
>
> 06:15:49 EET db 14563 DETAIL: Process 14563 waits for ShareLock on transaction 1017405468; blocked by process 36589.
> Process 36589 waits for ShareLock on transaction 1017403840; blocked by process 14563.
> Process 14563: delete from tableB where a in (select id from tableA where c in (....)
> Process 36589: delete from tableA where c in (....)
> 06:15:49 EET db 14563 HINT: See server log for query details.
> 06:15:49 EET db 14563 STATEMENT: delete from tableB where a in (select id from tableA where c in (....)
> 06:15:49 EET db 36589 LOG: process 36589 acquired ShareLock on transaction 1017403840 after 1110158.778 ms
> 06:15:49 EET db 36589 STATEMENT: delete from tableA where c in (....)
> 06:15:49 EET db 36589 LOG: duration: 1110299.539 ms execute <unnamed>: delete from tableA where c in (...)
>
> tableA : (id int, c int references c(id))
> tableB : (id int, a int references a(id) on delete cascade)
> tableC(id int...)
>
> One A can have Many B`s connected to (One A to Many B).
>
> deadlock_timeout is set to 5s.
>
> Now I'm trying to understand what might cause this deadlock. I think that its related to the foreign keys...

You can get that if the foreign key is defined as ON CASCADE DELETE or ON CASCADE SET NULL:

CREATE TABLE a (a_id integer PRIMARY KEY);

INSERT INTO a VALUES (1), (2);

CREATE TABLE b (b_id integer PRIMARY KEY, a_id integer NOT NULL REFERENCES a ON DELETE CASCADE);

INSERT INTO b VALUES (100, 1), (101, 1), (102, 2), (103, 2);

Transaction 1:

BEGIN;

DELETE FROM b WHERE b_id = 100;

Transaction 2:

BEGIN;

DELETE FROM a WHERE a_id = 2;

DELETE FROM a WHERE a_id = 1; -- hangs

Transaction 1:

DELETE FROM b WHERE b_id = 102;

ERROR: deadlock detected
DETAIL: Process 10517 waits for ShareLock on transaction 77325; blocked by process 10541.
Process 10541 waits for ShareLock on transaction 77323; blocked by process 10517.
HINT: See server log for query details.
CONTEXT: while deleting tuple (0,3) in relation "b"

Yours,
Laurenz Albe
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