Re: Foreign Key locking / deadlock issue. - Mailing list pgsql-general

Hello Phil,


On Tue, 2018-03-20 at 11:46 +0000, HORDER Phil wrote:
> Hi,
> I’m trying to understand why I’m getting a deadlock issue, and how to
> work around it.
>  
> At base, I think the problem is:
> 1.       Updates to a parent table are creating row level write
> locks,
> 2.       updates to a child table set the foreign key value to the
> parent table, which are then blocked.
>  
> While investigating, I found the row locking documentation, which
> says that I can request read locks that don’t block.
> But my sample code still gets blocked.
> https://www.postgresql.org/docs/9.6/static/explicit-locking.html
>  
> Can someone explain what I’m missing please?
>  
> parent process
> -------------------
>  
> start transaction;
> select * from pl where pl_id in (2,3) for no key update of pl;  (1)
> update pl set m_number = '234' where pl_id = 2;                 (2)
> update pl set m_number = '345' where pl_id = 3;                 (3)
>  
> child process
> -------------------
>  
> start transaction;
> select pl_id from pl where pl_id in (2,3) for key share of pl;  (4)
> update eln set pl_id = 3 where event_id = 303;                  (5)
> update eln set pl_id = 2 where event_id = 302;                  (6)
>  
>  
> My Parent process inserts and updates on the PL table, but never
> changes the key value.
> My Child process inserts and updates on the ELN table, and can set
> the FK reference value to the PL table.
>  
> I can understand that the default lock on the PL update will block
> the foreign key check from the ELN table.
> Why does this example still get blocked?
>  
> Is there a way around this without dropping the foreign key?
>  
> (And if so…. How can I get this to work in Spring Data / JPA?)
>  
>  
> Phil Horder
> Database Mechanic
>  
> Thales
> Land and Air Systems
> Horizon House, Throop Road, Templecombe, Somerset, BA8 0DH, UK
>  


The details are a bit sparse. I don't understand "why" you are using
"no key update" or "for key share".

Anyway, if the business rules dictate that table pl and table eln
should be updated in a single transaction, the way I'd do this would be
to create a method or a sub-routine where three arguments are specified
being the connection handle, the values of p_id, m_number and event_id
to be used. Then:-

BEGIN;
SELECT m_number FROM pl WHERE pl_id = $2 FOR UPDATE;
UPDATE pl SET m_number = $3 WHERE pl_id = $2;
Test for success.
If successful, then:-
SELECT pl_id FROM eln WHERE event_id = $4 FOR UPDATE;
UPDATE eln SET pl_id = $2 WHERE event_id = $4;
If successful then COMMIT;

If either of the updates fail, ROLLBACK; and inform the user that the
transaction failed.

If you are using Java then remember that auto-commit is usually set
"on" by default.

Doing a SELECT . . . FOR UPDATE on a single row just locks that row and
if another session also tries to do the same thing, then it just waits
until it times out.

You haven't said if this code is being generated dynamically by the
application. 

HTH,
Rob