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Re: Foreign key quandries

From: Rod Taylor <rbt(at)rbt(dot)ca>
To: Stephan Szabo <sszabo(at)megazone23(dot)bigpanda(dot)com>
Cc: PostgreSQL Development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Foreign key quandries
Date: 2003-03-01 05:11:37
Message-ID: 1046495497.26763.52.camel@jester (view raw, whole thread or download thread mbox)
Lists: pgsql-hackers
I'm not sure I understand the question. The case as described simply has
to deadlock because your approaching the same values with conflicting
tasks from opposite directions.

Detect it, kill one, and continue on.

Same problem as:

T1 insert into pk values (1);
T2 insert into pk values (2);
T2 insert into pk values (1);
T1 insert into pk values (2);

It's up to the application(s) to minimize risk by approaching values in
a common order for a given set of work.

On Fri, 2003-02-28 at 16:58, Stephan Szabo wrote:
> Going through the issues in doing dirty reads in foreign keys I've come up
> with a few cases that I'm fairly uncertain about how to handle with
> regards to deadlocks and figured I should ask for advice because I think
> I'm missing something painfully obvious, but don't get large enough blocks
> of time to think about it to figure out what it is.
> I'd thought maybe it'd be enough to say which type of
> thing on which constraint and use that to basically say that
> we don't need to wait on a transaction that's waiting on us
> due to a modification to the other table, but AFAICS
> that lets through a bad case:
>  T1: insert into fk values (2);
>  T2: delete from pk where key=3;
>  T2: delete from pk where key=2;
>  T1: insert into fk values (3);
> If T1 doesn't wait in this case, you can get into a case where
> a bad row is inserted into fk if you then have:
>  T1: delete from fk where key=2;
>  T1: commit;
> Now there's no row to make the second delete fail but
> transaction 2 still can't commit due to the fk row with key 3.
> I'd then thought of doing something based on what row/value
> transaction 2 was waiting on, but that has problems.
> Given a foreign key with no referential actions and a
> sequence like:
>  Transaction 1 inserts into the foreign key table a row
>   with a referencing key of 2.
>  Transaction 1 checks the foreign key
>  Transaction 2 deletes the primary key rows having keys
>   2 and 3
>  Transaction 1 inserts another row into the foreign key
>   table with a referencing key of 2.
>  Transactions 1 and 2 start checking the foreign key.
> AFAICS, transaction 2 needs to wait since there's already a
> row it can see in the foreign key table that's not yet committed
> (so it doesn't know if the delete works or not).  We can tell
> transaction 1 that it doesn't need to wait on transaction 2
> because transaction 1 is inserting a value that transaction 2
> will see in its check, thus we're saved from the first case.
> However, this has the potential to deadlock if we had for example,
> inserted a foreign key table row of 3 rather than 2 as the second
> insert in transaction 1 and the delete check for 2 went first.  If
> we knew that it was also going to be checking the 3 rows, we'd be
> safe, but then we've got to keep that information in some way that's
> visible to other transactions AFAICS.  And, if the checks were
> done in the order delete check for 3, delete check for 2(t2 blocks),
> insert check for 3, we'd be back in the state of the first example.

Rod Taylor <rbt(at)rbt(dot)ca>

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Subject: Re: Foreign key quandries
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