Re: blocking INSERTs

On Tue, 7 Jun 2005, Joseph Shraibman wrote:

> BEGIN;
> SELECT ... FROM table WHERE a = 1 FOR UPDATE;
> UPDATE table SET ... WHERE a = 1;
> if that resturns zero then
> INSERT INTO table (...) VALUES (...);
> END;
>
> The problem is that I need to avoid race conditions.  Sometimes I get
> primary key exceptions on the INSERT.

PG uses row locking and the problem above is that if there is no row with
a=1 then there is no row to lock in the first select. The update will
update zero rows and then the you come to the insert and nothing is locked
so 2 transactions can do the insert at the same time. This means that one
of the 2 transactions will fail.

If you use pg 8.0 maybe this example might help you:

http://developer.postgresql.org/docs/postgres/plpgsql-control-structures.html#PLPGSQL-UPSERT-EXAMPLE

and if not you are correct that you need to lock the table (or just accept
that it fail sometimes and handle that failure in the client).

--
/Dennis