Re: blocking INSERTs

Joseph Shraibman <jks@selectacast.net> writes:

> I want to do the following:
>
> 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.
>
> I think I need to lock the table in share mode to keep inserts from
> happening, but that blocks vacuums, and blocked vacuums block other
> things behind them.  So how do I get around this?

Ummm...  Don't lock the table for very long?  :)  If you commit
your transaction promptly then the table will be unlocked again.

-Doug