Re: Select for update Question

On 09/12/2003 11:47 John Sidney-Woollett wrote:
> Hi
> 
> I have a function called from a java app (via jdbc) which identifies
> images awaiting processing. This is determined by checking the
> WPImageStateID field on the WPImageHeader record (1=awaiting, 2=being
> processed, 3=complete).
> 
> The (jdbc) connection to the database is a standard one so I suspect that
> the transaction isolation level is Read uncommitted.

Unlikely as PostgreSQL doesn't support read uncommitted...

> 
> What I need is for the call to GetNextChangedImageHeader() to return the
> WDResourceID of the next WPImageHeader record awaiting processing.
> 
> The way it is written (I think that) it will either return the ID of a
> WPImageHeader record that genuinely is awaiting processing (if one is
> available), or will return -1 because it waited on a row lock which was
> released by another transaction on the same WPImageHeader record, but
> whose WPImageStateID is now no longer 1.
> 
> Does this look correct?

I think you need to play with a couple of psql sessions to sort this out. 
I think you might have a race condition here. 
-- 
Paul Thomas
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