Select unlocked row in Postgresql

Question

Is there a way to select rows in Postgresql that aren\'t locked? I have a multi-threaded app that will do:

Select... order by id desc limit 1 for update
         


        

Answer 1

How about the following? It might be treated more atomically than the other examples but should still be tested to make sure my assumptions aren't wrong.

UPDATE users SET flags = 1 WHERE id = ( SELECT id FROM users WHERE flags = 0 ORDER BY id DESC LIMIT 1 ) RETURNING ...;

You'll probably still be stuck with whatever locking scheme postgres uses internally to supply consistent SELECT results in the face of a simultaneous UPDATEs.