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

^^ that works. consider having an "immediate" status of "locked".

Let's say your table is like that:

id | name | surname | status

And possible statuses for example are: 1=pending, 2=locked, 3=processed, 4=fail, 5=rejected

Every new record gets inserted with status pending(1)

Your program does: "update mytable set status = 2 where id = (select id from mytable where name like '%John%' and status = 1 limit 1) returning id, name, surname"

Then your program does its thing and if it cames up with the conclusion that this thread shouldn't had processed that row at all, it does: "update mytable set status = 1 where id = ?"

Otherside it updates to the other statuses.

Answer 2

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.