Postgres SELECT COUNT ... FOR UPDATE

Question

Here is pseudo code for what I'm trying to do:

rate_count = SELECT COUNT(id) FROM job WHERE last_processed_at >= ?

current_limit = rate_limit - rate_count
if current_limit > 0
  UPDATE job SET state='processing'
  WHERE id IN(
    SELECT id FROM job
    WHERE state='pending'
    LIMIT :current_limit
  )

I have it working except for concurrency issues. When run from multiple sessions at the same time, both sessions SELECT and therefore update the same stuff :(

I'm able to get the 2nd query atomic by adding FOR UPDATE in it's SELECT subquery. But I can't add FOR UPDATE to the first query because FOR UPDATE isn't allowed with aggregate functions

How can I make this piece an atomic transaction?

Answer 1

You can do FOR UPDATE within a subquery:

rate_count := COUNT(id) 
              FROM (
                SELECT id FROM job
                WHERE last_processed_at >= ? FOR UPDATE
                ) a;

You can also do this whole thing in a single query:

UPDATE job SET state='processing'
WHERE id IN (
  SELECT id FROM job
  WHERE state='pending'
  LIMIT (SELECT GREATEST(0, rate_limit - COUNT(id))
         FROM (SELECT id FROM job
               WHERE last_processed_at >= ? FOR UPDATE) a
        )
)