queue_size.sql

WITH persisted_rate AS (
    SELECT
      date_trunc('minute', persisted_time) AS persisted_minute,
      count(*)                             AS num_persisted
    FROM persistent_queue_table
    GROUP BY persisted_minute
),
    polled_rate AS (
      SELECT
        date_trunc('minute', poll_time) AS poll_minute,
        count(*)                        AS num_polled
      FROM persistent_queue_table
      GROUP BY poll_minute
  ),
    total_objects_persisted_by_minute AS (
      SELECT
        persisted_minute,
        (
          SELECT sum(num_persisted)
          FROM persisted_rate persisted_rate_2
          WHERE persisted_rate_2.persisted_minute <= persisted_rate_1.persisted_minute
        ) AS running_persisted_sum
      FROM persisted_rate persisted_rate_1
  ),
    total_objects_polled_by_minute AS (
      SELECT
        poll_minute,
        (
          SELECT sum(num_polled)
          FROM polled_rate polled_rate_2
          WHERE polled_rate_2.poll_minute <= polled_rate_1.poll_minute
        ) AS running_polled_sum
      FROM polled_rate polled_rate_1
  )
SELECT
  persisted_minute                             AS minute,
  (running_persisted_sum - running_polled_sum) AS queue_size
FROM total_objects_persisted_by_minute
  INNER JOIN total_objects_polled_by_minute
    ON persisted_minute = poll_minute
ORDER BY minute;