refactor: De-complexify 'count' metric

Given that `live_count_grouped` is already necessary for computing all
the other live queue metrics, we can reduce the work on MySQL by running
`failed_count_grouped` (and filtering the scan down to fewer rows)
instead of an entire count against the table.

stack-info: PR: #97, branch: smudge/stack/8

Author: smudge

lib/delayed/monitor.rb

@@ -112,11 +112,7 @@ def grouped_count(scope, include_db_time: false, **kwargs)
     end
 
     def count_grouped
-      if Job.connection.supports_partial_index?
-        failed_count_grouped.merge(live_count_grouped) { |_, l, f| l + f }
-      else
-        grouped_count(jobs).transform_values(&:count)
-      end
+      failed_count_grouped.merge(live_count_grouped) { |_, l, f| l + f }
     end
 
     def live_count_grouped

spec/delayed/__snapshots__/monitor_spec.rb.snap

@@ -441,6 +441,7 @@ SELECT SUM(count) AS count,
        queue AS queue
   FROM (SELECT `delayed_jobs`.`priority`, `delayed_jobs`.`queue`, COUNT(*) AS count
   FROM `delayed_jobs`
+  WHERE `delayed_jobs`.`failed_at` IS NOT NULL
   GROUP BY `delayed_jobs`.`priority`, `delayed_jobs`.`queue`) subquery
   GROUP BY CASE WHEN priority < 10 THEN 0 WHEN priority < 20 THEN 10 WHEN priority < 30 THEN 20 WHEN priority >= 30 THEN 30 END, `queue`
 
@@ -450,10 +451,9 @@ SELECT SUM(count) AS count,
             -> Materialize  (cost=...)
                 -> Table scan on <temporary>
                     -> Aggregate using temporary table
-                        -> Covering index scan on delayed_jobs using idx_delayed_jobs_live  (cost=...)
+                        -> Filter: (delayed_jobs.failed_at is not null)  (cost=...)
+                            -> Covering index range scan on delayed_jobs using idx_delayed_jobs_live over (NULL < failed_at)  (cost=...)
 ---
--- QUERIES FOR `future_count`:
----------------------------------
 SELECT SUM(future_count) AS future_count,
        SUM(erroring_count) AS erroring_count,
        MIN(run_at) AS run_at,
@@ -479,6 +479,9 @@ SELECT SUM(future_count) AS future_count,
                         -> Filter: (delayed_jobs.failed_at is null)  (cost=...)
                             -> Covering index lookup on delayed_jobs using idx_delayed_jobs_live (failed_at = NULL)  (cost=...)
 ---
+-- QUERIES FOR `future_count`:
+---------------------------------
+-- (no new queries)
 -- QUERIES FOR `locked_count`:
 ---------------------------------
 SELECT SUM(claimed_count) AS claimed_count,
@@ -513,24 +516,7 @@ SELECT SUM(claimed_count) AS claimed_count,
 -- (no new queries)
 -- QUERIES FOR `failed_count`:
 ---------------------------------
-SELECT SUM(count) AS count,
-       CASE WHEN priority < 10 THEN 0 WHEN priority < 20 THEN 10 WHEN priority < 30 THEN 20 WHEN priority >= 30 THEN 30 END AS priority,
-       queue AS queue
-  FROM (SELECT `delayed_jobs`.`priority`, `delayed_jobs`.`queue`, COUNT(*) AS count
-  FROM `delayed_jobs`
-  WHERE `delayed_jobs`.`failed_at` IS NOT NULL
-  GROUP BY `delayed_jobs`.`priority`, `delayed_jobs`.`queue`) subquery
-  GROUP BY CASE WHEN priority < 10 THEN 0 WHEN priority < 20 THEN 10 WHEN priority < 30 THEN 20 WHEN priority >= 30 THEN 30 END, `queue`
-
--> Table scan on <temporary>
-    -> Aggregate using temporary table
-        -> Table scan on subquery  (cost=...)
-            -> Materialize  (cost=...)
-                -> Table scan on <temporary>
-                    -> Aggregate using temporary table
-                        -> Filter: (delayed_jobs.failed_at is not null)  (cost=...)
-                            -> Covering index range scan on delayed_jobs using idx_delayed_jobs_live over (NULL < failed_at)  (cost=...)
----
+-- (no new queries)
 -- QUERIES FOR `max_lock_age`:
 ---------------------------------
 -- (no new queries)
@@ -556,6 +542,7 @@ SELECT SUM(count) AS count,
        queue AS queue
   FROM (SELECT `delayed_jobs`.`priority`, `delayed_jobs`.`queue`, COUNT(*) AS count
   FROM `delayed_jobs`
+  WHERE `delayed_jobs`.`failed_at` IS NOT NULL
   GROUP BY `delayed_jobs`.`priority`, `delayed_jobs`.`queue`) subquery
   GROUP BY CASE WHEN priority < 10 THEN 0 WHEN priority < 20 THEN 10 WHEN priority < 30 THEN 20 WHEN priority >= 30 THEN 30 END, `queue`
 
@@ -565,10 +552,9 @@ SELECT SUM(count) AS count,
             -> Materialize  (cost=...)
                 -> Table scan on <temporary>
                     -> Aggregate using temporary table
-                        -> Table scan on delayed_jobs  (cost=...)
+                        -> Filter: (delayed_jobs.failed_at is not null)  (cost=...)
+                            -> Table scan on delayed_jobs  (cost=...)
 ---
--- QUERIES FOR `future_count`:
----------------------------------
 SELECT SUM(future_count) AS future_count,
        SUM(erroring_count) AS erroring_count,
        MIN(run_at) AS run_at,
@@ -594,6 +580,9 @@ SELECT SUM(future_count) AS future_count,
                         -> Filter: (delayed_jobs.failed_at is null)  (cost=...)
                             -> Table scan on delayed_jobs  (cost=...)
 ---
+-- QUERIES FOR `future_count`:
+---------------------------------
+-- (no new queries)
 -- QUERIES FOR `locked_count`:
 ---------------------------------
 SELECT SUM(claimed_count) AS claimed_count,
@@ -628,24 +617,7 @@ SELECT SUM(claimed_count) AS claimed_count,
 -- (no new queries)
 -- QUERIES FOR `failed_count`:
 ---------------------------------
-SELECT SUM(count) AS count,
-       CASE WHEN priority < 10 THEN 0 WHEN priority < 20 THEN 10 WHEN priority < 30 THEN 20 WHEN priority >= 30 THEN 30 END AS priority,
-       queue AS queue
-  FROM (SELECT `delayed_jobs`.`priority`, `delayed_jobs`.`queue`, COUNT(*) AS count
-  FROM `delayed_jobs`
-  WHERE `delayed_jobs`.`failed_at` IS NOT NULL
-  GROUP BY `delayed_jobs`.`priority`, `delayed_jobs`.`queue`) subquery
-  GROUP BY CASE WHEN priority < 10 THEN 0 WHEN priority < 20 THEN 10 WHEN priority < 30 THEN 20 WHEN priority >= 30 THEN 30 END, `queue`
-
--> Table scan on <temporary>
-    -> Aggregate using temporary table
-        -> Table scan on subquery  (cost=...)
-            -> Materialize  (cost=...)
-                -> Table scan on <temporary>
-                    -> Aggregate using temporary table
-                        -> Filter: (delayed_jobs.failed_at is not null)  (cost=...)
-                            -> Table scan on delayed_jobs  (cost=...)
----
+-- (no new queries)
 -- QUERIES FOR `max_lock_age`:
 ---------------------------------
 -- (no new queries)