Remove O(N) queries from workspace status API

Previously this was doing O(N) queries, which we accepted when the API
was addede because we did not, at the time, have workspaces with more
than low-10s of jobs.  We now have workspaces with hundreds of jobs
defined.  On a particularly large workspace (long-covid-symptoms) this
endpoint could take as long as 5s (and ~300 queries) to calculate.

This change uses raw SQL so we can make use of a GROUP BY in the CTE to
get the latest actions and then JOIN to it in the outer query to match
those up with their statuses.

jobserver/models/core.py

@@ -10,7 +10,7 @@
 from django.contrib.auth.models import UserManager as DjangoUserManager
 from django.contrib.auth.validators import UnicodeUsernameValidator
 from django.contrib.postgres.fields import ArrayField
-from django.db import models, transaction
+from django.db import connection, models, transaction
 from django.db.models import Min, Q, prefetch_related_objects
 from django.db.models.functions import Lower, NullIf
 from django.urls import reverse
@@ -1363,21 +1363,30 @@ def get_action_status_lut(self, backend=None):
 
         We need to get the latest status for each action run inside this
         Workspace.
-        """
-        jobs = Job.objects.filter(job_request__workspace=self)
-
-        if backend:
-            jobs = jobs.filter(job_request__backend__slug=backend)
-
-        # get all known actions
-        actions = set(jobs.values_list("action", flat=True))
 
-        action_status_lut = {}
-        for action in actions:
-            # get the latest status for an action
-            job = jobs.filter(action=action).order_by("-created_at").first()
-            action_status_lut[action] = job.status
-        return action_status_lut
+        We're doing this in raw SQL so we can use a CTE to get the latest
+        actions, and then join that with the Jobs table to get the status of
+        those actions.
+        """
+        sql = """
+        WITH latest_actions AS (
+          SELECT
+            job.action AS name,
+            job.status AS status,
+            row_number() OVER (PARTITION BY job.action ORDER BY job.created_at DESC) AS row_num
+          FROM
+            jobserver_job job
+            INNER JOIN jobserver_jobrequest job_request ON (job.job_request_id = job_request.id)
+            INNER JOIN jobserver_workspace workspace ON (job_request.workspace_id = workspace.id)
+          WHERE workspace.name = %s
+        )
+        SELECT name, status
+        FROM latest_actions
+        WHERE row_num = 1
+        """
+        with connection.cursor() as cursor:
+            cursor.execute(sql, [self.name])
+            return dict(cursor.fetchall())
 
     @property
     def is_interactive(self):