⚡ [#1451] -- ensure that rendering a submission does not lead to an e…

…xplosion of queries

src/openforms/submissions/form_logic.py

@@ -77,7 +77,15 @@ def evaluate_form_logic(
     if _evaluated:
         return configuration
 
-    rules = FormLogic.objects.filter(form=step.form_step.form)
+    # renderer evaluates logic for all steps at once, so we can avoid repeated queries
+    # by caching the rules on the form instance.
+    # Note that form.formlogic_set.all() is never cached by django, so we can't rely
+    # on that.
+    rules = getattr(submission.form, "_cached_logic_rules", None)
+    if rules is None:
+        rules = FormLogic.objects.filter(form=submission.form)
+        submission.form._cached_logic_rules = rules
+
     submission_state = submission.load_execution_state()
 
     for rule in rules:

src/openforms/submissions/rendering/renderer.py

@@ -57,11 +57,12 @@ def get_children(self) -> Iterator["SubmissionStepNode"]:
         """
         Produce only the direct child nodes.
         """
+        submission_data = self.submission.data
         for step in self.steps:
             new_configuration = evaluate_form_logic(
                 submission=self.submission,
                 step=step,
-                data=self.submission.data,
+                data=submission_data,
                 dirty=False,
                 request=self.dummy_request,
             )

src/openforms/submissions/tests/renderer/test_renderer.py

@@ -1,4 +1,4 @@
-from django.test import TestCase
+from django.test import TestCase, override_settings
 
 from rest_framework.reverse import reverse
 
@@ -103,3 +103,19 @@ def test_renderer_with_form_logic_and_disabled_step(self):
         self.assertEqual(len(nodes), 2)
         enabled_step_node = nodes[1]
         self.assertEqual(enabled_step_node.step, self.sstep1)
+
+    def test_performance_num_queries(self):
+        """
+        Assert that the number of queries stays low while rendering a submission.
+        """
+        renderer = Renderer(
+            submission=self.submission, mode=RenderModes.pdf, as_html=True
+        )
+
+        # Expected queries:
+        # 1. Getting the merged data of the submission steps
+        # 2. Getting the submission steps for the given submission
+        # 3 & 4. Loading the submission execution state
+        # 5. Query the form logic rules for the submission form (and this is cached)
+        with self.assertNumQueries(5):
+            list(renderer)