Issue Paginator: don't attempt to count the Issues

Counting incurs looking at all records which is too expensive if you have e.g.
1_000_000 issues.

Note that we take a different approach than the one for Events (where we
count-with-timeout). Reason for switching:
https://sqlite.org/forum/forumpost/fa65709226

For Events we have a known count for the non-query case (denormalized/counted
value), so we preserve what we had there. For Issues the trouble of keeping
counts right for muted/etc. is not (currently) worth it.

issues/templates/issues/issue_list.html

@@ -200,22 +200,22 @@
                 <svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 16 16" fill="currentColor" class="w-6 h-6 text-slate-200"><path fill-rule="evenodd" d="M9.78 4.22a.75.75 0 0 1 0 1.06L7.06 8l2.72 2.72a.75.75 0 1 1-1.06 1.06L5.47 8.53a.75.75 0 0 1 0-1.06l3.25-3.25a.75.75 0 0 1 1.06 0Z" clip-rule="evenodd" /></svg>
                 {% endif %}
 
-                {% if page_obj.paginator.num_pages > 1 %}
+                {% if page_obj.object_list|length > 0 %}
-                Issues {{ page_obj.start_index|intcomma }}–{{ page_obj.end_index|intcomma }} of {{ page_obj.paginator.count|intcomma }}
+                Issues {{ page_obj.start_index|intcomma }} – {{ page_obj.end_index|intcomma }}
-                {% elif page_obj.paginator.count > 0 %}
+                {% else %}
-                {{ page_obj.paginator.count|intcomma }} Issues
+                    {% if page_obj.number > 1 %}
+                    Less than {{ page_obj.start_index }} Issues  {# corresponds to the 1/250 case of having an exactly full page and navigating to an empty page after that #}
+                    {% else %}
+                    0 Issues
+                    {% endif %}
                 {% endif %}
 
                 {% if page_obj.has_next %}
                 <a href="?{% add_to_qs page=page_obj.next_page_number %}" class="inline-flex" title="Next page">
                 <svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 16 16" fill="currentColor" class="w-6 h-6"><path fill-rule="evenodd" d="M6.22 4.22a.75.75 0 0 1 1.06 0l3.25 3.25a.75.75 0 0 1 0 1.06l-3.25 3.25a.75.75 0 0 1-1.06-1.06L8.94 8 6.22 5.28a.75.75 0 0 1 0-1.06Z" clip-rule="evenodd" /></svg>
                 </a>
-                <a href="?{% add_to_qs page=page_obj.paginator.num_pages %}" class="inline-flex" title="Last page">
-                <svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 16 16" fill="currentColor" class="w-6 h-6"><path fill-rule="evenodd" d="M12.78 7.595a.75.75 0 0 1 0 1.06l-3.25 3.25a.75.75 0 0 1-1.06-1.06l2.72-2.72-2.72-2.72a.75.75 0 0 1 1.06-1.06l3.25 3.25Zm-8.25-3.25 3.25 3.25a.75.75 0 0 1 0 1.06l-3.25 3.25a.75.75 0 0 1-1.06-1.06l2.72-2.72-2.72-2.72a.75.75 0 0 1 1.06-1.06Z" clip-rule="evenodd" /></svg>
-                </a>
                 {% else %}
                 <svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 16 16" fill="currentColor" class="w-6 h-6 text-slate-200"><path fill-rule="evenodd" d="M6.22 4.22a.75.75 0 0 1 1.06 0l3.25 3.25a.75.75 0 0 1 0 1.06l-3.25 3.25a.75.75 0 0 1-1.06-1.06L8.94 8 6.22 5.28a.75.75 0 0 1 0-1.06Z" clip-rule="evenodd" /></svg>
-                <svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 16 16" fill="currentColor" class="w-6 h-6 text-slate-200"><path fill-rule="evenodd" d="M12.78 7.595a.75.75 0 0 1 0 1.06l-3.25 3.25a.75.75 0 0 1-1.06-1.06l2.72-2.72-2.72-2.72a.75.75 0 0 1 1.06-1.06l3.25 3.25Zm-8.25-3.25 3.25 3.25a.75.75 0 0 1 0 1.06l-3.25 3.25a.75.75 0 0 1-1.06-1.06l2.72-2.72-2.72-2.72a.75.75 0 0 1 1.06-1.06Z" clip-rule="evenodd" /></svg>
                 {% endif %}
         </div>
 

issues/views.py

@@ -15,6 +15,7 @@ from django.http import Http404
 from django.core.paginator import Paginator, Page
 from django.db.utils import OperationalError
 from django.conf import settings
+from django.utils.functional import cached_property
 
 from sentry.utils.safe import get_path
 from sentry_sdk_extensions import capture_or_log_exception
@@ -87,6 +88,35 @@ class KnownCountPaginator(EagerPaginator):
         return self._count
 
 
+class UncountablePage(Page):
+    """The Page subclass to be used with UncountablePaginator."""
+
+    @cached_property
+    def has_next(self):
+        # hack that works 249/250 times: if the current page is full, we have a next page
+        return len(self.object_list) == self.paginator.per_page
+
+    @cached_property
+    def end_index(self):
+        return (self.paginator.per_page * (self.number - 1)) + len(self.object_list)
+
+
+class UncountablePaginator(EagerPaginator):
+    """optimization: counting is too expensive; to be used in a template w/o .count and .last"""
+
+    def __init__(self, *args, **kwargs):
+        super().__init__(*args, **kwargs)
+
+    def _get_page(self, *args, **kwargs):
+        object_list = args[0]
+        object_list = list(object_list)
+        return UncountablePage(object_list, *(args[1:]), **kwargs)
+
+    @property
+    def count(self):
+        return 1_000_000_000  # big enough to be bigger than what you can click through or store in the DB.
+
+
 def _request_repr(parsed_data):
     if "request" not in parsed_data:
         return ""
@@ -268,7 +298,7 @@ def _issue_list_pt_2(request, project, state_filter, unapplied_issue_ids):
     if request.GET.get("q"):
         issue_list = search_issues(project, issue_list, request.GET["q"])
 
-    paginator = EagerPaginator(issue_list, 250)
+    paginator = UncountablePaginator(issue_list, 250)
     page_number = request.GET.get("page")
     page_obj = paginator.get_page(page_number)