From 9ecda5b2666979eba81f1c0ec12ce24b49783afc Mon Sep 17 00:00:00 2001
From: Jared Lewis <jared@jared.kiwi.nz>
Date: Sun, 25 Oct 2026 10:15:29 +1100
Subject: [PATCH 1/2] perf(diagnostics): load providers and group ids with the
 listing

The listing and catalog lazy loaded each diagnostic's execution groups, one query per diagnostic.
---
 .../src/ref_backend/api/routes/diagnostics.py | 22 ++++++++++++++-----
 .../test_api/test_routes/test_diagnostics.py  | 22 ++++++++++++++++++-
 2 files changed, 37 insertions(+), 7 deletions(-)

diff --git a/backend/src/ref_backend/api/routes/diagnostics.py b/backend/src/ref_backend/api/routes/diagnostics.py
index 82d2c576..2d268524 100644
--- a/backend/src/ref_backend/api/routes/diagnostics.py
+++ b/backend/src/ref_backend/api/routes/diagnostics.py
@@ -1,6 +1,7 @@
 from typing import Literal, NamedTuple
 
 from fastapi import APIRouter, HTTPException, Query, Request
+from sqlalchemy.orm import joinedload, selectinload
 from starlette.responses import StreamingResponse
 
 from climate_ref import models
@@ -59,8 +60,15 @@ async def _get_diagnostic(
     return diagnostic
 
 
-def _listed_diagnostics(app_context: AppContextDep) -> list[models.Diagnostic]:
-    query = app_context.session.query(models.Diagnostic)
+def _listed_diagnostics(
+    app_context: AppContextDep, *, with_group_ids: bool = False
+) -> list[models.Diagnostic]:
+    query = app_context.session.query(models.Diagnostic).options(joinedload(models.Diagnostic.provider))
+    if with_group_ids:
+        # One query for every diagnostic's group ids, rather than one per diagnostic.
+        query = query.options(
+            selectinload(models.Diagnostic.execution_groups).load_only(models.ExecutionGroup.id)
+        )
     if app_context.settings.DIAGNOSTIC_PROVIDERS:
         query = query.join(models.Provider).filter(
             models.Provider.slug.in_(app_context.settings.DIAGNOSTIC_PROVIDERS)
@@ -77,10 +85,12 @@ class _Scope(NamedTuple):
     filters: DiagnosticFilter
 
 
-def _scoped_diagnostics(app_context: AppContextDep, mip_era: MipEra | None, source_id: str | None) -> _Scope:
+def _scoped_diagnostics(
+    app_context: AppContextDep, mip_era: MipEra | None, source_id: str | None, *, with_group_ids: bool = False
+) -> _Scope:
     """Get the listed diagnostics, counting only the groups that draw on `mip_era` and ran `source_id`"""
     return _Scope(
-        _listed_diagnostics(app_context),
+        _listed_diagnostics(app_context, with_group_ids=with_group_ids),
         DiagnosticFilter(
             include_superseded=False,
             include_retracted=False,
@@ -146,7 +156,7 @@ async def _list(
 
     Counts cover the promoted version of each diagnostic and leave out retracted executions.
     """
-    scope = _scoped_diagnostics(app_context, mip_era, source_id)
+    scope = _scoped_diagnostics(app_context, mip_era, source_id, with_group_ids=True)
     value_types = _value_types(app_context, scope)
     return Collection(
         data=[
@@ -172,7 +182,7 @@ async def catalog(
     Checking for metric values is most of the cost of the full listing.
     Fetch those from `/diagnostics/value-flags`.
     """
-    scope = _scoped_diagnostics(app_context, mip_era, source_id)
+    scope = _scoped_diagnostics(app_context, mip_era, source_id, with_group_ids=True)
     return Collection(data=_catalog_entries(app_context, scope))
 
 
diff --git a/backend/tests/test_api/test_routes/test_diagnostics.py b/backend/tests/test_api/test_routes/test_diagnostics.py
index 768de038..bb30d756 100644
--- a/backend/tests/test_api/test_routes/test_diagnostics.py
+++ b/backend/tests/test_api/test_routes/test_diagnostics.py
@@ -2,7 +2,7 @@
 
 import pytest
 from fastapi.testclient import TestClient
-from sqlalchemy import select
+from sqlalchemy import event, select
 
 from climate_ref import models
 from climate_ref.database import Database
@@ -992,3 +992,23 @@ def test_value_facets_of_an_unknown_diagnostic(client: TestClient) -> None:
     r = client.get("/api/v1/diagnostics/nope/nope/values/facets", params={"value_type": "scalar"})
 
     assert r.status_code == 404
+
+
+@pytest.mark.parametrize("path", ["/diagnostics/", "/diagnostics/catalog", "/diagnostics/value-flags"])
+def test_listing_does_not_query_per_diagnostic(client: TestClient, settings, path: str):
+    engine = deps._get_database_dependency(settings, test_ref_config())._engine
+    statements: list[str] = []
+
+    def record(conn, cursor, statement, *_):
+        statements.append(statement)
+
+    event.listen(engine, "before_cursor_execute", record)
+    try:
+        r = client.get(f"{settings.API_V1_STR}{path}")
+    finally:
+        event.remove(engine, "before_cursor_execute", record)
+
+    assert r.status_code == 200
+    assert len(r.json()["data"]) > 10
+    # One query per diagnostic would scale with the catalogue, so stay under a small fixed number.
+    assert len(statements) <= 10, statements

From 83a2e95260cfcb522253d3eab1caed62fb4a0107 Mon Sep 17 00:00:00 2001
From: Jared Lewis <jared@jared.kiwi.nz>
Date: Sun, 25 Oct 2026 10:15:41 +1100
Subject: [PATCH 2/2] docs: add changelog for #157

---
 changelog/157.improvement.md | 1 +
 1 file changed, 1 insertion(+)
 create mode 100644 changelog/157.improvement.md

diff --git a/changelog/157.improvement.md b/changelog/157.improvement.md
new file mode 100644
index 00000000..57fff2a4
--- /dev/null
+++ b/changelog/157.improvement.md
@@ -0,0 +1 @@
+Sped up the diagnostics listing by loading every diagnostic's provider and execution groups in a fixed number of queries.