Fix list_persons N+1 (the ~4s person-page load)

Opening any person page on a large tree took 4-5s on an idle server. Root cause:
list_persons looped over every person calling privacy.person_visibility (which
issues TWO get_membership_role queries per call) AND _attach_primary_name (one
name query per person). On the reporter's 2,324-person tree that's ~7,000
serialized DB round-trips per page load — the person page fetches the full
person list to build its name-lookup map.

Fix:
- Resolve the viewer's membership role ONCE. Members see the whole tree (full),
  so skip the per-person privacy engine entirely.
- Add _attach_primary_names: one batched names query (person_id IN (...),
  ordered the same as the single-person query so it picks the same name) instead
  of one per person.
- Apply the same batching to the non-member path, search_persons, the deleted-
  persons list, and public_view_service.list_public_persons.

Member-path list_persons goes from ~3·N queries to ~3 total. Other tree-wide
list endpoints (events/relationships/media/citations) were already flat selects.

Adds a regression test that asserts list_persons issues a constant number of
queries (not proportional to person count). Suite: 103 passing.

Signed-off-by: Justin Paul <justin@jpaul.me>

backend/app/services/person_service.py

@@ -45,6 +45,29 @@ async def _attach_primary_name(session: AsyncSession, person: Person) -> None:
     person.primary_name = _format_name(name) if name is not None else None
 
 
+async def _attach_primary_names(session: AsyncSession, persons: list[Person]) -> None:
+    """Batch version of ``_attach_primary_name`` — ONE query for the whole list
+    instead of one per person (the difference between 1 and N queries when
+    rendering a 2k-person tree). The global order (is_primary desc, sort_order)
+    matches the single-person query, so the first row seen per person is the same
+    name ``_attach_primary_name`` would pick."""
+    if not persons:
+        return
+    rows = (
+        await session.execute(
+            select(Name)
+            .where(Name.person_id.in_([p.id for p in persons]), Name.deleted_at.is_(None))
+            .order_by(Name.is_primary.desc(), Name.sort_order)
+        )
+    ).scalars().all()
+    best: dict[uuid.UUID, Name] = {}
+    for n in rows:
+        best.setdefault(n.person_id, n)
+    for p in persons:
+        n = best.get(p.id)
+        p.primary_name = _format_name(n) if n is not None else None
+
+
 async def create_person(
     session: AsyncSession,
     *,
@@ -336,15 +359,18 @@ async def list_deleted_persons(
         .order_by(Person.deleted_at.desc())
     )
     persons = list((await session.execute(stmt)).scalars().all())
-    for person in persons:
-        await _attach_primary_name(session, person)
+    await _attach_primary_names(session, persons)
     return persons
 
 
 async def list_persons(
     session: AsyncSession, *, viewer_id: uuid.UUID, tree: Tree
 ) -> list[Person]:
-    if not await privacy.can_view_tree(session, user_id=viewer_id, tree=tree):
+    # Resolve the viewer's role ONCE. Members see the whole tree (full), so we
+    # skip the per-person privacy engine entirely and batch the name fetch — the
+    # difference between ~3 queries and ~3·N queries on a 2k-person tree.
+    role = await privacy.get_membership_role(session, viewer_id, tree.id)
+    if role is None and not await privacy.can_view_tree(session, user_id=viewer_id, tree=tree):
         raise Forbidden("not permitted to view this tree")
 
     stmt = (
@@ -354,7 +380,15 @@ async def list_persons(
     )
     persons = list((await session.execute(stmt)).scalars().all())
 
+    if role is not None:
+        await _attach_primary_names(session, persons)
+        return persons
+
+    # Non-member on a viewable (public/unlisted/site_members) tree: redact per
+    # person. Names are batched for the non-redacted ones; redacted ones already
+    # have their display name overwritten by _redact.
     visible: list[Person] = []
+    full: list[Person] = []
     for person in persons:
         vis = await privacy.person_visibility(
             session, user_id=viewer_id, tree=tree, person=person
@@ -364,8 +398,9 @@ async def list_persons(
         if vis == Visibility.redacted:
             _redact(person)
         else:
-            await _attach_primary_name(session, person)
+            full.append(person)
         visible.append(person)
+    await _attach_primary_names(session, full)
     return visible
 
 
@@ -406,7 +441,11 @@ async def search_persons(
         .order_by(sub.c.score.desc())
     )
     persons = list((await session.execute(stmt)).scalars().all())
+    if await privacy.get_membership_role(session, viewer_id, tree.id) is not None:
+        await _attach_primary_names(session, persons)
+        return persons
     out: list[Person] = []
+    full: list[Person] = []
     for person in persons:
         vis = await privacy.person_visibility(
             session, user_id=viewer_id, tree=tree, person=person
@@ -416,6 +455,7 @@ async def search_persons(
         if vis == Visibility.redacted:
             _redact(person)
         else:
-            await _attach_primary_name(session, person)
+            full.append(person)
         out.append(person)
+    await _attach_primary_names(session, full)
     return out

backend/app/services/public_view_service.py

@@ -33,7 +33,11 @@ from app.models.source import Citation, Source
 from app.models.tree import Tree
 from app.services import privacy
 from app.services.exceptions import NotFound
-from app.services.person_service import _attach_primary_name, _redact
+from app.services.person_service import (
+    _attach_primary_name,
+    _attach_primary_names,
+    _redact,
+)
 from app.services.privacy import Visibility
 
 
@@ -78,6 +82,7 @@ async def list_public_persons(
     session: AsyncSession, *, viewer_id: uuid.UUID | None, tree: Tree
 ) -> list[Person]:
     out: list[Person] = []
+    full: list[Person] = []
     for p in await _persons(session, tree):
         vis = await privacy.person_visibility(session, user_id=viewer_id, tree=tree, person=p)
         if vis == Visibility.hidden:
@@ -85,8 +90,9 @@ async def list_public_persons(
         if vis == Visibility.redacted:
             _redact(p)
         else:
-            await _attach_primary_name(session, p)
+            full.append(p)
         out.append(p)
+    await _attach_primary_names(session, full)  # one query, not one per person
     return out
 
 

backend/tests/test_person_list_performance.py

@@ -0,0 +1,39 @@
+"""Regression guard: list_persons must batch — a constant number of queries,
+not one (or three) per person. A 2k-person tree took ~4s before this was fixed."""
+
+import sqlalchemy as sa
+
+from tests.conftest import auth, register
+
+
+async def test_list_persons_does_not_n_plus_one(client, engine):
+    owner = auth(await register(client, "perf-owner@ex.com"))
+    tid = (await client.post("/api/v1/trees", json={"name": "Perf"}, headers=owner)).json()["id"]
+    n = 25
+    for i in range(n):
+        await client.post(
+            f"/api/v1/trees/{tid}/persons",
+            json={"given": f"P{i}", "surname": "X"},
+            headers=owner,
+        )
+
+    selects = 0
+
+    def _count(conn, cursor, statement, params, context, executemany):
+        nonlocal selects
+        if statement.lstrip().upper().startswith("SELECT"):
+            selects += 1
+
+    sa.event.listen(engine.sync_engine, "before_cursor_execute", _count)
+    try:
+        resp = await client.get(f"/api/v1/trees/{tid}/persons", headers=owner)
+    finally:
+        sa.event.remove(engine.sync_engine, "before_cursor_execute", _count)
+
+    assert resp.status_code == 200
+    body = resp.json()
+    assert len(body) == n
+    assert all(p["primary_name"] for p in body)  # names still resolve correctly
+    # Batched: a small constant (auth, role, persons, one names query, …) — NOT
+    # proportional to n. The old per-person path was ~3·n SELECTs.
+    assert 0 < selects < n, f"expected a constant query count, got {selects} for {n} people"