perf: optimize historical communication fallback

tests/test_historical_communication_fallback_performance.py

@@ -0,0 +1,99 @@
+from contextlib import nullcontext
+from pathlib import Path
+
+import app.communication_service as service
+
+
+class _Result:
+    def __init__(self, *, scalar_value=None, rows=()):
+        self.scalar_value = scalar_value
+        self.rows = list(rows)
+
+    def scalar(self):
+        return self.scalar_value
+
+    def mappings(self):
+        return self
+
+    def all(self):
+        return self.rows
+
+
+class _Connection:
+    def __init__(self, *, possible, rows=()):
+        self.possible = possible
+        self.rows = rows
+        self.statements = []
+
+    def execute(self, statement, params=None):
+        self.statements.append((str(statement), params))
+        if len(self.statements) == 1:
+            return _Result(scalar_value=self.possible)
+        return _Result(rows=self.rows)
+
+
+class _Engine:
+    def __init__(self, connection):
+        self.connection = connection
+
+    def begin(self):
+        return nullcontext(self.connection)
+
+
+def test_no_historical_candidates_returns_after_one_statement(monkeypatch):
+    connection = _Connection(possible=False)
+    monkeypatch.setattr(service, "engine", _Engine(connection))
+
+    assert service._list_message_backed_chatwoot_items_for_opportunity(
+        "dc3c020e-33eb-4067-b7da-ab684533a425"
+    ) == []
+    assert len(connection.statements) == 1
+    assert "SELECT EXISTS" in connection.statements[0][0]
+
+
+def test_positive_fallback_uses_two_statements_and_preserves_rows(monkeypatch):
+    expected = [{"id": "message-1", "source_system": "chatwoot", "created_at": "2026-08-14"}]
+    connection = _Connection(possible=True, rows=expected)
+    monkeypatch.setattr(service, "engine", _Engine(connection))
+
+    result = service._list_message_backed_chatwoot_items_for_opportunity(
+        "dc3c020e-33eb-4067-b7da-ab684533a425", limit=12
+    )
+
+    assert result == expected
+    assert len(connection.statements) == 2
+    assert connection.statements[1][1]["limit"] == 12
+
+
+def test_relationship_paths_are_unioned_without_or_join_conditions(monkeypatch):
+    connection = _Connection(possible=True)
+    monkeypatch.setattr(service, "engine", _Engine(connection))
+    service._list_message_backed_chatwoot_items_for_opportunity(
+        "dc3c020e-33eb-4067-b7da-ab684533a425"
+    )
+    sql = connection.statements[1][0]
+
+    assert "re.id = m.raw_event_id OR re.message_id = m.id" not in sql
+    assert "ar.message_id = m.id OR ar.raw_event_id = re.id" not in sql
+    assert "t.message_id = m.id OR t.raw_event_id = re.id" not in sql
+    assert sql.count(" UNION\n") >= 10
+    assert "SELECT DISTINCT ON (m.id)" in sql
+    assert "ORDER BY m.id, COALESCE(t.created_at, m.created_at) DESC" in sql
+    assert "ORDER BY created_at DESC" in sql
+
+
+def test_only_missing_relationship_indexes_are_added():
+    migration = Path("migrations/008_historical_communication_lookup_indexes.sql").read_text()
+    expected = {
+        "messages(raw_event_id)",
+        "raw_events(message_id)",
+        "action_runs(message_id)",
+        "action_runs(raw_event_id)",
+        "tasks(message_id)",
+        "tasks(raw_event_id)",
+        "tasks(action_run_id)",
+    }
+    assert all(columns in migration for columns in expected)
+    assert "messages(conversation_id)" not in migration
+    assert "raw_events(conversation_id)" not in migration
+    assert "tasks(opportunity_id)" not in migration