perf(checkpoint-sqlite): pin the ancestor walk's join order

Without ANALYZE stats sqlite chose checkpoints as the outer loop of the
recursive step and scanned the whole thread per ancestor, so the walk was
quadratic (4000 steps: 0.48s). CROSS JOIN keeps ancestors outside and each
step becomes one primary key lookup (0.006s).

libs/checkpoint-sqlite/langgraph/checkpoint/sqlite/_delta.py

@@ -36,6 +36,10 @@ from langgraph.checkpoint.base import DeltaChannelHistory, PendingWrite
 # descendant's config makes the chain a loop. `step_walk_with_row` stops on a
 # repeated id; sqlite yields recursive rows lazily, so abandoning the cursor
 # ends the recursion.
+#
+# `CROSS JOIN` pins `ancestors` as the outer loop, so each step is one primary
+# key lookup. With a plain `JOIN` and no `ANALYZE` stats, sqlite can put
+# `checkpoints` outside and scan the whole thread per step.
 DELTA_STAGE1_SQL = (
     "WITH RECURSIVE ancestors(checkpoint_id, parent_checkpoint_id, type, "
     "checkpoint) AS ("
@@ -44,7 +48,7 @@ DELTA_STAGE1_SQL = (
     "WHERE thread_id = ? AND checkpoint_ns = ? AND checkpoint_id = ? "
     "UNION ALL "
     "SELECT c.checkpoint_id, c.parent_checkpoint_id, c.type, c.checkpoint "
-    "FROM checkpoints c JOIN ancestors a "
+    "FROM ancestors a CROSS JOIN checkpoints c "
     "ON c.checkpoint_id = a.parent_checkpoint_id "
     "WHERE c.thread_id = ? AND c.checkpoint_ns = ?"
     ") "

libs/checkpoint-sqlite/tests/test_delta_parent_walk.py

@@ -11,6 +11,7 @@ from langgraph.checkpoint.base import (
 )
 
 from langgraph.checkpoint.sqlite import SqliteSaver
+from langgraph.checkpoint.sqlite._delta import DELTA_STAGE1_SQL
 from langgraph.checkpoint.sqlite.aio import AsyncSqliteSaver
 
 CHANNEL = "ch"
@@ -87,3 +88,17 @@ def test_walk_terminates_when_put_makes_the_parent_chain_cycle() -> None:
 
         got = saver.get_delta_channel_history(config=b, channels=[CHANNEL])
         assert got[CHANNEL] == {"writes": []}
+
+
+def test_walk_step_looks_up_the_parent_by_primary_key() -> None:
+    with SqliteSaver.from_conn_string(":memory:") as saver:
+        saver.setup()
+        plan = [
+            row[3]
+            for row in saver.conn.execute(
+                f"EXPLAIN QUERY PLAN {DELTA_STAGE1_SQL}", ("t", "", "id", "t", "")
+            )
+        ]
+    assert any(
+        step.startswith("SEARCH c ") and "checkpoint_id=?" in step for step in plan
+    ), f"recursive step should look up the parent by key, got {plan}"