[postgres] Sort Ascending (#2594)

Adds a few of preliminaries:
1. Makes the returned "score" actually the result of the requested
operation (cosine, inner_product, l2)
2. Sorts asc, etc. so that if you were to add an HNSW index (and not
have any WHERE filters), it would be used
3. Drop the inner WHERE statement if no namespace or other filters are
provided. See (2) for why.
I don't yet add an index to the migrations since I think we need to
agree on the right balance to ensure it's actually used in common query
patterns.

libs/checkpoint-postgres/langgraph/store/postgres/base.py

@@ -321,7 +321,7 @@ class BasePostgresStore(Generic[C]):
             if op.query and self.index_config:
                 embedding_requests.append((idx, op.query))
 
-                score_operator = _get_distance_operator(self)
+                score_operator, post_operator = _get_distance_operator(self)
                 vector_type = (
                     cast(PostgresIndexConfig, self.index_config)
                     .get("ann_index_config", {})
@@ -351,18 +351,28 @@ class BasePostgresStore(Generic[C]):
                     if not filter_conditions
                     else " AND " + " AND ".join(filter_conditions)
                 )
+                if op.namespace_prefix:
+                    prefix_filter_str = f"WHERE s.prefix LIKE %s {filter_str} "
+                    ns_args: Sequence = (f"{_namespace_to_text(op.namespace_prefix)}%",)
+                else:
+                    ns_args = ()
+                    if filter_str:
+                        prefix_filter_str = f"WHERE {filter_str} "
+                    else:
+                        prefix_filter_str = ""
+
                 base_query = f"""
                     WITH scored AS (
-                        SELECT s.prefix, s.key, s.value, s.created_at, s.updated_at, {score_operator} AS score
+                        SELECT s.prefix, s.key, s.value, s.created_at, s.updated_at, {score_operator} AS neg_score
                         FROM store s
                         JOIN store_vectors sv ON s.prefix = sv.prefix AND s.key = sv.key
-                        WHERE s.prefix LIKE %s {filter_str}
-                        ORDER BY {score_operator} DESC 
+                        {prefix_filter_str}
+                        ORDER BY {score_operator} ASC 
                         LIMIT %s
                     )
                     SELECT * FROM (
                         SELECT DISTINCT ON (prefix, key) 
-                            prefix, key, value, created_at, updated_at, score 
+                            prefix, key, value, created_at, updated_at, {post_operator} as score 
                         FROM scored 
                         ORDER BY prefix, key, score DESC
                     ) AS unique_docs
@@ -372,7 +382,7 @@ class BasePostgresStore(Generic[C]):
                 """
                 params = [
                     _PLACEHOLDER,  # Vector placeholder
-                    f"{_namespace_to_text(op.namespace_prefix)}%",
+                    *ns_args,
                     *filter_params,
                     _PLACEHOLDER,
                     expanded_limit,
@@ -702,7 +712,6 @@ class PostgresStore(BaseStore, BasePostgresStore[_pg_internal.Conn]):
                         _paramslist[i] = embedding
 
         for (idx, _), (query, params) in zip(search_ops, queries):
-            # Execute the actual query
             cur.execute(query, params)
             rows = cast(list[Row], cur.fetchall())
             results[idx] = [
@@ -915,7 +924,7 @@ def _decode_ns_bytes(namespace: Union[str, bytes, list]) -> tuple[str, ...]:
     return tuple(namespace.split("."))
 
 
-def _get_distance_operator(store: Any) -> str:
+def _get_distance_operator(store: Any) -> tuple[str, str]:
     """Get the distance operator and score expression based on config."""
     # Note: Today, we are not using ANN indices due to restrictions
     # on PGVector's support for mixing vector and non-vector filters
@@ -936,12 +945,22 @@ def _get_distance_operator(store: Any) -> str:
     config = cast(PostgresIndexConfig, store.index_config)
     distance_type = config.get("distance_type", "cosine")
 
+    # Return the operator and the score expression
+    # The operator is used in the CTE and will be compatible with an ASCENDING ORDER
+    # sort clause.
+    # The score expression is used in the final query and will be compatible with
+    # a DESCENDING ORDER sort clause and the user's expectations of what the similarity score
+    # should be.
     if distance_type == "l2":
-        return "1 - (sv.embedding <-> %s::%s)"
+        # Final: "-(sv.embedding <-> %s::%s)"
+        # We return the "l2 similarity" so that the sorting order is the same
+        return "sv.embedding <-> %s::%s", "-scored.neg_score"
     elif distance_type == "inner_product":
-        return "-(sv.embedding <#> %s::%s)"
-    else:  # cosine
-        return "1 - (sv.embedding <=> %s::%s)"
+        # Final: "-(sv.embedding <#> %s::%s)"
+        return "sv.embedding <#> %s::%s", "-(scored.neg_score)"
+    else:  # cosine similarity
+        # Final:  "1 - (sv.embedding <=> %s::%s)"
+        return "sv.embedding <=> %s::%s", "1 - scored.neg_score"
 
 
 def _ensure_index_config(

libs/checkpoint-postgres/tests/test_store.py

@@ -634,6 +634,7 @@ def test_embed_with_path_operation_config(
     distance_type: str,
 ) -> None:
     """Test operation-level field configuration for vector search."""
+
     with _create_vector_store(
         vector_type,
         distance_type,
@@ -695,3 +696,89 @@ def test_embed_with_path_operation_config(
         # assert len(results) == 3
         # doc5_result = next(r for r in results if r.key == "doc5")
         # assert doc5_result.score is None
+
+
+def _cosine_similarity(X: list[float], Y: list[list[float]]) -> list[float]:
+    """
+    Compute cosine similarity between a vector X and a matrix Y.
+    Lazy import numpy for efficiency.
+    """
+
+    similarities = []
+    for y in Y:
+        dot_product = sum(a * b for a, b in zip(X, y))
+        norm1 = sum(a * a for a in X) ** 0.5
+        norm2 = sum(a * a for a in y) ** 0.5
+        similarity = dot_product / (norm1 * norm2) if norm1 > 0 and norm2 > 0 else 0.0
+        similarities.append(similarity)
+
+    return similarities
+
+
+def _inner_product(X: list[float], Y: list[list[float]]) -> list[float]:
+    """
+    Compute inner product between a vector X and a matrix Y.
+    Lazy import numpy for efficiency.
+    """
+
+    similarities = []
+    for y in Y:
+        similarity = sum(a * b for a, b in zip(X, y))
+        similarities.append(similarity)
+
+    return similarities
+
+
+def _neg_l2_distance(X: list[float], Y: list[list[float]]) -> list[float]:
+    """
+    Compute l2 distance between a vector X and a matrix Y.
+    Lazy import numpy for efficiency.
+    """
+
+    similarities = []
+    for y in Y:
+        similarity = sum((a - b) ** 2 for a, b in zip(X, y)) ** 0.5
+        similarities.append(-similarity)
+
+    return similarities
+
+
+@pytest.mark.parametrize(
+    "vector_type,distance_type",
+    [
+        ("vector", "cosine"),
+        ("vector", "inner_product"),
+        ("halfvec", "l2"),
+    ],
+)
+@pytest.mark.parametrize("query", ["aaa", "bbb", "ccc", "abcd", "poisson"])
+def test_scores(
+    fake_embeddings: CharacterEmbeddings,
+    vector_type: str,
+    distance_type: str,
+    query: str,
+) -> None:
+    """Test operation-level field configuration for vector search."""
+    with _create_vector_store(
+        vector_type,
+        distance_type,
+        fake_embeddings,
+        text_fields=["key0"],
+    ) as store:
+        doc = {
+            "key0": "aaa",
+        }
+        store.put(("test",), "doc", doc, index=["key0", "key1"])
+
+        results = store.search((), query=query)
+        vec0 = fake_embeddings.embed_query(doc["key0"])
+        vec1 = fake_embeddings.embed_query(query)
+        if distance_type == "cosine":
+            similarities = _cosine_similarity(vec1, [vec0])
+        elif distance_type == "inner_product":
+            similarities = _inner_product(vec1, [vec0])
+        elif distance_type == "l2":
+            similarities = _neg_l2_distance(vec1, [vec0])
+
+        assert len(results) == 1
+        assert results[0].score == pytest.approx(similarities[0], abs=1e-3)