Order by field support FTS (#1132)

This PR adds support for passing through a set of ordering fields at
index time (unsigned ints that tantivity can use as fast_fields) that at
query time you can sort your results on. This is useful for cases where
you want to get related hits, i.e by keyword, but order those hits by
some other score, such as popularity.

I.e search for songs descriptions that match on "sad AND jazz AND 1920"
and then order those by number of times played. Example usage can be
seen in the fts tests.

---------

Co-authored-by: Nat Roth <natroth@Nats-MacBook-Pro.local>
Co-authored-by: Chang She <759245+changhiskhan@users.noreply.github.com>

python/python/tests/test_fts.py

@@ -43,6 +43,7 @@ def table(tmp_path) -> ldb.table.LanceTable:
         )
         for _ in range(100)
     ]
+    count = [random.randint(1, 10000) for _ in range(100)]
     table = db.create_table(
         "test",
         data=pd.DataFrame(
@@ -52,6 +53,7 @@ def table(tmp_path) -> ldb.table.LanceTable:
                 "text": text,
                 "text2": text,
                 "nested": [{"text": t} for t in text],
+                "count": count,
             }
         ),
     )
@@ -79,6 +81,39 @@ def test_search_index(tmp_path, table):
     assert len(results[1]) == 10  # _distance
 
 
+def test_search_ordering_field_index_table(tmp_path, table):
+    table.create_fts_index("text", ordering_field_names=["count"])
+    rows = (
+        table.search("puppy", ordering_field_name="count")
+        .limit(20)
+        .select(["text", "count"])
+        .to_list()
+    )
+    for r in rows:
+        assert "puppy" in r["text"]
+    assert sorted(rows, key=lambda x: x["count"], reverse=True) == rows
+
+
+def test_search_ordering_field_index(tmp_path, table):
+    index = ldb.fts.create_index(
+        str(tmp_path / "index"), ["text"], ordering_fields=["count"]
+    )
+
+    ldb.fts.populate_index(index, table, ["text"], ordering_fields=["count"])
+    index.reload()
+    results = ldb.fts.search_index(
+        index, query="puppy", limit=10, ordering_field="count"
+    )
+    assert len(results) == 2
+    assert len(results[0]) == 10  # row_ids
+    assert len(results[1]) == 10  # _distance
+    rows = table.to_lance().take(results[0]).to_pylist()
+
+    for r in rows:
+        assert "puppy" in r["text"]
+    assert sorted(rows, key=lambda x: x["count"], reverse=True) == rows
+
+
 def test_create_index_from_table(tmp_path, table):
     table.create_fts_index("text")
     df = table.search("puppy").limit(10).select(["text"]).to_pandas()
@@ -94,6 +129,7 @@ def test_create_index_from_table(tmp_path, table):
                 "text": "gorilla",
                 "text2": "gorilla",
                 "nested": {"text": "gorilla"},
+                "count": 10,
             }
         ]
     )
@@ -166,6 +202,7 @@ def test_null_input(table):
                 "text": None,
                 "text2": None,
                 "nested": {"text": None},
+                "count": 7,
             }
         ]
     )