@@ -29,8 +29,12 @@ Unreleased
   dropped. Now a warning is issued, thanks to `Jisang Han <pull 2234_>`_.
   Closes `issue 2200`_.
 
+- A performance improvement in the low-level line number bookkeeping when
+  combining data files, thanks to `Kevin Turcios <pull 2239_>`_.
+
 .. _issue 2200: https://github.com/coveragepy/coveragepy/issues/2200
 .. _pull 2234: https://github.com/coveragepy/coveragepy/pull/2234
+.. _pull 2239: https://github.com/coveragepy/coveragepy/pull/2239
 
 
 .. start-releases
 
@@ -140,6 +140,7 @@ Karthikeyan Singaravelan
 Kassandra Keeton
 Ken Schackart
 Kevin Brown-Silva
+Kevin Turcios
 Kjell Braden
 Krystian Kichewko
 Kumar Aditya
 
@@ -132,15 +132,19 @@ class NumbitsUnionAgg:
     """SQLite aggregate function for computing union of numbits."""
 
     def __init__(self) -> None:
-        self.result = b""
+        self.result = 0
+        self.result_nbytes = 0
 
     def step(self, value: bytes) -> None:
         """Process one value in the aggregation."""
-        self.result = numbits_union(self.result, value)
+        self.result |= int.from_bytes(value, "little")
+        self.result_nbytes = max(self.result_nbytes, len(value))
 
     def finalize(self) -> bytes:
         """Return the final aggregated result."""
-        return self.result
+        if not self.result_nbytes:
+            return b""
+        return self.result.to_bytes(self.result_nbytes, "little")
 
 
 class CoverageData:
 
@@ -20,6 +20,7 @@
     num_in_numbits,
     register_sqlite_functions,
 )
+from coverage.sqldata import NumbitsUnionAgg
 
 from tests.coveragetest import CoverageTest
 from tests.hypo import example, given, settings
@@ -112,6 +113,11 @@ def setUp(self) -> None:
         conn = sqlite3.connect(":memory:")
         register_sqlite_functions(conn)
         self.cursor = conn.cursor()
+        conn.create_aggregate(
+            "numbits_union_agg",
+            1,
+            NumbitsUnionAgg,  # type: ignore[arg-type]
+        )
         self.cursor.execute("create table data (id int, numbits blob)")
         self.cursor.executemany(
             "insert into data (id, numbits) values (?, ?)",
@@ -155,6 +161,14 @@ def test_numbits_union(self) -> None:
         answer = numbits_to_nums(list(res)[0][0])
         assert expected == answer
 
+    def test_numbits_union_aggregate(self) -> None:
+        res = self.cursor.execute(
+            "select numbits_union_agg(numbits) from data where id in (7, 9)",
+        )
+        expected = set(range(7, 100, 7)) | set(range(9, 100, 9))
+        answer = set(numbits_to_nums(list(res)[0][0]))
+        assert expected == answer
+
     def test_numbits_intersection(self) -> None:
         res = self.cursor.execute(
             "select numbits_intersection("