BUG/PERF: Series.combine_first converting int64 to float64

doc/source/whatsnew/v2.1.0.rst

@@ -108,6 +108,7 @@ Performance improvements
 - Performance improvement in :meth:`DataFrame.where` when ``cond`` is backed by an extension dtype (:issue:`51574`)
 - Performance improvement in :meth:`read_orc` when reading a remote URI file path. (:issue:`51609`)
 - Performance improvement in :meth:`~arrays.ArrowExtensionArray.isna` when array has zero nulls or is all nulls (:issue:`51630`)
+- Performance improvement in :meth:`Series.combine_first` (:issue:`51777`)
 
 .. ---------------------------------------------------------------------------
 .. _whatsnew_210.bug_fixes:
@@ -193,7 +194,7 @@ Groupby/resample/rolling
 
 Reshaping
 ^^^^^^^^^
--
+- Bug in :meth:`Series.combine_first` converting ``int64`` dtype to ``float64`` and losing precision on very large integers (:issue:`51764`)
 -
 
 Sparse

pandas/core/series.py

@@ -3266,13 +3266,23 @@ def combine_first(self, other) -> Series:
         falcon      NaN
         dtype: float64
         """
+        from pandas.core.reshape.concat import concat
+
         new_index = self.index.union(other.index)
-        this = self.reindex(new_index, copy=False)
-        other = other.reindex(new_index, copy=False)
+
+        this = self
+        null_mask = isna(this)
+        if null_mask.any():
+            drop = this.index[null_mask].intersection(other.index[notna(other)])
+            if len(drop):
+                this = this.drop(drop)
+
+        other = other.reindex(other.index.difference(this.index), copy=False)
         if this.dtype.kind == "M" and other.dtype.kind != "M":
             other = to_datetime(other)
-
-        return this.where(notna(this), other)
+        combined = concat([this, other]).reindex(new_index, copy=False)
+        combined.name = self.name
+        return combined
 
     def update(self, other: Series | Sequence | Mapping) -> None:
         """

pandas/tests/series/methods/test_combine_first.py

@@ -112,3 +112,11 @@ def test_combine_first_timezone_series_with_empty_series(self):
         s2 = Series(index=time_index)
         result = s1.combine_first(s2)
         tm.assert_series_equal(result, s1)
+
+    def test_combine_first_preserves_dtype(self):
+        # GH51764
+        s1 = Series([np.iinfo(np.int64).min, np.iinfo(np.int64).max])
+        s2 = Series([1, 2, 3])
+        result = s1.combine_first(s2)
+        expected = Series([np.iinfo(np.int64).min, np.iinfo(np.int64).max, 3])
+        tm.assert_series_equal(result, expected)