fix: keep nulls in nullable int32 and int64 decimal parquet columns

dataframe-parquet/src/DataFrame/IO/Parquet.hs

@@ -674,18 +674,14 @@ applyLogicalType (Just (LT_DECIMAL f)) col =
         scale = unField dt.decimal_scale
         precision = unField dt.decimal_precision
      in if precision <= 9
-            then case DI.toVector @Int32 @VU.Vector col of
-                Right xs ->
-                    DI.fromUnboxedVector $
-                        VU.map (\raw -> fromIntegral @Int32 @Double raw / 10 ^ scale) xs
-                Left _ -> col
+            then
+                fromRight col $
+                    DI.mapColumn (\raw -> fromIntegral @Int32 @Double raw / 10 ^ scale) col
             else
                 if precision <= 18
-                    then case DI.toVector @Int64 @VU.Vector col of
-                        Right xs ->
-                            DI.fromUnboxedVector $
-                                VU.map (\raw -> fromIntegral @Int64 @Double raw / 10 ^ scale) xs
-                        Left _ -> col
+                    then
+                        fromRight col $
+                            DI.mapColumn (\raw -> fromIntegral @Int64 @Double raw / 10 ^ scale) col
                     else col
 applyLogicalType _ col = col
 

tests/Parquet.hs

@@ -823,6 +823,19 @@ int64Decimal =
             )
         )
 
+nullableIntDecimal :: Test
+nullableIntDecimal = testBothReadParquetPaths $ \readParquet ->
+    TestCase
+        ( assertEqual
+            "nullableIntDecimal"
+            ( D.fromNamedColumns
+                [ ("int32_dec", D.fromList [Just (1 :: Double), Nothing, Just 2.5, Just 0.75])
+                , ("int64_dec", D.fromList [Nothing, Just (1234.5 :: Double), Nothing, Just (-5)])
+                ]
+            )
+            (unsafePerformIO (readParquet "./tests/data/nullable_int_decimal.parquet"))
+        )
+
 byteArrayDecimal :: Test
 byteArrayDecimal =
     TestCase
@@ -1394,6 +1407,7 @@ tests =
     , -- Group 7: decimal types
       int32Decimal
     , int64Decimal
+    , nullableIntDecimal
     , byteArrayDecimal
     , fixedLengthDecimal
     , fixedLengthDecimalLegacy