BUG: sql ignores dtype when chunksize is set and result is empty (pandas-dev#50288)

Author: phofl

doc/source/whatsnew/v2.0.0.rst

@@ -874,6 +874,7 @@ I/O
 - Bug in :func:`read_sas` caused fragmentation of :class:`DataFrame` and raised :class:`.errors.PerformanceWarning` (:issue:`48595`)
 - Improved error message in :func:`read_excel` by including the offending sheet name when an exception is raised while reading a file (:issue:`48706`)
 - Bug when a pickling a subset PyArrow-backed data that would serialize the entire data instead of the subset (:issue:`42600`)
+- Bug in :func:`read_sql_query` ignoring ``dtype`` argument when ``chunksize`` is specified and result is empty (:issue:`50245`)
 - Bug in :func:`read_csv` for a single-line csv with fewer columns than ``names`` raised :class:`.errors.ParserError` with ``engine="c"`` (:issue:`47566`)
 - Bug in displaying ``string`` dtypes not showing storage option (:issue:`50099`)
 - Bug in :func:`DataFrame.to_string` with ``header=False`` that printed the index name on the same line as the first row of the data (:issue:`49230`)

pandas/io/sql.py

@@ -1617,6 +1617,7 @@ def _query_iterator(
                         index_col=index_col,
                         coerce_float=coerce_float,
                         parse_dates=parse_dates,
+                        dtype=dtype,
                         use_nullable_dtypes=use_nullable_dtypes,
                     )
                 break
@@ -2182,9 +2183,12 @@ def _query_iterator(
             if not data:
                 cursor.close()
                 if not has_read_data:
-                    yield DataFrame.from_records(
+                    result = DataFrame.from_records(
                         [], columns=columns, coerce_float=coerce_float
                     )
+                    if dtype:
+                        result = result.astype(dtype)
+                    yield result
                 break
 
             has_read_data = True

pandas/tests/io/test_sql.py

@@ -2364,6 +2364,21 @@ def nullable_expected(self, storage) -> DataFrame:
             }
         )
 
+    def test_chunksize_empty_dtypes(self):
+        # GH#50245
+        dtypes = {"a": "int64", "b": "object"}
+        df = DataFrame(columns=["a", "b"]).astype(dtypes)
+        expected = df.copy()
+        df.to_sql("test", self.conn, index=False, if_exists="replace")
+
+        for result in read_sql_query(
+            "SELECT * FROM test",
+            self.conn,
+            dtype=dtypes,
+            chunksize=1,
+        ):
+            tm.assert_frame_equal(result, expected)
+
 
 class TestSQLiteAlchemy(_TestSQLAlchemy):
     """