fix(mysql): handle string typed decimal results (#24241)

tests/unit_tests/db_engine_specs/test_mysql.py

@@ -16,6 +16,7 @@
 # under the License.
 
 from datetime import datetime
+from decimal import Decimal
 from typing import Any, Optional
 from unittest.mock import Mock, patch
 
@@ -220,3 +221,42 @@ def test_get_schema_from_engine_params() -> None:
         )
         == "db1"
     )
+
+
+@pytest.mark.parametrize(
+    "data,description,expected_result",
+    [
+        (
+            [("1.23456", "abc")],
+            [("dec", "decimal(12,6)"), ("str", "varchar(3)")],
+            [(Decimal("1.23456"), "abc")],
+        ),
+        (
+            [(Decimal("1.23456"), "abc")],
+            [("dec", "decimal(12,6)"), ("str", "varchar(3)")],
+            [(Decimal("1.23456"), "abc")],
+        ),
+        (
+            [(None, "abc")],
+            [("dec", "decimal(12,6)"), ("str", "varchar(3)")],
+            [(None, "abc")],
+        ),
+        (
+            [("1.23456", "abc")],
+            [("dec", "varchar(255)"), ("str", "varchar(3)")],
+            [("1.23456", "abc")],
+        ),
+    ],
+)
+def test_column_type_mutator(
+    data: list[tuple[Any, ...]],
+    description: list[Any],
+    expected_result: list[tuple[Any, ...]],
+):
+    from superset.db_engine_specs.mysql import MySQLEngineSpec as spec
+
+    mock_cursor = Mock()
+    mock_cursor.fetchall.return_value = data
+    mock_cursor.description = description
+
+    assert spec.fetch_data(mock_cursor) == expected_result