fix: columns/index rebuild (apache#16355)

superset/charts/post_processing.py

@@ -296,7 +296,9 @@ def apply_post_process(
         query["coltypes"] = extract_dataframe_dtypes(processed_df)
         query["rowcount"] = len(processed_df.index)
 
-        # flatten columns/index so we can encode data as JSON
+        # Flatten hierarchical columns/index since they are represented as
+        # `Tuple[str]`. Otherwise encoding to JSON later will fail because
+        # maps cannot have tuples as their keys in JSON.
         processed_df.columns = [
             " ".join(str(name) for name in column).strip()
             if isinstance(column, tuple)

superset/utils/csv.py

@@ -96,10 +96,14 @@ def get_chart_dataframe(
 
     result = simplejson.loads(content.decode("utf-8"))
     df = pd.DataFrame.from_dict(result["result"][0]["data"])
+
+    # rebuild hierarchical columns and index
     df.columns = pd.MultiIndex.from_tuples(
-        tuple(colname) for colname in result["result"][0]["colnames"]
+        tuple(colname) if isinstance(colname, list) else (colname,)
+        for colname in result["result"][0]["colnames"]
     )
     df.index = pd.MultiIndex.from_tuples(
-        tuple(indexname) for indexname in result["result"][0]["indexnames"]
+        tuple(indexname) if isinstance(indexname, list) else (indexname,)
+        for indexname in result["result"][0]["indexnames"]
     )
     return df