[NOTICKET][doc] fix aggregate count operation when value_col is not specified

src/query/executors/tabular.py

@@ -178,9 +178,15 @@ def _apply_operation(df: pd.DataFrame, op: TabularOperation, limit: int) -> pd.D
             raise ValueError(f"sort requires sort_col, got {op}")
         return df.sort_values(op.sort_col, ascending=op.ascending).head(limit)
     elif op.operation == "aggregate":
-        if not op.value_col or not op.agg_func:
-            raise ValueError(f"aggregate requires value_col and agg_func, got {op}")
-        funcs = {"sum": "sum", "avg": "mean", "min": "min", "max": "max", "count": "count"}
+        if not op.agg_func:
+            raise ValueError(f"aggregate requires agg_func, got {op}")
+        if op.agg_func == "count":
+            if not op.value_col:
+                return pd.DataFrame([{"column_name": c, "dtype": str(df[c].dtype)} for c in df.columns])
+            return pd.DataFrame([{"count": int(df[op.value_col].count()), "operation": "count"}])
+        if not op.value_col:
+            raise ValueError(f"aggregate requires value_col for {op.agg_func}, got {op}")
+        funcs = {"sum": "sum", "avg": "mean", "min": "min", "max": "max"}
         value = getattr(df[op.value_col], funcs[op.agg_func])()
         return pd.DataFrame([{op.value_col: value, "operation": op.agg_func}])
     else:  # "raw"