The bug is caused by the following reasons:

* spider_group_by_handler::init_scan() generates a query for a data node.
* The function adds DISTINCT if and only if
  spider_group_by_handler::query::distinct is TRUE.
* spider_group_by_handler::query::distinct is set to the value of
  JOIN::select_distinct in JOIN::make_aggr_tables_info().
* In the test case, DISTINCT is not added because JOIN::select_distinct
  is FALSE at the call of JOIN::make_aggr_tables_info().

Why JOIN::select_distinct is set to FALSE? That is because the function
JOIN::optimize_stage2() convert DISTINCT into GROUP BY and then optimizes
away GROUP BY.