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MDEV-26585 Wrong query results when
using index for group-by
The problem was that "group_min_max optimization" does not work if some aggregate functions, like COUNT(*), is used. The function get_best_group_min_max() is using the join->sum_funcs array to check which aggregate functions are used. The bug was that aggregates in HAVING where not yet added to join->sum_funcs at the time get_best_group_min_max() was called. Fixed by populate join->sum_funcs already in prepare, which means that all sum functions will be in join->sum_funcs in get_best_group_min_max(). A benefit of this approach is that we can remove several calls to make_sum_func_list() from the code and simplify the function. I removed some wrong setting of 'sort_and_group'. This variable is set when alloc_group_fields() is called, as part of allocating the cache needed by end_send_group() and does not need to be set by other functions. One problematic thing was that Spider is using *join->sum_funcs to detect at which stage the optimizer is and do internal calculations of aggregate functions. Updating join->sum_funcs early caused Spider to fail when trying to find min/max values in opt_sum_query(). Fixed by temporarily resetting sum_funcs during opt_sum_query(). Reviewer: Sergei Petrunia
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