Sergei Petrunia added a comment - 2015-04-13 18:44 Debugging on 10.0, looking at this line of code in JOIN::save_explain_data_intern: if (distinct & test_all_bits(used_tables, join->select_list_used_tables)) select_list_used_tables==0 when we come here... which is weird, JOIN::optimize_inner does call JOIN::eval_select_list_used_tables which should have put a correct value there. in eval_select_list_used_tables we have: (gdb) p item $11 = (Item *) 0x7fffa4006618 (gdb) p dbug_print_item( item) $12 = 0x1842700 "count(0)" (gdb) p item->used_tables() $13 = 0 One can debate what (count )->used_tables() should return, but this is not the point. The point is, if we run SELECT DISTINCT COUNT(*) then it is obvious that Distinct optimization is not applicable. Short-circuiting logic applies when running SELECT DISTINCT func(table_column) but not when one runs SELECT DISTINCT aggregate_func. (unless maybe some cases with Loose Scan).

Elena Stepanova added a comment - 2015-04-13 19:06 On 5.3 and 5.5, it looks like this: elenst@wheezy-64:~/git/5.5$ ml test -e "explain select distinct count(*) from t2 where a between 10 and 20 group by a;" + ------+-------------+-------+-------+---------------+------+---------+------+------+---------------------------------------------------------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | + ------+-------------+-------+-------+---------------+------+---------+------+------+---------------------------------------------------------------------+ | 1 | SIMPLE | t2 | range | a | a | 5 | NULL | 10 | Using where ; Using index ; Using temporary ; Using filesort; Distinct | + ------+-------------+-------+-------+---------------+------+---------+------+------+---------------------------------------------------------------------+

Sergei Petrunia added a comment - 2016-04-17 19:46 This is fixed in 10.2 tree by post- MDEV-8646 fixes.