Item_cond::fix_fields() and Item_func_between::fix_fields() update these optimizer related SELECT_LEX members:
The purpose of these members is to allocate optimizer related buffers in:
- update_ref_and_keys() in sql_select.cc and
- check_func_dependency() in opt_table_elimination,cc
and it seems they have no any other purposes..
This looks like a dead code and the collected values are never used:
- cond_count and_between_count are later always initialized to 0 before the walk(count_sargable_conds) call in update_ref_and_keys().
- moreover, using the values collected during fix_fields() would not be correct to allocate optimized related buffers, because the code in fix_fields() does not distinguish between different query parts, so the items can be just a part of the SELECT list rather than a part of WHERE/HAVING/ON.