Bug #13111584 PERFORMANCE REGRESSION DUE TO SUBQUERY MATERIALIZATION:
when there is choice between IN(subquery)->EXISTS and
subquery materialization, 5.6.5 always chose the latter;
this patch introduces a cost-based choice, controlled by a new switch:
optimizer_switch='subquery_materialization_cost_based=on/off';
ON (the default): do cost-based choice;
OFF: do subquery materialization (as in 5.6.5).
See commit comment of item_subselect.cc for details.