select: clean up slices/drilldowns execution code
select: clean up slices/drilldowns execution code

groonga/groonga,groonga/groonga,groonga/groonga,kenhys/groonga,groonga/groonga,groonga/groonga,groonga/groonga,kenhys/groonga,kenhys/groonga,kenhys/groonga,kenhys/groonga,kenhys/groonga,kenhys/groonga,groonga/groonga,kenhys/groonga,groonga/groonga