starting on a fresh system, zabbix housekeeper still does execute a large amount of delete queries. excerpt from debug log :
5374:20130107:151212.512 In delete_history() table:'trends' itemid:30668 keep_history:365 now:1357564317
5374:20130107:151212.512 query [txnlev:0] [select min(clock) from trends where itemid=30668]
5374:20130107:151212.512 query without transaction detected
5374:20130107:151212.512 query [txnlev:0] [delete from trends where itemid=30668 and clock<1326028317]
as can be seen, first a select is made, and even though we do not have any data that should be deleted, a delete query is executed.
also, for every item warning "query without transaction detected" is printed, which can't be good