when to update pg_statistic relation? is it when inserting a new tuple of any user's relations? and the relation pg_stats at document 8.4 chapter 44.55.pg_stats hasn't been used anymore ? another question is: I noticed when i send the sql :"select catcode from pois goup by catcode", the function :double estimate_num_groups(PlannerInfo *root, List *groupExprs, double input_rows) uses pg_statistic.staattnum to set Agg.numGroups, then if the pg_statistic.staattnum is incorrect ,does postgresql still use this infomation to set Agg.numGroups ?