x and inserting tuples require locating the bucket
  where a given tuple ought to be located. To do this, we need the bucket
  count, highmask, and lowmask from the metapage; however, it's undesirable
  for performance reasons to have to have to lock and pin the metapage for
  every such operation. Instead, we retain a cached copy of the metapage
  in each backend's relcache entry. This will produce the correct bucket
  mapping as long as the target bucket hasn't been split since the last
  cache refresh.
 