Re: Tid scan improvements

So we'd extend that to:
  - Include in the OR-list "range" subquals of the form (ctid > ? AND
ctid < ?) (either side could be optional, and we have to deal with >=
and <= and having ctid on the rhs, etc.).
  - Cost the range subquals by assuming they don't overlap, and
estimating how many blocks and tuples they span.
  - When beginning the scan, evaluate all the ?s and build an array of
"tid ranges" to fetch.  A tid range is a struct with a starting tid,
and an ending tid, and might just be a single tid item.
  - Sort and remove duplicates.
  - Iterate over the array, using a single fetch for single-item tid
ranges, and starting/ending a heap scan for multi-item tid ranges.

I think I'll try implementing this.