Allow planner to use Merge Append to efficiently implement UNION

Allow planner to use Merge Append to efficiently implement UNION

Until now, UNION queries have often been suboptimal as the planner has
only ever considered using an Append node and making the results unique
by either using a Hash Aggregate, or by Sorting the entire Append result
and running it through the Unique operator.  Both of these methods
always require reading all rows from the union subqueries.

Here we adjust the union planner so that it can request that each subquery
produce results in target list order so that these can be Merge Appended
together and made unique with a Unique node.  This can improve performance
significantly as the union child can make use of the likes of btree
indexes and/or Merge Joins to provide the top-level UNION with presorted
input.  This is especially good if the top-level UNION contains a LIMIT
node that limits the output rows to a small subset of the unioned rows as
cheap startup plans can be used.

Author: David Rowley
Reviewed-by: Richard Guo, Andy Fan
Discussion: https://postgr.es/m/CAApHDvpb_63XQodmxKUF8vb9M7CxyUyT4sWvEgqeQU-GB7QFoQ@mail.gmail.com