handle UNION/INTERSECT/EXCEPT, GROUP BY, HAVING, aggregates,

ORDER BY, DISTINCT, LIMIT

--query_planner()

pull out constant quals, which can be used to gate execution of the

whole plan (if any are found, we make a top-level Result node

to do the gating)

make list of base relations used in query

split up the qual into restrictions (a=1) and joins (b=c)

find qual clauses that enable merge and hash joins

Back at standard_join_search(), apply set_cheapest() to extract the

cheapest path for each newly constructed joinrel.

Loop back if this wasn't the top join level.

Back at query_planner:

put back any constant quals by adding a Result node

Back at grouping_planner:

convert Path tree returned by query_planner into a Plan tree

do grouping(GROUP)

do aggregates

do window functions

make unique(DISTINCT)

make sort(ORDER BY)

make limit(LIMIT/OFFSET)