preprocess target list for non-SELECT queries

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

ORDER BY, DISTINCT, LIMIT

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

find selectivity of columns used in joins

make_rel_from_joinlist()

hand off join subproblems to a plugin, GEQO, or standard_join_search()

call join_search_one_level() for each level of join tree needed

join_search_one_level():

For each joinrel of the prior level, do make_rels_by_clause_joins()