Adjust join_search_one_level's handling of clauseless joins.
Adjust join_search_one_level's handling of clauseless joins.

For an initial relation that lacks any join clauses (that is, it has to be
cartesian-product-joined to the rest of the query), we considered only
cartesian joins with initial rels appearing later in the initial-relations
list.  This creates an undesirable dependency on FROM-list order.  We would
never fail to find a plan, but perhaps we might not find the best available
plan.  Noted while discussing the logic with Amit Kapila.

Improve the comments a bit in this area, too.

Arguably this is a bug fix, but given the lack of complaints from the
field I'll refrain from back-patching.

kmjungersen/PostgresXL,ashwinstar/gpdb,arcivanov/postgres-xl,Postgres-XL/Postgres-XL,jmcatamney/gpdb,arcivanov/postgres-xl,greenplum-db/gpdb,adam8157/gpdb,ashwinstar/gpdb,50wu/gpdb,ashwinstar/gpdb,adam8157/gpdb,snaga/postgres-xl,adam8157/gpdb,lisakowen/gpdb,oberstet/postgres-xl,ashwinstar/gpdb,yazun/postgres-xl,postmind-net/postgres-xl,xinzweb/gpdb,ashwinstar/gpdb,pavanvd/postgres-xl,xinzweb/gpdb,postmind-net/postgres-xl,pavanvd/postgres-xl,50wu/gpdb,ovr/postgres-xl,postmind-net/postgres-xl,adam8157/gpdb,arcivanov/postgres-xl,pavanvd/postgres-xl,50wu/gpdb,zeroae/postgres-xl,lisakowen/gpdb,kmjungersen/PostgresXL,adam8157/gpdb,pavanvd/postgres-xl,xinzweb/gpdb,ovr/postgres-xl,jmcatamney/gpdb,ovr/postgres-xl,50wu/gpdb,Postgres-XL/Postgres-XL,techdragon/Postgres-XL,xinzweb/gpdb,jmcatamney/gpdb,techdragon/Postgres-XL,lisakowen/gpdb,jmcatamney/gpdb,snaga/postgres-xl,xinzweb/gpdb,arcivanov/postgres-xl,lisakowen/gpdb,postmind-net/postgres-xl,50wu/gpdb,oberstet/postgres-xl,zeroae/postgres-xl,tpostgres-projects/tPostgres,ashwinstar/gpdb,techdragon/Postgres-XL,ovr/postgres-xl,ashwinstar/gpdb,50wu/gpdb,greenplum-db/gpdb,tpostgres-projects/tPostgres,adam8157/gpdb,oberstet/postgres-xl,xinzweb/gpdb,xinzweb/gpdb,lisakowen/gpdb,jmcatamney/gpdb,ovr/postgres-xl,snaga/postgres-xl,postmind-net/postgres-xl,yazun/postgres-xl,zeroae/postgres-xl,Postgres-XL/Postgres-XL,tpostgres-projects/tPostgres,techdragon/Postgres-XL,zeroae/postgres-xl,adam8157/gpdb,jmcatamney/gpdb,jmcatamney/gpdb,kmjungersen/PostgresXL,greenplum-db/gpdb,tpostgres-projects/tPostgres,Postgres-XL/Postgres-XL,arcivanov/postgres-xl,yazun/postgres-xl,oberstet/postgres-xl,pavanvd/postgres-xl,kmjungersen/PostgresXL,oberstet/postgres-xl,Postgres-XL/Postgres-XL,greenplum-db/gpdb,greenplum-db/gpdb,lisakowen/gpdb,greenplum-db/gpdb,ashwinstar/gpdb,tpostgres-projects/tPostgres,arcivanov/postgres-xl,50wu/gpdb,snaga/postgres-xl,xinzweb/gpdb,snaga/postgres-xl,zeroae/postgres-xl,kmjungersen/PostgresXL,yazun/postgres-xl,yazun/postgres-xl,jmcatamney/gpdb,50wu/gpdb,greenplum-db/gpdb,lisakowen/gpdb,adam8157/gpdb,greenplum-db/gpdb,techdragon/Postgres-XL,lisakowen/gpdb