Never distribute a CTAS table by a resjunk column

[Alena0704]

Never distribute a CTAS table by a resjunk column

Some queries need auxiliary resjunk columns in the targetlist, an ORDER BY expression that is not in the select list for instance. They are needed to execute the query but are not columns of the resulting relation.

When CREATE TABLE AS has no explicit DISTRIBUTED BY clause and the distribution key cannot be deduced from the flow, the policy falls back to hashing on the first hashable column of the plan targetlist. That scan did not skip resjunk entries, so if every visible column was of a type with no hash operator class while a resjunk one was hashable, the resjunk column became the distribution key. Its resno then pointed past the last attribute of the new table, and reading the table crashed the backend:

create table resjunk_test as
with cte as (select point(1, 2) as a, random() as b)
select a from cte order by b;
select * from resjunk_test; -- segmentation fault

Skip resjunk entries, as get_partitioned_policy_from_flow() already does, so that such a table ends up randomly distributed instead.

Adapted from GreengageDB/greengage#328.