Support MVs without key column.

[big-andy-coates]

The rejected alternatives section of KLIP-25 talks about supporting creating MVs without key columns, e.g.

CREATE TABLE OUTPUT AS SELECT COUNT() FROM INPUT GROUP BY X;

The above should create a table, (well, MV really), which internally tracks X as the primary key to allow correct processing of updates, but externally has no X column.

This means X should not appear in the output of SELECT * FROM OUTPUT or DESCRIBE SOURCE OUTPUT, etc.

Nor should X be available in downstream queries.

Both tables and streams MVs should support this.