Add a 'NO EMPTY RESULTS' filter to SELECT

It is the semantic of CQL that a (CQL) row exists as long as it has one non-null column (including the PK columns, which, given that no PK columns can be null, means that it's enough to have the PK set for a row to exist). This does means that the result to

CREATE TABLE test (k int PRIMARY KEY, v1 int, v2 int);
INSERT INTO test(k, v1) VALUES (0, 4);
SELECT v2 FROM test;

must be (and is)

 v2
------
 null

That fact does mean however that when we only select a few columns of a row, we still need to find out rows that exist but have no values for the selected columns. Long story short, given how the storage engine works, this means we need to query full (CQL) rows even when only some of the columns are selected because that's the only way to distinguish between "the row exists but have no value for the selected columns" and "the row doesn't exist". I'll note in particular that, due to CASSANDRA-5762, we can't unfortunately rely on the row marker to optimize that out.

Now, when you selects only a subsets of the columns of a row, there is many cases where you don't care about rows that exists but have no value for the columns you requested and are happy to filter those out. So, for those cases, we could provided a new SELECT filter. Outside the potential convenience (not having to filter empty results client side), one interesting part is that when this filter is provided, we could optimize a bit by only querying the columns selected, since we wouldn't need to return rows that exists but have no values for the selected columns.

For the exact syntax, there is probably a bunch of options. For instance:

• SELECT NON EMPTY(v2, v3) FROM test: the vague rational for putting it in the SELECT part is that such filter is kind of in the spirit to DISTINCT. Possibly a bit ugly outside of that.
• SELECT v2, v3 FROM test NO EMPTY RESULTS or SELECT v2, v3 FROM test NO EMPTY ROWS or SELECT v2, v3 FROM test NO EMPTY: the last one is shorter but maybe a bit less explicit. As for RESULTS versus ROWS, the only small object to NO EMPTY ROWS could be that it might suggest it is filtering non existing rows (I mean, the fact we never ever return non existing rows should hint that it's not what it does but well...) while we're just filtering empty "resultSet rows".

Of course, if there is a pre-existing SQL syntax for that, it's even better, though a very quick search didn't turn anything. Other suggestions welcome too.