Show stats for aggregation queries in SHOW STATS

[kokosing]

Currently (after #3109, see TestShowStats):

        assertQuery("SHOW STATS FOR (SELECT avg(totalprice) FROM orders GROUP BY orderkey)",
                "SELECT * FROM (VALUES " +
                        "   ('_col0', null, null, null, null, null, null), " +
                        "   (null, null, null, null, null, null, null))");
 assertQuery(
                "SHOW STATS FOR (SELECT DISTINCT * FROM orders)",
                "VALUES " +
                        "   ('orderkey', null, null, null, null, null, null), " +
                        "   ('custkey', null, null, null, null, null, null), " +
                        "   ('orderstatus', null, null, null, null, null, null), " +
                        "   ('totalprice', null, null, null, null, null, null), " +
                        "   ('orderdate', null, null, null, null, null, null), " +
                        "   ('orderpriority', null, null, null, null, null, null), " +
                        "   ('clerk', null, null, null, null, null, null), " +
                        "   ('shippriority', null, null, null, null, null, null), " +
                        "   ('comment', null, null, null, null, null, null), " +
                        "   (null, null, null, null, null, null, null)");
        assertQuery(
                "SHOW STATS FOR (SELECT DISTINCT regionkey FROM region)",
                "VALUES " +
                        "   ('regionkey', null, null, null, null, null, null), " +
                        "   (null, null, null, null, null, null, null)");
    }

However, we should be able to the row count as NDV of columns are known.

[findepi]

Calculate row count statistic for aggregations

The title can be misunderstood a bit.

We do calculate the row count for aggregations and this is important for CBO. The reason why it didn't work in #3109 is -- i am guessing -- because of partial aggregations.
A potential solution could be disabling partial aggregations when planning for SHOW STATS.

[findepi]

#6998 could help with this.