Commits on Jun 1, 2020

1. opt: add rule to fold two grouping operators into one …

   Previously, the optimizer could not fold two grouping operators into
   a single equivalent grouping operator.
   
   This patch adds a rule that effects this transformation under the
   following conditions:
   1. All of the outer aggregates are aggregating on the output columns
      of the inner aggregates.
   2. Every inner-outer aggregation pair can be replaced with an
      equivalent single aggregate.
   3. The inner grouping columns functionally determine the outer
      grouping columns.
   4. Both grouping operators are unordered.
   
   As an example, the following query pairs are equivalent:
   ```
   SELECT sum(t) FROM (SELECT sum(b) FROM ab GROUP BY a) AS g(t);
   SELECT sum(b) FROM ab;
   
   SELECT max(t) FROM (SELECT max(b) FROM ab GROUP BY a) AS g(t);
   SELECT max(b) FROM ab;
   
   SELECT sum_int(t) FROM (SELECT count(b) FROM ab GROUP BY a) AS g(t);
   SELECT count(b) FROM ab;
   ```
   This situation is rare in direct SQL queries, but can arise when
   composing views and queries.
   
   Release note (sql change): The optimizer can now fold two grouping
   operators together when they are aggregating over functions like sum.

   

   DrewKimball committed Jun 1, 2020
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