Commits on Feb 6, 2019

1. opt: fix zero-column groups in joins colliding …

   It's fundamental to the optimizer that any two unrelated (i.e., one is
   not the parent of the other) expressions in the tree have disjoint
   column sets. Usually, this prevents the same relational expression from
   occurring in the tree more than once, but in the case of a group with
   zero columns, this is not true.
   
   This allows the following group collision to occur:
   
   Let A be the 0-column values node with two rows `VALUES (), ()`,
   and B be the 0-column values node with three rows `VALUES (), (), ()`.
   
   Then consider the following query:
   ```
   (A JOIN B) UNION (B JOIN A)
   ```
   
   During build, we add A JOIN B and B JOIN A to *separate memo groups*.
   Then, during exploration, we apply the `CommuteJoin` rule to transform
   `A JOIN B` to `B JOIN A`. This attempts to get interned into the same
   group, but the interner finds that `B JOIN A` already exists in a
   different group, and panics.
   
   As a side note, a values node with no columns is not valid SQL, but it
   can be constructed via the `MergeProjectWithValues` rule:
   ```
   SELECT 1 FROM (VALUES (1))
   => MergeProjectWithValues
   SELECT 1 FROM (VALUES ())
   ```
   
   This commit fixes the problem by simply disabling these types of
   explorations on groups with zero columns. This isn't a perfect fix,
   since the subtle problem is still lurking, and people going forward need
   to understand its implications, but it will fix the problem for the time
   being.
   
   Release note (bug fix): fixed several related panics in the optimizer
   related to plan exploration.

   Justin Jaffray committed Feb 6, 2019
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