Commits on Aug 6, 2019

1. sql: allow cascading action when default is set implicitly to null …

   Add support for implicitly setting the default value of a column
   to null, for cascading tables for both ON DELETE and ON UPDATE.
   
   Previously, if a user set a reference to a column and added a
   SET DEFAULT for an ON DELETE/UPDATE cascading action, without an
   explicit default expression, CRDB would forbid it. Postgres does not
   distinguish between SET NULL and the absence of a default expression.
   Thus, the change is to also have CRDB not distinguish between the two
   cases.
   
   In the situation where a user creates a table with an explicit
   default null via SET NULL, CRDB does not allow the user to create the
   table at all. This deviates from how postgres handles it. Postgres
   allows the user to create the table but throws an error if a
   cascading action occurs that uses the default value. The new changes
   now prevent the user from creating a table when they constrain the
   columns to be non-nullable, and implicitly set the default to NULL.
   This is done to match the behavior for the explicit case of setting
   the default to NULL.
   
   Resolves: cockroachdb#38975
   
   Release note (sql change): Columns without an explicit default value now
   support foreign keys with the SET DEFAULT action, in the same way as the
   SET NULL and SET DEFAULT NULL cases.

   Tyler314 committed Aug 6, 2019
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