The current approach of handling alter table commands does not allow
getting the result of the command; for instance, the object address of
a created constraint on a hypertable. Thus there is no way to get the
auto-generated name of a constraint, which is needed when the
corresponding constraints are created on the hypertable's chunks.
Therefore, this change blocks the ability to create constraints
without an explicit name. When a better approach to handling alter
table is deviced, it is possible to remove this restriction.
Adding a constraint using an existing index changes the existing
index's name as a side effect (PostgreSQL wants the index name to
match the constraint name). This creates a mismatch between the
index's name and our metadata in the chunk_index table. Further, since
the name change happens as a result of an internal command (and not
via DDL command that we can capture) there is currently no way to sync
up this information.
This PR add support for primary-key, foreign-key, unique, and exclusion constraints.
Previously supported are CHECK and NOT NULL constraints. Now, foreign key
constraints where a hypertable references a plain table is support
(while vice versa, with a plain table references a hypertable, is still not).
