Allow INITIALLY IMMEDIATE constraints in CREATE TABLE statements

[andrew-farries]

Convert INITIALLY IMMEDIATE constraints appearing in CREATE TABLE column constraints to OpCreateTable operations rather than raw SQL.

The documentation is a little misleading, as it makes it sound like INITIALLY IMMEDIATE implies that the constraint is DEFERRABLE, but in practice it looks like INITIALLY IMMEDIATE without DEFERRABLE is a no-op:

CREATE TABLE foo(a int PRIMARY KEY GENERATED ALWAYS AS IDENTITY)

-- Specify an INITIALLY IMMEDIATE constraint without DEFERRABLE
ALTER TABLE foo ADD COLUMN bar int UNIQUE INITIALLY IMMEDIATE

-- Query the catalog to see the constraint
SELECT conname AS constraint_name,
        condeferrable AS is_deferrable,
        condeferred AS is_deferred
FROM pg_constraint
WHERE conrelid = 'foo'::regclass;

Result:

+-----------------+---------------+-------------+
| constraint_name | is_deferrable | is_deferred |
|-----------------+---------------+-------------|
| foo_pkey        | False         | False       |
| foo_bar_key     | False         | False       |
+-----------------+---------------+-------------+

ie, the UNIQUE constraint added to bar is not DEFERRABLE or DEFERRED.

In contrast, if the column bar is defined:

ALTER TABLE foo ADD COLUMN bar int UNIQUE DEFERRABLE INITIALLY IMMEDIATE

Then the same catalog query shows:

+-----------------+---------------+-------------+
| constraint_name | is_deferrable | is_deferred |
|-----------------+---------------+-------------|
| foo_pkey        | False         | False       |
| foo_bar_key     | True          | False       |
+-----------------+---------------+-------------+

ie, the constraint is DEFERRABLE.

So specifying INITIALLY IMMEDIATE without DEFERRABLE is a no-op.