Promote check constraint to not null on PG >= 12

[solebared]

Addresses #34 .

References

• Mailing list discussion: Feature idea: use index when checking for NULLs before SET NOT NULL.
• Commit introducing the feature to PostgreSQL.
• SO answer from author of commit above.
• Recipe, with links to further reading

Note on author's minimal expertise :)

I'm not really qualified to evaluate any risks associated with this approach. I've read through the references above and done some minimal testing locally, but don't have much personal expertise in PostgreSQL.

As such, i'm relying on reviewers to weigh in on the approach and potential gotchas.

[solebared]

Tested locally on v13.11, and am seeing the desired log output:

LOG:  statement: ALTER TABLE "some_numbers" VALIDATE CONSTRAINT "chk_rails_0171ff2f52" /*application:Wrapbook*/
DEBUG:  verifying table "some_numbers"
LOG:  statement: SET statement_timeout TO '0' /*application:Wrapbook*/
LOG:  statement: BEGIN /*application:Wrapbook*/
LOG:  statement: ALTER TABLE "some_numbers"
        ALTER "number_for_type_change"
        SET NOT NULL
         /*application:Wrapbook*/
DEBUG:  existing constraints on column "some_numbers.number_for_type_change" are sufficient to prove that it does not contain nulls
LOG:  execute <unnamed>: SELECT conname, pg_get_constraintdef(c.oid, true) AS constraintdef, c.convalidated AS valid
                    FROM pg_constraint c
                    JOIN pg_class t ON c.conrelid = t.oid
                    JOIN pg_namespace n ON n.oid = c.connamespace
                    WHERE c.contype = 'c'
                      AND t.relname = 'some_numbers'
                      AND n.nspname = ANY (current_schemas(false))
         /*application:Wrapbook*/
LOG:  statement: ALTER TABLE "some_numbers" DROP CONSTRAINT "chk_rails_0171ff2f52" /*application:Wrapbook*/
LOG:  statement: COMMIT /*application:Wrapbook*/

Note in particular the second DEBUG entry.

[fatkodima]

Thanks for the PR! 💪
I will review it today/tomorrow, make some inline changes (if necessary) and release a new version.