Add safe_make_column_not_nullable

[alexspeller]

Addresses #106

I also added some quoting for some related methods where it seemed to be missing

[jcoleman]

I'm wondering if we want to also (whether it's in this PR or not) add a method that converts a validated check constraint to a regular NOT NULL constraint (the benefit basically being that it would assert that the constraint exists and is already validated).

[jcoleman]

Are constraint names global? If so, do we want to add a random tag on the end of this?

[rkrage]

Yeah, I think constraint names need to be unique within a schema. Because this method is doing multiple things non-transactionally, there is a possibility we could error out before the constraint is removed. So a) I agree with @jcoleman that we should add some random characters to this string and b) we should document that this might leave behind a constraint on failure, similar to creating a partitioned index.

[alexspeller]

The documentation says that constraints are only unique per-table:

https://www.postgresql.org/docs/current/sql-set-constraints.html#:~:text=Because%20PostgreSQL%20does%20not%20require,will%20act%20on%20all%20matches.

I don't have a strong opinion on the naming of the constraint or if it should still have randomness regardless

[jcoleman]

I think adding a random tag would make re-running a failed migration here safer, albeit at the cost of you not discovering what's been left behind by the previous run.

I think I'd probably say this: let's add a tag using a the first N chars of a sha of the column name (so it's stable), and at the beginning of the method raise a specific error if that constraint already exists.

[jcoleman]

@alexspeller Apologies, for some reason I'm not getting notifications on this repo anymore; I'll take a look at this again.

[jcoleman]

@alexspeller I think @rkrage are going to add wrapping this PR up to our internal development list, and after we wrap it up, we'll merge.