Placeholder for supporting migration cases

[chuwy]

Here I'm collecting possible Schema-migrations and ways to handle it

• New property (column) added. Addition. Will be implemented in 0.6.0.

ALTER TABLE ADD COLUMN ...

• Property become nullable. Revision. See #147

ALTER TABLE ADD COLUMN customer_id_new string null;
INSERT INTO customer_id_new SELECT from customer_id;
ALTER TABLE DROP COLUMN customer_id;
ALTER TABLE RENAME COLUMN customer_id_new TO customer_id
END TRANSACTION;

• Property become union-type: string -> [string, number]. Revision. See #137

[alexanderdean]

How about:

1. A default is added to a property (Addition)
2. A minimum and/or maximum value is removed from a numeric (Revision)

[chuwy]

Migrated to https://github.com/snowplow/iglu/issues/241