Add support for ALTER TABLE .. ALTER COLUMN .. { SET | DROP } NOT NULL

[lukaseder]

This syntax is supported natively in PostgreSQL. In many other databases, it is rather difficult to emulate, as the nullability of a column can only be modified together with the data type. We'd have to query the dictionary views prior to generating the ALTER statement...

Hence, emulation is done in a separate issue: #6476

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See also #3805