Handle NULLs in ALTER MODIFY COLUMN

[vdimir]

Changelog category (leave one):

• Backward Incompatible Change

Changelog entry (a user-readable short description of the changes that goes into CHANGELOG.md):

ALTER MODIFY COLUMN now requires explicit DEFAULT when converting nullable columns to non-nullable types. Previously such ALTERs could get stuck with cannot convert null to not null errors, now NULLs are replaced with column's default expression. Resolves #5985

Details

Close #5985

[clickhouse-gh]

Workflow [PR], commit [cd53684]

Summary: ❌

job_name	test_name	status	info	comment
BuzzHouse (amd_msan)		failure
	Logical error: '(isConst() || isSparse() || isReplicated()) ? getDataType() == rhs.getDataType() : typeid(*this) == typeid(rhs)'	FAIL	cidb, issue
Integration tests (amd_asan, db disk, old analyzer, 3/6)		error
	OOM in dmesg	FAIL	cidb

[antaljanosbenjamin]

LGTM.

[den-crane]

• Alter of nullable columns to not nullable is not supported #5985

[vdimir]

Integration tests (amd_asan, db disk, old analyzer, 3/6) error
OOM in dmesg FAIL cidb

Seems like #84881

[bakwc]

Hi team,
just a heads-up that the recent changes appear to have broken backward compatibility.
We spent quite a bit of time investigating an issue on our side before realizing it was related to changes in ClickHouse.
It would be greatly appreciated if the impact on existing integrations and dependent projects could be considered in future updates. Thank you!

[antaljanosbenjamin]

just a heads-up that the recent changes appear to have broken backward compatibility.

Hey! Did it broke in a way that is not mentioned in the changelog item? If yes, we have a problem. If not, then it is kind of expected.

Unfortunately from time to time we have to bite the bullet to introduce backward incompatible changes in order to fix big issues, in this case stuck mutations. We try to keep backward incompatible changes minimal and try to always add a proper changelog item, however we might mistakes (not noticing backward incompatibility or missing parts of the scope).