Describe the bug

This is an issue and I can't get the stack trace somehow. Also, while the ALTER statement is stuck, querying the system.mutations table from another client gets stuck too.

A side note. A projection gets accepted with either a GROUP BY or ORDER BY clause, but if I run this: https://fiddle.clickhouse.com/df18b861-0baf-40f2-bdae-3b72c4022f9e, the error message DB::Exception: ORDER BY cannot be empty is misleading, because it can be empty.

How to reproduce

Run:

CREATE TEMPORARY TABLE t0 (c0 Int, c1 Int, PROJECTION p0 (SELECT c0 GROUP BY c0)) ENGINE = MergeTree() ORDER BY tuple();
ALTER TABLE t0 CLEAR COLUMN c0; --stuck

Then from another client try:

SELECT * FROM system.mutations; --stuck

Error message and/or stacktrace

No response