SQL Standard behavior for DROP COLUMN xxx RESTRICT:
* If a constraint (UNIQUE or CHECK) uses only the dropped column,
it's automatically dropped too. If it uses many columns - an error.
* don't issue an error for ER_KEY_BASED_ON_GENERATED_VIRTUAL_COLUMN
* support keyread on vcols
* callback into the server to compute vcol values from mi_check/mi_repair
* DMLs just work. Automatically.