fix(mysql)!: full MODIFY COLUMN support.

[burnison]

The semantics of MODIFY COLUMN in MySQL are not the same as ALTER COLUMN, but have been treated so since #3189. The two ALTER TABLE expressions are fundamentally incompatible:

  | MODIFY [COLUMN] col_name column_definition
        [FIRST | AFTER col_name]

  | ALTER [COLUMN] col_name {
        SET DEFAULT {literal | (expr)}
      | SET {VISIBLE | INVISIBLE}
      | DROP DEFAULT
    }

Most importantly, the MODIFY COLUMN syntax takes a full column_definition and column position. The actual execution behaviour of MODIFY does not merge the column definition, but completely redefines it.

When deciding what approach to take for implementation, I evaluated 3 approaches:

• Adding a column_definition to AlterColumn, but this would be parallel to the other attributes leading to surprising syntax.

• Similarly, adding constraints to AlterColumn, which, again, would lead to surprising, inconsistent outcomes.

• This implementation, a dedicated ModifyColumn, which potentially breaks existing MODIFY COLUMN deployments that might depend on it parsing to AlterColumn.

This breaking change is defensible, however, in that MODIFY COLUMN c DROP DEFAULT isn't valid syntax in MySQL. I'm led to believe the behaviour of existing deployments would likely be questionable if not broken (example: I had been working around it for more than a year by rewriting to ADD COLUMN).