In phpBB 3.3.x, it is possible to add index key length in migrations, f.e.

	public function update_schema()

	{

		return [

			'add_index'	=> [

				$this->table_prefix . 'my_table'		=> [

					'my_index'	=> ['column1', 'column2(191)'],

				],

			]

		];

	}

Index length is a mandatory in some cases, f.e. in MySQL for the TEXT data type index, otherwise it will throw MySQL exceptions like "An exception occurred while executing a query: BLOB/TEXT column \'column_name\' used in key specification without a key length".

Currently specifying index key length in migration won't work in phpBB 4.