$table->unique('email');
$table->unique('email', 'optional_custom_index_name');
$table->index('email');
$table->index('email', 'optional_custom_index_name');
$table->dropUnique('contacts_email_unique');
$table->dropIndex('contacts_email_unique');
$table->dropForeign('contacts_user_id_foreign');

$table->integer('customer_id')->index();
$table->foreign('customer_id')->references('id')->on('customers')->onDelete('cascade');
Schema::table('customers', function (Blueprint $table) {
    $table->dropColumn('customer_group_id');
});