magento-test/dev/tests/integration/_files/Magento/TestModuleMview/etc/mview.xml

<?xml version="1.0" encoding="UTF-8"?>
<!--
/**
 * Copyright © Magento, Inc. All rights reserved.
 * See COPYING.txt for license details.
 */
-->
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Mview/etc/mview.xsd">
    <view id="test_view_with_additional_columns" class="Magento\Framework\Indexer\Action\Dummy" group="indexer">
        <subscriptions>
            <table name="test_mview_table" entity_column="entity_id">
                <additionalColumns>
                    <column name="additional_column" cl_name="test_additional_column" />
                </additionalColumns>
            </table>
        </subscriptions>
    </view>
</config>