Vendor\ModuleName\etc\mview.xml

mview.xml

<?xml version="1.0" encoding="UTF-8"?>
<!--
/**
 * @author Reena Parekh <reena1.parekh@gmail.com>
 */
-->
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Mview/etc/mview.xsd">
    <view id="my_custom_indexer" class="Vendor\ModuleName\Model\Indexer\CustomIndexer" group="indexer">
        <subscriptions>
            <table name="my_custom_module_table" entity_column="entity_id"/>
        </subscriptions>
    </view>
</config>