Magento – Magento Most Viewed Products Report empty

The event is being fired from core. You can disable this event observer in a custom module (rather then editing core files, which is evil ;) ). Put this in your app/code/local/(VendorName)/(ModuleName)/etc/config.xml:

<config>
    <frontend>
        <events>
            <catalog_controller_product_view>
                <observers>
                    <reports>
                        <type>disabled</type>
                    </reports>
                </observers>
            </catalog_controller_product_view>
        </events>
    </frontend>
</config>

The magic happens in merging your config.xml with the one from core that is defining this observer. Your XML overrides the core <type> node and sets the type to "disabled" which results in the event observer still being there, but not being fired.

Is there any downside to this (disabling viewed products counting)?

I can't think of any downside in disabling this other than that you can't see product view reports anymore...

Also see the answer by Fra here explaining how to disable reports completely (if you don't need them at all) and other Magento code that puts a load on your database.

By default, Magento 2 shows the most viewed product in Magento Admin > DASHBOARD > choose tab Most Viewed Products. The _prepareCollection() method in vendor/magento/module-backend/Block/Dashboard/Tab/Products/Viewed.php is used for selecting products.

We can copy some code lines from _prepareCollection() above. For example, in your block, use the following code below:

class Index extends \Magento\Framework\View\Element\Template
{
    /**
     * @var \Magento\Reports\Model\ResourceModel\Product\CollectionFactory
     */
    protected $_productsFactory;

    /**
     * @param \Magento\Framework\View\Element\Template\Context $context
     * @param \Magento\Reports\Model\ResourceModel\Product\CollectionFactory $productsFactory
     * @param array $data
     */
    public function __construct(
        \Magento\Framework\View\Element\Template\Context $context,
        \Magento\Reports\Model\ResourceModel\Product\CollectionFactory $productsFactory,
        array $data = []
    ) {
        $this->_productsFactory = $productsFactory;
        parent::__construct($context, $data);
    }

    /**
     * Getting most viewed products
     */
    public function getCollection()
    {

        $currentStoreId = $this->_storeManager->getStore()->getId();

        $collection = $this->_productsFactory->create()
        ->addAttributeToSelect(
            '*'
        )->addViewsCount()->setStoreId(
                $currentStoreId
        )->addStoreFilter(
                $currentStoreId
        );
        $items = $collection->getItems();
        return $items;
    }
}

The initial class \Magento\Reports\Model\ResourceModel\Product\CollectionFactory is the most important class because this class will get our products report. The getCollection method will return the most viewed items by current store.

In your template, you can get the items. For example:

<?php
    $items = $this->getCollection();
    foreach($items as $item) {
        echo $item->getName() .'<br/>';
    }
?>