Why getData() and toArray() on collection have a different behaviour for EAVs tables such as Product?

Question

 $collection = $this->productCollectionFactory->create();
 $collection->addAttributeToSelect('*');

• collection->getData() -> returns only a few data (most of the main table until explicitly provided in addAttributeToSelect('my_attr_code').
• collection->toArray() -> returns complete data.

This behaviour is same on the Flat tables, but why different on EAV table such as product?

Answer 1

Eav collection: $collection->addAttributeToSelect('*'); 
Flat collection: $collection->addFieldToSelect('*'); 

Collections of flat tables are inherited from:

Magento\Framework\Data\Collection

   /**
     * Convert collection to array
     */
    public function toArray($arrRequiredFields = [])
    {
        $arrItems = [];
        $arrItems['totalRecords'] = $this->getSize();

        $arrItems['items'] = [];
        foreach ($this as $item) {
            $arrItems['items'][] = $item->toArray($arrRequiredFields);
        }
        return $arrItems;
    }

Collection of EAV entities have abstract layer from:

Magento\Eav\Model\Entity\Collection\AbstractCollection Magento\Framework\Data\Collection\AbstractDb

   /**
     * Retrieve array of attributes
     */
    public function toArray($arrAttributes = [])
    {
        $arr = [];
        foreach ($this->getItems() as $key => $item) {
            $arr[$key] = $item->toArray($arrAttributes);
        }
        return $arr;
    }


    /**
     * Retrieve collection items
     *
     * @return \Magento\Framework\DataObject[]
     */
    public function getItems()
    {
        $this->load();
        return $this->_items;
    }

    /**
     * Load data
     *
     */
    public function load($printQuery = false, $logQuery = false)
    {
        return $this->loadData($printQuery, $logQuery);
    }