Simple and light way to build an array with the values of an collection withoud loading the complete objects in Magento

collection_values.php

/** @var array $_productSkus */
    protected $_productSkus;

    /**
     * Return the sku for a given productId
     *
     * @param $productId
     * @return bool
     */
    public function getProductSku($productId)
    {
        if (empty($this->_productSkus)) {
            $collection = Mage::getModel('catalog/product')->getCollection()
                ->addAttributeToSelect(array('sku'), 'inner')
                ->addAttributeToFilter('type_id', array(Mage_Catalog_Model_Product_Type::TYPE_CONFIGURABLE));
            $select = $collection->getConnection()
                ->query($collection->getSelect());
            while ($row = $select->fetch()) {
                $this->_productSkus[$row['entity_id']] = $row['sku'];
            }
            unset($select, $collection);
        }
        if (isset($this->_productSkus[$productId])) {
            return $this->_productSkus[$productId];
        } else {
            return false;
        }
    }