public function modifySelect(\Magento\Framework\DB\Select $select)
    {
        /* aliases for tables ... */
        $tblEntity = 'e'; // this is alias for 'catalog_product_entity' table
        $tblStockItem = $this->_resource->getTableName(self::TBL_STOCK_ITEM);
        $tblWrhsQty = $this->_resource->getTableName(self::TBL_WRHS_QTY);
        /* ... and fields */
        $fldStockItemProdId = StockItem::PRODUCT_ID;
        $fldStockItemId = StockItem::ITEM_ID;
        $fldEntityId = \Magento\Eav\Model\Entity::DEFAULT_ENTITY_ID_FIELD;
        $fldQty = self::FLD_QTY;
        $fldStockItemRef = Quantity::A_STOCK_ITEM_REF;

        /* LEFT JOIN `cataloginventory_stock_item` */
        $on = "`$tblStockItem`.`$fldStockItemProdId`=`$tblEntity`.`$fldEntityId`";
        $fields = [];
        $select->joinLeft($tblStockItem, $on, $fields);

        /* LEFT JOIN `prxgt_wrhs_qty` */
        $on = "`$tblWrhsQty`.`$fldStockItemRef`=`$tblStockItem`.`$fldStockItemId`";
        $fields = [$fldQty => $this->getEquationQty()];
        $select->joinLeft($tblWrhsQty, $on, $fields);

        /* GROUP BY */
        $select->group("$tblEntity.$fldEntityId");
        return $select;
    }