问优化Slick生成的SQL查询

EN

SELECT
  c.id,
  count(cp.product_id)
FROM cart c LEFT OUTER JOIN cart_product cp ON c.id = cp.cart_id
WHERE c.id = 3
GROUP BY c.id;

Cart.joinLeft(CartProduct)
  .on { case (c, cp) => c.id === cp.cartId }
  .filter { case (c, cp) => c.id === 3 }
  .groupBy { case (c, cp) => c.id }
  .map { case (c, pr) => (c, pr.length)
}

SELECT
  x2.x3,
  count(1)
FROM (SELECT
        x4.x5  AS x3,
        x4.x6  AS x7,
        x8.x9  AS x10,
        x8.x11 AS x12,
        x8.x13 AS x14,
        x8.x15 AS x16
      FROM (SELECT
              x17."id"      AS x5,
              x17."user_id" AS x6
            FROM "cart" x17) x4 LEFT OUTER JOIN (SELECT
                                                   1                AS x9,
                                                   x18."id"         AS x11,
                                                   x18."cart_id"    AS x13,
                                                   x18."product_id" AS x15
                                                 FROM "cart_product" x18) x8 ON x4.x5 = x8.x13) x2
WHERE x2.x3 = 3
GROUP BY x2.x3;