求大佬告知union分页如何优化?

SELECT title,
       img_url,
       img_border,
       url,
       ext,
       product_id,
       product_type,
       up_time,
       min_price,
       max_price
FROM (
        (SELECT title,
                re_group_content.img_url,
                img_border,
                url,
                re_group_content.ext,
                product_id,
                product_type,
                now() AS up_time,
                0 AS min_price,
                0 AS max_price
         FROM re_group_content
         LEFT JOIN re_product ON re_group_content.product_id = re_product.id
         AND re_group_content.product_type = 'common_product'
         LEFT JOIN re_blind_box ON re_group_content.product_id = re_blind_box.id
         AND re_group_content.product_type = 'blind_box'
         WHERE re_group_content.status = 1
           AND group_id = 12
           AND ((re_product.status = 1
                 AND re_product.up_time <= now())
                OR re_blind_box.status = 1) )
      UNION
        (SELECT '' AS title,
                img_url,
                '' AS img_border,
                '' AS url,
                '' AS ext,
                j.id AS product_id,
                'common_product' AS product_type,
                j.up_time,
                IFNULL(min(sale_price), 0) AS min_price,
                IFNULL(min(prime_price), 0) AS max_price
         FROM re_product j
         LEFT JOIN re_product_package k ON j.id = k.product_id
         AND k.package_type != 'url'
         WHERE j.status = 1
           AND up_time <= now()
         GROUP BY j.id)
      UNION
        (SELECT '' AS title,
                img_url,
                '' AS img_border,
                '' AS url,
                '' AS ext,
                id AS product_id,
                'blind_box' AS product_type,
                created_at AS up_time,
                sale_price AS min_price,
                sale_price AS max_price
         FROM re_blind_box
         WHERE status = 1
           AND sale_price != 0 )) m
ORDER BY up_time DESC
LIMIT 0, 10

上面SQL不考虑索引优化，MYSQL函数优化的问题哈。只针对UNION，对于多union分页例子，有什么优化的思路吗？
另外，使用 union 的话，表的数据大概多少以内用union做分页好点？