结合业务特性巧用临时表

太阳不下山

　　拿到某开发sql如下

SELECT  p.products_id FROM products AS p
JOIN products_to_categories AS pc USING(products_id)
JOIN categories AS c USING(categories_id)
JOIN products_realtime_quantity AS prq ON prq.sku_or_poa = p.products_model
WHERE products_status =1 AND categories_status =1   AND prq.msg != 'Temporary out stock.'
ORDER BY p.products_date_added DESC LIMIT 10

　　一般看到这种sql,在where中只有status类似的字段(可选择性非常低,数据两极分化非常明显)而且需要order by的语句,我们就应该想到使用force index(order_by_column)来进行优化.

　　explian

+----+-------------+-------+--------+------------------------------+-------------------+---------+--------------------------------+--------+----------------------------------------------+
| id | select_type | table | type   | possible_keys        | key         | key_len | ref              | rows   | Extra                    |
+----+-------------+-------+--------+------------------------------+-------------------+---------+--------------------------------+--------+----------------------------------------------+
|  1 | SIMPLE    | pc  | index  | PRIMARY,categories_id    | PRIMARY       | 8     | NULL               | 1009510 | Using index; Using temporary; Using filesort |
|  1 | SIMPLE    | p   | eq_ref | PRIMARY,products_model     | PRIMARY       | 4     | banggood_work.pc.products_id   |    1 | Using where                  |
|  1 | SIMPLE    | c   | eq_ref | PRIMARY            | PRIMARY       | 4     | banggood_work.pc.categories_id |    1 | Using where                  |
|  1 | SIMPLE    | prq   | ref  | ix_prg_sku_or_poa,ix_prq_msg | ix_prg_sku_or_poa | 152   | banggood_work.p.products_model |    1 | Using where                  |
+----+-------------+-------+--------+------------------------------+-------------------+---------+--------------------------------+--------+----------------------------------------------+

　　发现mysql优化器选择了pc表的主键,虽然使用了索引,但是进行了全索引扫描，效果还是不理想!
　　强制使用force index后,explain

EXPLAIN
  -> SELECT  p.products_id FROM products AS p   FORCE INDEX(products_date_added)
  -> JOIN products_to_categories AS pc USING(products_id)
  -> JOIN categories AS c USING(categories_id)
  -> JOIN products_realtime_quantity AS prq ON prq.sku_or_poa = p.products_model
  -> WHERE products_status =1 AND categories_status =1   AND prq.msg != 'Temporary out stock.'
  -> ORDER BY p.products_date_added DESC LIMIT 10
  -> ;
+----+-------------+-------+--------+------------------------------+---------------------+---------+--------------------------------+------+-------------+
| id | select_type | table | type   | possible_keys        | key         | key_len | ref              | rows | Extra     |
+----+-------------+-------+--------+------------------------------+---------------------+---------+--------------------------------+------+-------------+
|  1 | SIMPLE    | p   | index  | NULL             | products_date_added | 8     | NULL               |  1 | Using where |
|  1 | SIMPLE    | prq   | ref  | ix_prg_sku_or_poa,ix_prq_msg | ix_prg_sku_or_poa   | 152   | banggood_work.p.products_model |  1 | Using where |
|  1 | SIMPLE    | pc  | ref  | PRIMARY,categories_id    | PRIMARY       | 4     | banggood_work.p.products_id  | 1009 | Using index |
|  1 | SIMPLE    | c   | eq_ref | PRIMARY            | PRIMARY       | 4     | banggood_work.pc.categories_id |  1 | Using where |
+----+-------------+-------+--------+------------------------------+---------------------+---------+--------------------------------+------+-------------+

　　发现索引已经变成productsw_date_added,而执行时间，前者是2s,后者是0.003s.
　　

　　这是我们一贯的优化方法,但是我们可以根据sql语句的特性和业务特性,结合临时表进行一些淫邪的优化，虽然并不通用，但是可以开阔sql优化者的思维。
　　我们可以看到这条语句是需要根据产品添加时间拿取符合(products_status =1 AND categories_status =1   AND prq.msg != 'Temporary out stock.')条件的10个最新上架产品.而我们知道，最新上架的产品一般状态都是不可能马上下架,而且对应的类别id也是可用,而且库存也是充足的(要不然何必上架),这个特性站到了99.9%以上.所以，我们利用这个特性，先从产品表中找出不带任何条件的200个产品,放到临时表,然后再用临时表结果集,和拿取条件进行匹配,取出最新的10条.
　　(200条是一个参考值，根据各自的逻辑特性来取)
　　sql如下

SELECT DISTINCT p.products_id FROM
(SELECT products_id,products_model,products_status,products_date_added
  FROM products
  ORDER BY products_date_added DESC LIMIT 200
) AS p
JOIN products_to_categories AS pc USING(products_id)
JOIN categories AS c USING(categories_id)
JOIN products_realtime_quantity AS prq ON prq.sku_or_poa = p.products_model
WHERE products_status =1 AND categories_status =1   AND prq.msg != 'Temporary out stock.'
ORDER BY products_date_added DESC LIMIT 10;

　　explain后发现

|  1 | PRIMARY   | <derived2> | ALL  | NULL             | NULL        | NULL  | NULL               |  200 | Using where; Using temporary; Using filesort |
|  1 | PRIMARY   | prq    | ref  | ix_prg_sku_or_poa,ix_prq_msg | ix_prg_sku_or_poa   | 152   | p.products_model         |  1 | Using where; Distinct            |
|  1 | PRIMARY   | pc     | ref  | PRIMARY,categories_id    | PRIMARY       | 4     | p.products_id          | 1009 | Using index; Distinct            |
|  1 | PRIMARY   | c      | eq_ref | PRIMARY            | PRIMARY       | 4     | banggood_work.pc.categories_id |  1 | Using where; Distinct            |
|  2 | DERIVED   | products   | index  | NULL             | products_date_added | 8     | NULL               |  200 |                        |

　　我们可以看到，已经利用products表中的products_date_added字段排序取出200条,整个sql语句变成一个只有200行的临时表驱动查询了，性能相对于原来的语句,提高上百倍！
　　执行时间大约是0.02s(可能比force index略慢)。