Question

我一直在努力解决这个问题。它耗时超过12秒并且无法弄清楚原因，因为所有三张桌子都有礼服，所以它会慢得多。

SELECT SUM(quantity), rprice, media_file, url, title
FROM inventory, ndxz_objects, ndxz_media
WHERE productid = ndxz_objects.id
AND media_ref_id = ndxz_objects.id
AND productid = media_ref_id
AND media_order = (SELECT MIN(media_order) FROM ndxz_media WHERE media_ref_id = ndxz_objects.id )
GROUP BY productid
ORDER BY productid DESC

Theres是ndxz_objects.id和productid

的索引

Answer 1

缓慢是由嵌套查询引起的，因为每个media_ref_id = ndxz_objects.id都需要进行表扫描

你需要在ndxz_media表的media_ref_id和media_order上创建一个索引

我还建议使用连接重写查询

Answer 2

我猜你还需要ndxz_media.media_ref_id上的索引。（我也猜测那是media_ref_id列的来源，因为你没有非常清楚地列出列名。）

相同的索引应该包含media_order作为第二列。

Answer 3

问题几乎可以肯定是相关子查询，每行执行一次，即使索引此类查询也不是很好。我认为以下应该会达到相同的效果。

SELECT SUM(quantity), rprice, media_file, url, title
FROM inventory, ndxz_objects, ndxz_media,
(SELECT MIN(media_order) as order FROM ndxz_media GROUP BY media_ref_id) min_orders
WHERE productid = ndxz_objects.id
AND media_ref_id = ndxz_objects.id
AND productid = media_ref_id
AND media_order = min_orders.order 
GROUP BY productid
ORDER BY productid DESC

虽然您提供的查询实际上无效，因为rprice, media_file, url, title未汇总或在group by子句中

非常慢的Mysql SELECT查询

3 个答案: