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这一条SQL怎么优化

SQL code
<!--Code highlighting produced by Actipro CodeHighlighter (freeware)http://www.CodeHighlighter.com/-->select * from dx_gd_goods where g_label >= 100101000 and g_label


上面的SQL语句执行要2秒。太慢了。

我如果变成这样,但是实际需求是一定要排序的。
SQL code
<!--Code highlighting produced by Actipro CodeHighlighter (freeware)http://www.CodeHighlighter.com/-->select * from dx_gd_goods where g_label >= 100101000 and g_label


这样做后,程序只需要0.16秒就可以返回

我怎样做才能到排序后,执行的时间又短呢。

------解决方案--------------------
g_label 和 g_likenum 字段都建立索引没有
------解决方案--------------------
我觉得你的测试不准确,没理由相差这么大。你是多次运行过取平均值吗?
------解决方案--------------------
g_likenum 没有用到索引
------解决方案--------------------
前一个语句执行慢,后一个执行快,我猜测原因是这样的:后者在扫描数据记录的时候,找到符合 where 条件的记录,只要凑够 30 个就好了,即使 g_label 没有索引,只要符合条件的记录在总的数据集合里所占的比例不是特别低,也没有什么问题;而前者则不行,它必须找到符合 where 条件的所有记录,然后再按照 g_likenum 排序,才能取出前面的 30 个,这个过程如果没有适当索引的帮助,很可能导致全表扫描。

解决问题的办法当然也只能是设置适当的索引。但是什么样的索引是适当的,还跟你的 dx_gd_goods 表中的数据分布特点有关系。比如,当符合 where 条件的记录所占的比例足够高,而 g_likenum 字段的区分度也很高的情况下,(g_likenum) 这样的索引就已经很好了;但如果符合 where 条件的记录只是相对很少一部分,那么 (g_label, g_likenum) 这样的索引可能效果会更好一点。

另外,有的时候数据库(比如 mysql)可能并不一定按你的想像来使用索引,所以要用 explain 进行考察,必要时可以通过 index hint 等手段进行干预。


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------解决方案--------------------
g_likenum 是不是数据很集中的那种?enum?
------解决方案--------------------
这是前段的sql吗?如果是的话你可以给g_label 和 g_likenum建立个符合索引,你的mysql是什么版本?可以的话创建分区呢
------解决方案--------------------
探讨

哈哈,
还好,刚才强制了一下索引,速度还可以接受了,0.7秒就返回了。
虽然不是很好,但是还可以接受引用:

这是前段的sql吗?如果是的话你可以给g_label 和 g_likenum建立个符合索引,你的mysql是什么版本?可以的话创建分区呢

------解决方案--------------------
在 g_label 和 g_likenum 上建联合索引
------解决方案--------------------
探讨
有点大

------解决方案--------------------
探讨

唠叨大哥,
我有这样的语句三条,

[code=SQL]
select * from dx_gd_goods where g_label > = 100101000 and g_label
select * from dx_……

------解决方案--------------------
探讨

哦,
我现在没有建立复合,我是分开建的。
引用:

引用:
有点大


能不能建分区呢,只是你已经给这两个字段建立复合索引了,不知道建分区效果是否明显了,不过你可以尝试先,如果没啥大效果再把分区删除了

------解决方案--------------------
那也就只能这样了,再优化就是大动作了
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