If multiple rows have identical values in theORDER BYcolumns, the server is free to return those rows in any order, and may do so differently depending on the overall execution plan. In other words, the sort or
Re: Order by two columns Gaby Lopez October 31, 2014 04:26AM Re: Order by two columns Rick James October 31, 2014 08:24AM Re: Order by two columns Peter Brawley October 31, 2014 11:05AM Sorry, you can't reply to this topic. It has been closed....
二、“order by”引发的乱序 经过一番排查,发现罪魁祸首其实是order by。当使用order by的字段存在多行相同值时,就会导致上面出现的乱序问题,这里我们来看下官方的解释[1]: ❝If multiple rows have identical values in theORDER BYcolumns, the server is free to return those rows in any order, ...
ORDER BY userid; MySQL会直接遍历上图userid索引的叶子节点链表,不需要进行额外的排序操作。这就是用索引扫描来排序。 但如果userid字段上没有任何索引,图1的B+树结构不存在,MySQL就只能先扫表筛选出符合条件的数据,再将筛选结果根据userid排序。这个排序过程就是filesort。
the server is free to return those rows in any order, and may do so differently depending on the overall execution plan. In other words, the sort order of those rows is nondeterministic with respect to the nonordered columns. 如果order by的字段有多个行都有相同的值,mysql是会随机的顺序返回...
ORDER BY 排序列存在相同值时返回顺序是不固定的 If multiple rows have identical values in the ORDER BY columns, the server is free to return those rows in any order, and may do so differently depending on the overall execution plan. In other words, the sort order of those rows is nondeter...
这种行为的一种表现是带有和不带有 LIMIT 的 ORDER BY 查询可能会以不同的顺序返回行,如本节后面所述。 原文: If multiple rows have identical values in the ORDER BY columns, the server is free to return those rows in any order, and may do so differently depending on the overall execution plan...
If multiple rows have identical values in theORDER BYcolumns, the server is free to return those rows in any order, and may do so differently depending on the overall execution plan. In other words, the sort order of those rows is nondeterministic with respect to the nonordered columns. ...
Whether the optimizeractually does so depends on whether reading the index is more efficient than a table scanif columns not in the index must also be read. 官方文档告诉我们optimizer会帮我们选择一种高效的ORDER BY方式。 但也不能完全依赖optimizer的判断,这时合理建立索引、引导它使用指定索引可能是更...
Subject Written By Posted order by 2 columns avancha mailto November 01, 2012 09:18PM Re: order by 2 columns laptop alias November 02, 2012 01:19AM Sorry, you can't reply to this topic. It has been closed.