mysql - SQL 查询中的 RAND() 是如何工作的

Question

下面的查询实际上是如何工作的？

SELECT * FROM t ORDER BY RAND() LIMIT 1;

它是否首先对源数据库中的所有 记录进行排序，然后截断它以获得 1 行？

Answer 1

这个问题与 LIMIT 查询优化有关，而不是 RAND() 是如何工作的。

来自 manual (大胆强调我的):

If you combine LIMIT row_count with ORDER BY, MySQL ends the sorting as soon as it has found the first row_count rows of the sorted result, rather than sorting the entire result.

If ordering is done by using an index, this is very fast. If a filesort must be done, all rows that match the query without the LIMIT clause are selected, and most or all of them are sorted, before the first row_count are found. After the initial rows have been found, MySQL does not sort any remainder of the result set.