mysql8.0.18中的hash join原理（官网）

hash连接算法：https://dev.mysql.com/doc/refman/8.0/en/hash-joins.html

嵌套连接算法：https://dev.mysql.com/doc/refman/8.0/en/nested-loop-joins.html#block-nested-loop-join-algorithm

A hash join is usually faster than and is intended to be used in such cases instead of the block nested loop algorithm (see Block Nested-Loop Join Algorithm) employed in previous versions of MySQL. Beginning with MySQL 8.0.20, support for block nested loop is removed, and the server employs a hash join wherever a block nested loop would have been used previously.

哈希连接通常比以前版本的 MySQL 中使用的块嵌套循环算法( block nested loop algorithm)（请参阅块嵌套循环连接算法） 更快，并且旨在在这种情况下使用散列连接 。

从 MySQL 8.0.20 开始，删除了对块嵌套循环的支持，并且服务器在以前使用块嵌套循环的任何地方都采用哈希连接。