@nekobest 香港 YouTube 上车吗？

换小米吧。

@he110comex CPU 拆了都比这个贵的吧。。。

@wsseo 中文的暂时没。。主要大多数都是论文。。使用价值是指？

没有收获一个新的 star，伤心 (T_T)
看来 v 站 DBA 或者数据库开发不多？

五点半是不有点太早了。。。

@demonps 问题应该在 where 里面前两个条件没有索引，可以拆开成 explain select it_id where c_user_id = 0 and l_user_id != 3333; 然后继续拆，单独看一下数据情况，估计会有至少一个估计不准确的（应该是 l_user_id 没有索引）。

我写这句的时候突然有个想法，你可以试下：

select it_id from assign_idx where (c_type, cat_id) in ((30, 1), (30, 2), (30, 3), (20, 4), (20, 6), (20, 9), (20, 10)) and c_user_id = 0 and l_user_id != 3333 order by create_time asc limit 1;

试一下 UPDATE aaa SET kk = null WHERE xx != 0

给一下
explain select it_id where (c_type, cat_id) in ((30, 1), (30, 2), (30, 3), (20, 4), (20, 6), (20, 9), (20, 10));
结果？

自己打下脸，好像 (c_type, cat_id) 可以优化

https://dev.mysql.com/doc/refman/8.0/en/range-optimization.html#row-constructor-range-optimization

Only IN() predicates are used, not NOT IN().
On the left side of the IN() predicate, the row constructor contains only column references.
On the right side of the IN() predicate, row constructors contain only runtime constants, which are either literals or local column references that are bound to constants during execution.
On the right side of the IN() predicate, there is more than one row constructor.

https://dev.mysql.com/doc/refman/8.0/en/multiple-column-indexes.html 文档

MySQL can use multiple-column indexes for queries that test all the columns in the index, or queries that test just the first column, the first two columns, the first three columns, and so on. If you specify the columns in the right order in the index definition, a single composite index can speed up several kinds of queries on the same table.

(c_type, cat_id) 是索引的一部分，所以建的索引不能用，慢
where 里面的 col 都不能用索引，慢

@NewConn 不好意思我是妹子。

WITH E AS (
SELECT ID
FROM A
WHERE type = 2
AND DELETE_FLAG = 0
AND uid = 41
UNION
SELECT a.ID
FROM A a, B b, A c
WHERE a.type = 3
AND a.DELETE_FLAG = 0
AND a.uid = 41
AND a.ID = b.ID
AND b.parent_id = c.ID
)

这里面为什么不能先把 a.type in (2,3) and a.delete_flag =0 and a.uid = 41 先选择出来然后再别的操作？这一句的 selectivity 有多少？有多少符合条件的？

ALTER TABLE operations are processed using one of the following algorithms:

COPY: Operations are performed on a copy of the original table, and table data is copied from the original table to the new table row by row. Concurrent DML is not permitted.
不可以并发

INPLACE: Operations avoid copying table data but may rebuild the table in place. An exclusive metadata lock on the table may be taken briefly during preparation and execution phases of the operation. Typically, concurrent DML is supported.
可能加锁，一般可以并发

INSTANT: Operations only modify metadata in the data dictionary. No exclusive metadata locks are taken on the table during preparation and execution, and table data is unaffected, making operations instantaneous. Concurrent DML is permitted. (Introduced in MySQL 8.0.12)
允许并发

https://dev.mysql.com/doc/refman/8.0/en/alter-table.html

@yukiloh 我是学生。工作的时候碰到问题去搜索，其实就是把这个时间转嫁到实际操作的时候学习了。这个本科课程内容也没多少，认真一下，只了解概念，两三周绝对够了。

发现一个 http://sqlfiddle.com/ 就是广告有点明显。。。错误提示比 DB Fiddle 详细。