前言

MySQL 的 innodb 引擎之所以使用 B+tree 来存储索引，就是想尽量减少数据查询时磁盘 IO 次数。树的高度直接影响了查询的性能。一般树的高度在 3~4 层较为适宜。数据库分表的目的也是为了控制树的高度。那么如何获取树的高度呢？下面使用一个示例来说明如何获取树的高度。

示例数据准备

建表语句如下：

CREATETABLE`user` (
  `id` int(11) NOTNULLAUTO_INCREMENT,
  `name` varchar(100) CHARACTERSETlatin1 DEFAULTNULL,
  `age` int(11) DEFAULTNULL,
  PRIMARYKEY(`id`),
  KEY`name` (`name`),
  KEY`age` (`age`)
) ENGINE=InnoDB DEFAULTCHARSET=utf8

表中插入100万条数据。数据如下：

mysql> select* fromuserlimit 2\G
*************************** 1. row ***************************
  id: 110000
name: ab
 age: 100
*************************** 2. row ***************************
  id: 110001
name: ab
 age: 100
2 …

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Twitter! The most important and most common platform, where most of the educators tend to learn for their professional development and training.  It is basically a social networking microblogging service that allows you to send anecdote message/short messages mainly known as tweets! The twitter transfer news contains text and links, the basic tweet consists of only 140 characters, but has recently increased upto 280 characters.

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Anatomy of Tweet!

Tweet

One of the greatest things about Twitter is that it is …

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MySQL如何恢復表空間？

這不是新的信息，但我沒有多說，所以現在為那些需要它的人解決它。

如果您丟失了ibd文件......您將丟失數據。 因此，如果您有一個可用的副本..或者即使您從另一個數據庫同步，您仍然可以導入它。 什麼/你如何失去表空間？

這是一個恢復表空間的簡單示例。



mysql> Create database demo;

mysql> use demo;

mysql> CREATE TABLE `demotable` (
-> `id` int(11) NOT NULL AUTO_INCREMENT,
-> `dts` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
-> PRIMARY KEY (`id`)
-> ) ENGINE=InnoDB;

現在我們存儲一些數據......


mysql> INSERT INTO demotable (id) VALUES (NULL);
Query OK, 1 row affected (0.10 …

所以我意識到在最近出現這種情況後我沒有發表過關於此的帖子。

以下是場景：在午夜進行備份，他們使用每個數據庫的MySQL轉儲。 然後在第二天上午十點數據庫崩潰。 在我被調用之前發生了一系列事件，但他們把它帶到了MyISAM表的數據庫版本和表空間中缺少的IBD文件。

所以選項1，從備份恢復會讓我們到午夜，我們會丟失數小時的數據。 選項2，我們重新導入1000的ibd文件並保留所有內容。 然後我們有選項3，從備份恢復，然後應用binlogs進行最近的更改。

為了使它更有趣，他們沒有我被告知的所有ibd文件，我確實看到一些丟失。 所以不確定這是怎麼可能的，但是選項2變成了無效選項。 當然，他們希望盡可能減少數據丟失，因此我們選擇了3。

…

所以MySQL的組複製出來了MySQL 5.7。 現在已經有一段時間了，人們開始更多地詢問它。

• https://dev.mysql.com/doc/refman/8.0/en/group-replication.html
• …

因此，max_connections從my.cnf文件中設置的值下降到214的問題在Ubuntu上已經存在了一段時間。

作為一個例子，它在2015年被注意到

• https://codepoets.co.uk/2015/mysql-max_connections-stuck-on-214/

我最近又碰到了這個，並通過以下步驟解決了。


# cp /lib/systemd/system/mysql.service /etc/systemd/system/
# cd /etc/systemd/system/
# vi mysql.service

LimitNOFILE=infinity
LimitMEMLOCK=infinity

# systemctl …