MySQL8.0-INFORMATION_SCHEMA增强

Coinciding with the new native data dictionary in MySQL 8.0, we have made a number of useful enhancements to our INFORMATION_SCHEMA subsystem design in MySQL 8.0. In this post I will first go through our legacy implementation as it has stood since MySQL 5.1, and then cover what’s changed.

与MySQL 8.0原生数据字典一致，在MySQL 8.0的INFORMATION_SCHEMA子系统设计中，我们做了一些很有用的增强。在这篇文章中，我将会介绍自MySQL 5.1以来的旧的实现方式，然后介绍我们做了什么改变。

Background

INFORMATION_SCHEMA was first introduced into MySQL 5.0, as a standards compliant way of retrieving meta data from a running MySQL server. When we look at the history of INFORMATION_SCHEMA there have been a number of complaints about the performance of certain queries, particularly in the case that there are many database objects (schemas, tables etc).

INFORMATION_SCHEMA首次引入MySQL 5.0，作为一种从正在运行的MySQL服务器检索元数据的标准兼容方式。当我们回顾INFORMATION_SCHEMA的历史时，对于某些特定查询性能总是有很多的抱怨，特别是在有许多数据库对象（schema，表等）的情况下。

In an effort to address these reported issues, since MySQL 5.1 we have made a number of performance optimizations to speed up the execution of INFORMATION_SCHEMA queries. The optimizations are described in the MySQL manual, and apply when the user provides an explicit schema name or table name in the query.

为了解决这些上报的问题，从MySQL 5.1开始，我们进行了许多性能优化来加快INFORMATION_SCHEMA查询的执行速度。MySQL手册<链接1>中描述了这些优化，当用户在查询中提供显式schema名称或表名时，将会应用这些。

Alas, despite these improvements INFORMATION_SCHEMA performance is still a major pain point for many of our users. The key reason behind these performance issues in the current INFORMATION_SCHEMA implementation is that INFORMATION_SCHEMA tables are implemented as temporary tables that are created on-the-fly during query execution. These temporary tables are populated via:www.weixiu3721.com

1. Meta data from files, e.g. table definitions from .FRM files.
2. Details from storage engines, e.g. dynamic table statistics.
3. Data from global data structures in the MySQL server.

尽管有这些改进，INFORMATION_SCHEMA的性能仍然是我们许多用户的主要痛点。在当前INFORMATION_SCHEMA实现方式下产生的性能问题背后的关键原因是，INFORMATION_SCHEMA表的查询实现方式是在查询执行期间创建临时表。这些临时表通过以下方式填充：

1. 元数据来自文件，例如：表定义来自FRM文件
2. 细节来自于存储引擎，例如：动态表的统计信息
3. 来自MySQL server层中全局数据结构的数据

For a MySQL server having hundreds of database, each with hundreds of tables within them, the INFORMATION_SCHEMAquery would end-up doing lot of I/O reading each individual FRM files from the file system. And it would also end-up using more CPU cycles in effort to open the table and prepare related in-memory data structures. It does attempt to use the MySQL server table cache (the system variable ‘table_definition_cache‘), however in large server instances it’s very rare to have a table cache that is large enough to accommodate all of these tables.

对于一个MySQL实例来说可能有上百个库，每个库又有上百张表，INFORMATION_SCHEMA查询最终会从文件系统中读取每个单独的FRM文件，造成很多I/O读取。并且最终还会消耗更多的CPU来打开表并准备相关的内存数据结构。它确实尝试使用MySQL server层的表缓存（系统变量table_definition_cache ），但是在大型实例中，很少有一个足够大的表缓存来容纳所有的表。

One can easily face the above mentioned performance issue if the optimization is not used by the INFORMATION_SCHEMAquery. For example, let us consider the two queries below