my43_mysql内存相关概念

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相关参数

read_buffer_size

https://dev.mysql.com/doc/refman/8.0/en/server-system-variables.html#sysvar_read_buffer_size

If you do many sequential scans, you might want to increase this value, which defaults to 131072.

The value of this variable should be a multiple of 4KB. If it is set to a value that is not a multiple of 4KB, its value will be rounded down to the nearest multiple of 4KB.

This option is also used in the following context for all storage engines:
For caching the indexes in a temporary file (not a temporary table), when sorting rows for ORDER BY.
For bulk insert into partitions.
For caching results of nested queries.
read_buffer_size is also used in one other storage engine-specific way: to determine the memory block size for MEMORY tables.
For more information about memory use during different operations, see Section 8.12.3.1, “How MySQL Uses Memory”.

read_rnd_buffer_size

https://dev.mysql.com/doc/refman/8.0/en/server-system-variables.html#sysvar_read_rnd_buffer_size

This variable is used for reads from MyISAM tables, and, for any storage engine, for Multi-Range Read optimization.

When reading rows from a MyISAM table in sorted order following a key-sorting operation, the rows are read through this buffer to avoid disk seeks. See Section 8.2.1.16, “ORDER BY Optimization”. Setting the variable to a large value can improve ORDER BY performance by a lot. However, this is a buffer allocated for each client, so you should not set the global variable to a large value. Instead, change the session variable only from within those clients that need to run large queries.

Multi-Range Read：减少磁盘的随机访问，并且将随机访问转化为较为顺序的数据访问，详细说明请参见

https://www.cnblogs.com/olinux/p/5146868.html

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