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查询结果集很小,但 Created_tmp_disk_tables 不断增加

原创 MySQL 作者:G8bao7 时间:2020-05-19 17:13:16 0 删除 编辑

官方文档: https://dev.mysql.com/doc/refman/5.7/en/internal-temporary-tables.html


create table tb(id int , va varchar(10));

insert into tb(id, va) values (1, 'Created_tmp_disk_tables');


Some query conditions prevent the use of an in-memory temporary table, in which case the server uses an on-disk table instead:


Presence of a BLOB or TEXT column in the table. This includes user-defined variables having a string value because they are treated as BLOB or TEXT columns, depending on whether their value is a binary or nonbinary string, respectively.

# BLOB、TEXT列, 或者自定义变量被按BLOB、TEXT类型处理

alter table tb modify va text;

select * from (select * from tb) t;


Presence of any string column with a maximum length larger than 512 (bytes for binary strings, characters for nonbinary strings) in the SELECT list, if UNION or UNION ALL is used.

# 列长度定义超过512,在union时

alter table tb modify va varchar(513);

select * from tb union select * from tb;


The SHOW COLUMNS and DESCRIBE statements use BLOB as the type for some columns, thus the temporary table used for the results is an on-disk table.

# 查询表的列信息时。这个比较坑

SHOW columns from tb ;

DESCRIBE tb;


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