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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