查詢結果集很小，但 Created_tmp_disk_tables 不斷增加

官方文件: 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， 或者自定義變數被按BLOB、TEXT型別處理

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.

# 列長度定義超過512，在union時

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.

# 查詢表的列資訊時。這個比較坑

SHOW columns from tb ;

DESCRIBE tb;