如何解決查詢結果集很小但Created_tmp_disk_tables不斷增加的問題

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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， 或者自定義變量被按BLOB、TEXT類型處理

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.

# 查詢表的列信息時。這個比較坑

SHOW columns from tb ;

DESCRIBE tb;

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