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

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;