Delete in MySQL: the difference between delete, drop and truncate

The same thing ： Both have deletion function .

The three difference ：

	delete	truncate	drop
type	Belong to DML	Belong to DDL	Belong to DDL
Roll back	Roll back	Cannot roll back	Cannot roll back
Delete content	Delete all or part of a table's data rows	Delete all data in the table	Delete tables from the database
Delete speed	Slow to delete	Delete fast	Delete the fastest

Three diagrams ：

Detailed explanation of the three

delete

Delete content , Do not delete table structure , Don't free up space .

usage ：

--  Delete the whole table data 
DELETE FROM table_name

--  Delete some data 
DELETE FROM table_name
WHERE condition

1、DELETE Belongs to the database DML Operating language , Delete only the data, not the structure of the table , I can walk away from business , It will trigger trigger;
2、 stay InnoDB in ,DELETE It doesn't really delete the data ,MySQL In fact, it just marks the deleted data as deleted , therefore delete When deleting data from a table , Table files will not take up less space on disk , Storage will not be released , Just make the deleted data rows invisible . Although no disk space has been released , But the next time you insert data , You can still reuse this space （ reusing → Cover ）.
3、DELETE Execution time , It will cache the deleted data to rollback segement in , Business commit Effective after ;
4、delete from table_name Delete all data from the table , about MyISAM Will immediately free up disk space ,InnoDB It won't free up disk space ;
5、 about delete from table_name where xxx Conditional deletion , Whether it's InnoDB still MyISAM Will not free up disk space ;
6、delete Use after operation optimize table table_name Will immediately free up disk space . Whether it's InnoDB still MyISAM . So in order to free up disk space ,delete Later optimize table operation .

drop

drop Statement to delete the table structure and all data , And release all the space occupied by the table .

usage ：

--  Delete the whole table data 
DROP TABLE table_name

1、drop： Belongs to the database DDL Define language , Same as Truncate; Take effect immediately , Unable to retrieve
2、drop table table_name Free up disk space immediately , Whether it's InnoDB and MyISAM;
3、drop Statement will delete the constraint that the structure of the table is dependent on (constrain)、 trigger (trigger)、 Indexes (index); Stored procedures that depend on the table / The function will keep , But it became invalid state .

Truncate

Delete the contents of the table , Do not delete table structure , Release space .

usage ：

--  Delete the whole table data 
TRUNCATE TABLE table_name

1、truncate Belongs to the database DDL Define language , No business , The original data will not be put into rollback segment in , Operation does not trigger trigger, Take effect immediately , Unable to retrieve .
2、truncate table table_name Free up disk space immediately , Whether it's InnoDB and MyISAM .truncate table It's kind of like drop table then create, It's just this create table The process has been optimized , For example, the table structure file has already existed before, and so on . So the speed should be close to drop table The speed of .
3、truncate Can quickly clear a table . And reset auto_increment Value . But for different types of storage engines, what you need to pay attention to is ： about MyISAM,truncate Reset auto_increment（ Autogenous sequence ） The value of is 1. and delete The back table remains auto_increment. about InnoDB,truncate Reset auto_increment The value of is 1.delete The back table remains auto_increment. But doing delete Restart after the entire table MySQL Words , After the restart auto_increment Will be set to 1. in other words ,InnoDB The table itself cannot be persisted auto_increment.delete After the table auto_increment Still in memory , But it was lost after the restart , Only from 1 Start . Essentially after the restart auto_increment From SELECT 1+MAX(ai_col) FROM t Start .

Execution speed

drop> truncate > delete

Security

Careful use drop and truncate, Especially when there is no backup .
On use , Want to delete some data lines delete, Pay attention to take where Clause . The rollback segment should be large enough .

Use advice

Delete table completely [drop]
Want to keep the table and delete all the data . If it's not about business [truncate]
If it's about business , Or want to trigger trigger[delete]