What's the difference between delete, truncate, and drop, and what to do if you delete data by mistake

Here's the thing , Last week, the boss told me that the disk of the test server was full . He came to me and asked me to help clean it up .

Found to be MySQL Of data The catalog is too big , So I went into mysql, It turns out that there are several million level tables that are too big , So I deleted , A meal operation as fierce as a tiger , Space occupation or 95%.

bosses ：“ How did you delete ？”
I ：“ One of me rm -rf / * ”
bosses ：“？？？”
I ：“ incorrect , I use delete from Deleted ”
bosses ：“ You're not going to work tomorrow .”

Later, in order not to be disliked by the big man ,HaC I added some knowledge ,mysql It turns out that more than delete, also drop 、truncate.

1、delete

grammar ：

DELETE FROM t_table;

1) 、DELETE The sentence is DML（Data Manipulation Language, Data manipulation language ）, So it just deletes data , Do not delete table structure . Does not reduce the space occupied by tables or indexes .

2）、 The process of execution is to delete one row from the table at a time , It needs to be recorded in the log （binlog, If it's on binlog）, Execution is slow , You can add where Conditions .

Develop good habits ：delete Add limit Conditions , faster .
If you add limit 1 Hit the first one and return , You don't have to scan the whole table and go back .

3）、 The index will not be reset , Insert index is still from the last item you deleted +1 Start .

4）、MyISAM Will immediately free up disk space , and InnoDB It won't free up disk space , The data is just invisible to you . It's going to create holes , Mark as reusable , Next time you execute insert, It will cover this part of the space .

5） Go business , If you don't commit, have access to rollback . It also triggers triggers .

2、truncate

grammar ：

TRUNCATE TABLE t_table

1）、DDL(Data Definition Language, Database definition language ) operation , The space occupied by the tables and indexes will be restored to the original size .

2）、 No logging , Faster .

3）、 You can only delete table, Cannot be rolled back .

4）、 Delete all records in the table . Reset index position .

3、drop

grammar ：

DROP TABLE t_table

1）、DDL(Data Definition Language, Database definition language ) operation .

2）、 Delete everything about the table （ data / structure / constraint …）, And release all the space occupied by the table .

3）、 Cannot be rolled back , It doesn't trigger .

---

4、 Space release test

Create a new table , Insert 10w Data ,

CREATE TABLE `t_coke` (
  `id` bigint(20) NOT NULL AUTO_INCREMENT COMMENT ' Number ',
  `name` varchar(500) DEFAULT NULL COMMENT ' coke ',
  PRIMARY KEY (`id`) USING BTREE
) ENGINE=InnoDB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8 COMMENT=' coke ';

#  stored procedure , Insert  10w data 
CREATE DEFINER=`root`@`localhost` PROCEDURE `coke`()
begin
  declare i int;
  set i=1;
  while(i<=100000)do
    insert into t_coke values(i, "1984 Coca Cola ");
    set i=i+1;
  end while;
end

Copy two more tables ：

CREATE TABLE t_coke_copy1 SELECT * FROM t_coke;
CREATE TABLE t_coke_copy2 SELECT * FROM t_coke;

MySQL Of information_schema.TABLES Of DATA_LENGTH Records the data size of the table , We can use the size of this table .

SELECT
    concat( round( sum( DATA_LENGTH / 1024 / 1024 ), 2 ), 'M' ) AS table_size 
FROM
    information_schema.TABLES 
WHERE
    table_schema = 'test'  #  Database name 
    AND table_name = 't_coke_copy2'; #  Table name 

perform delete：

DELETE FROM t_coke_copy2;

Oh, Ho , The space is bigger , This is the table that creates holes .

Check the size of the hole ：

show table status from test like 't_coke_copy2';

The hole has 10m So big , It must have been before delete Through the data , There was no time to release .

perform delete After the statement , It doesn't immediately release space , You can use optimize table Instructions , But the instruction locks the table ; Or you can use MySQL Automatic cleaning of , It takes time .

OPTIMIZE TABLE t_coke_copy2;

OPTIMIZE after , Check the size again ：

It's the initialization size .

Although deleted , But your index continues to increase on the basis of the original ：

and truncate One step in place , Initialize the table directly .

5、 recovery

delete Recovery is easier （ The premise is to turn on binlog）：

# binlog Open or not 
show variables like 'log_bin';
# binlog  The format of 
SHOW VARIABLES LIKE 'binlog_format';

find binlog That's all right. , Reference resources ：

https://blog.csdn.net/huangba...

You can also use Flashback Tools for recovery .

If the production is accidentally used truncate and drop, How to recover data ？

Ready to run ？

duck No need ！ If your MySQL There's full backup , And back up in real time binlog.

1、 If HaC I was at noon 12 Click to delete a table . Last night in the morning 0 Point back up a library .
2、 Back to 0 Point Library ,
3、 take 12 Point to 0 Between the dots binlog To restore .（ find binlog Of drop table、truncate table Time nodes of ）
4、 Re execution drop table、truncate table Time nodes of At the current time node Of binlog The sentence of , This is equivalent to skipping the deletion statement .

therefore , Backup is important ！

6、 summary ：

	delete	truncate	drop
Security	low	in	high
Space	MyISAM Will immediately free up disk space , and InnoDB It won't free up disk space	Free up disk space immediately , Whether it's Innodb and MyISAM	Free up disk space immediately , Whether it's Innodb and MyISAM
recovery	recoverable	Irrecoverable	Irrecoverable
Speed	slow	in	fast
Business	Go business , Trigger trigger	No business , Don't trigger trigger	No business , Don't trigger trigger

To put it simply , I have a coke in my hand ,delete It's like hiding coke , Actually, the bottle is still , Coke is still there ;truncate It's to finish the coke , Left a bottle ;drop Just drink the coke and throw away the bottle .

Don't say the , The boss is going to urge me again , I'm going to have a coke , No , perform truncate.