An article to talk about the auto-incrementing primary key in MySQL
Jul 05, 2022 am 10:08 AMThis article will give you an in-depth understanding of the auto-incrementing primary key in MySQL. I hope it will be helpful to you!
#1. Where is the self-increased value stored?
Different engines have different strategies for saving auto-incremented values
1. The auto-incremented value of the MyISAM engine is stored in the data file
2.The auto-incremented value of the InnoDB engine , in MySQL5.7 and previous versions, the self-incremented value is stored in memory and is not persisted. After each restart, when you open the table for the first time, you will find the maximum value of the auto-increment max(id), and then use the max(id) step size as the current auto-increment value of the table
select?max(ai_col)?from?table_name?for?update;
In MySQL8 .0 version records the changes in the self-increasing value in the redo log. When restarting, rely on the redo log to restore the value before restarting
2. Self-increasing value modification mechanism
If the field id is defined as AUTO_INCREMENT, when inserting a row of data, the behavior of auto-increment is as follows:
1. If the id field is specified as 0, null or unspecified value when inserting data, then this The current AUTO_INCREMENT value of the table is filled in the auto-increment field
2. If the id field specifies a specific value when inserting data, use the value specified in the statement directly
Assume that a certain value is to be inserted The value is X, and the current auto-increment value is Y
1. If The self-increment value is modified to a new self-increment value
The new self-increment value generation algorithm is: starting from auto_increment_offset (initial value), taking auto_increment_increment (step size) as the step size, and continuing to superpose until the first value is found. A value greater than Field, c is the only index. The table creation statement is as follows:CREATE?TABLE?`t`?( ??`id`?int(11)?NOT?NULL?AUTO_INCREMENT, ??`c`?int(11)?DEFAULT?NULL, ??`d`?int(11)?DEFAULT?NULL, ??PRIMARY?KEY?(`id`), ??UNIQUE?KEY?`c`?(`c`) )?ENGINE=InnoDB;Assume that there is already a record (1,1,1) in table t, and then execute another insert data command:
insert?into?t?values(null,?1,?1);The execution process is as follows:
1. The executor calls the InnoDB engine interface to write a row. The value of the passed row is (0,1,1)
2.InnoDB Find the value for which the auto-increment id is not specified, and obtain the current auto-increment value of table t 23. Change the value of the incoming row to (2,1,1)4 .Change the auto-increment value of the table to 35. Continue to insert data. Since the record of c=1 already exists, a Duplicate key error (unique key conflict) is reported, and the statement returns The corresponding execution flow chart is as follows:After that, when inserting a new data row, the auto-incremented ID obtained is 3. There is a situation where the auto-incrementing primary key is not continuous
Unique key conflicts and transaction rollbacks will lead to the situation where the auto-incrementing primary key id is not continuous
4. Optimization of lock increase
But in MySQL5. In version 0, the scope of self-increasing locks is statement level. In other words, if a statement applies for a table auto-increment lock, the lock will not be released until the statement is executed.
MySQL version 5.1.22 introduces a new strategy, a new parameter innodb_autoinc_lock_mode, the default value is 11. This parameter is set to 0, which means the strategy of the previous MySQL5.0 version is adopted, that is, the lock is released only after the statement is executed.
2. This parameter is set to 1In ordinary insert statements, the self-increasing lock is released immediately after the application.
For statements such as insert...select that insert data in batches, the self-increasing lock still has to wait until the statement is completed before being released
3. This parameter is set to 2. All actions for applying for an auto-incremented primary key are to release the lock after application. For the sake of data consistency, the default setting is 1- If sessionB releases the auto-increment lock immediately after applying for the auto-increment value, then the following situation may occur:
, sessionB continued to execute and inserted two records ( 4,3,3), (5,4,4)
- Ideas to solve this problem:
- 1) Let the original library insert data statements in batches to generate continuous id values. Therefore, the self-increasing lock is not released until the execution of the statement is completed, just to achieve this purpose
- 2) Record the operations of inserting data in the binlog truthfully, so that when the standby database is executed, it no longer relies on the self-increasing lock. Add primary key to generate. That is, set innodb_autoinc_lock_mode to 2 and binlog_format to row
如果有批量插入數(shù)據(jù)(insert … select、replace … select和load data)的場景時(shí),從并發(fā)插入數(shù)據(jù)性能的角度考慮,建議把innodb_autoinc_lock_mode設(shè)置為2,同時(shí)binlog_format設(shè)置為row,這樣做既能并發(fā)性,又不會(huì)出現(xiàn)數(shù)據(jù)一致性的問題
對(duì)于批量插入數(shù)據(jù)的語句,MySQL有一個(gè)批量申請(qǐng)自增id的策略:
1.語句執(zhí)行過程中,第一次申請(qǐng)自增id,會(huì)分配1個(gè)
2.1個(gè)用完以后,這個(gè)語句第二次申請(qǐng)自增id,會(huì)分配2個(gè)
3.2個(gè)用完以后,還是這個(gè)語句,第三次申請(qǐng)自增id,會(huì)分配4個(gè)
4.依次類推,同一個(gè)語句去申請(qǐng)自增id,每次申請(qǐng)到的自增id個(gè)數(shù)都是上一次的兩倍
insert?into?t?values(null,?1,1); insert?into?t?values(null,?2,2); insert?into?t?values(null,?3,3); insert?into?t?values(null,?4,4); create?table?t2?like?t; insert?into?t2(c,d)?select?c,d?from?t; insert?into?t2?values(null,?5,5);
insert … select,實(shí)際上往表t2中插入了4行數(shù)據(jù)。但是,這四行數(shù)據(jù)是分三次申請(qǐng)的自增id,第一次申請(qǐng)到了id=1,第二次被分配了id=2和id=3,第三次被分配到id=4到id=7
由于這條語句實(shí)際上只用上了4個(gè)id,所以id=5到id=7就被浪費(fèi)掉了。之后,再執(zhí)行insert into t2 values(null, 5,5)
,實(shí)際上插入了的數(shù)據(jù)就是(8,5,5)
這是主鍵id出現(xiàn)自增id不連續(xù)的第三種原因
五、自增主鍵用完了
自增主鍵字段在達(dá)到定義類型上限后,再插入一行記錄,則會(huì)報(bào)主鍵沖突的錯(cuò)誤
CREATE?TABLE?t?(?id?INT?UNSIGNED?auto_increment?PRIMARY?KEY?)?auto_increment?=?4294967295; INSERT?INTO?t?VALUES(NULL); INSERT?INTO?t?VALUES(NULL);
第一個(gè)insert語句插入數(shù)據(jù)成功后,這個(gè)表的AUTO_INCREMENT沒有改變(還是4294967295),就導(dǎo)致了第二個(gè)insert語句又拿到相同的自增id值,再試圖執(zhí)行插入語句,報(bào)主鍵沖突錯(cuò)誤
【相關(guān)推薦:mysql視頻教程】
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