MySQL Locking

Abstract

看到孤獨煙回答粉絲的一個問題, 就想要研究一下MySQL 鎖的情況。下面是對MySQL manual locking的一個筆記。

Share & Exclusive Lock

• A shared (S) lock permits the transaction that holds the lock to read a row.

• An exclusive (X) lock permits the transaction that holds the lock to update or delete a row.

Intention Lock

• Intention locks are table-level locks that indicate which type of lock (shared or exclusive) a transaction requires later for a row in a table.
• SELECT ... LOCK IN SHARE MODE sets an IS lock, and SELECT ... FOR UPDATE sets an IX lock.(after version 8.0, share mode used this way select ... for share, consistent with update)

	X	S	IX	IS
X	Conflict	Conflict	Conflict	Conflict
S	Conflict	Compatible	Conflict	Compatible
IX	Conflict	Conflict	Compatible	Compatible
IS	Conflict	Compatible	Compatible	Compatible

The intention locking protocol is as follows:

• Before a transaction can acquire a shared lock on a row in a table, it must first acquire an IS lock or stronger on the table.

• Before a transaction can acquire an exclusive lock on a row in a table, it must first acquire an IX lock on the table.

Record Lock

• A record lock is a lock on an index record.
• Record locks always lock index records, even if a table is defined with no indexes. For such cases, InnoDB creates a hidden clustered index and uses this index for record locking

Example:

CREATE TABLE `tt` (
  `a` int(11) NOT NULL DEFAULT '0',
  `b` varchar(10) COLLATE utf8mb4_bin DEFAULT NULL,
  `c` varchar(10) COLLATE utf8mb4_bin DEFAULT NULL,
  PRIMARY KEY (`a`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin;

insert into tt values (1, 'aaa', 'ccc');
insert into tt values (2, 'bbb', 'ccc');
insert into tt values (3, 'ccc', 'ccc');


start transaction;
select * from tt where a = 1 for update;

show engine innodb status;

可以看到如下輸出,在 table 上有一個 IX 鎖，在行上有一個 X 的 record lock.

2 lock struct(s), heap size 376, 1 row lock(s)
MySQL thread id 127707691, OS thread handle 0x7efbeb65d700, query id 4117992277 10.16.163.157 mams_test
TABLE LOCK table `test1`.`tt` trx id A06213D6 lock mode IX
RECORD LOCKS space id 6784 page no 3 n bits 72 index `PRIMARY` of table `test1`.`tt` trx id A06213D6 lock_mode X locks rec but not gap
Record lock, heap no 6 PHYSICAL RECORD: n_fields 5; compact format; info bits 0
 0: len 4; hex 80000001; asc     ;;
 1: len 6; hex 0000a0600c90; asc    `  ;;
 2: len 7; hex 780000c61b1e0f; asc x      ;;
 3: len 3; hex 646464; asc ddd;;
 4: SQL NULL;

ps: 執行show engine innodb status之前需要先將lock 監控打開，需要創建兩張表，具體信息可見Enabling InnoDB Monitors

CREATE TABLE innodb_monitor (a INT) ENGINE=INNODB;
CREATE TABLE innodb_lock_monitor (a INT) ENGINE=INNODB;

無index Example:

create table ttp (a int, b varchar(10));
insert into ttp values (1, 'cc');
insert into ttp values (1, 'dd');

start transaction;
select * from ttp where b = 'dd' for update;

show engine innodb status;

輸出如下，可以看到MySQL自動生成了一個clustered index,而且由於沒有索引，這次將表裏的兩條數據全部加了 X 鎖：

TABLE LOCK table `test1`.`ttp` trx id A0622BA2 lock mode IX
RECORD LOCKS space id 6789 page no 3 n bits 72 index `GEN_CLUST_INDEX` of table `test1`.`ttp` trx id A0622BA2 lock_mode X
Record lock, heap no 1 PHYSICAL RECORD: n_fields 1; compact format; info bits 0
 0: len 8; hex 73757072656d756d; asc supremum;;

Record lock, heap no 2 PHYSICAL RECORD: n_fields 5; compact format; info bits 0
 0: len 6; hex 000001ae524d; asc     RM;;
 1: len 6; hex 0000a0622b13; asc    b+ ;;
 2: len 7; hex e1000014d20110; asc        ;;
 3: len 4; hex 80000001; asc     ;;
 4: len 2; hex 6363; asc cc;;

Record lock, heap no 3 PHYSICAL RECORD: n_fields 5; compact format; info bits 0
 0: len 6; hex 000001ae524e; asc     RN;;
 1: len 6; hex 0000a0622b2b; asc    b++;;
 2: len 7; hex f9000084630110; asc     c  ;;
 3: len 4; hex 80000001; asc     ;;
 4: len 2; hex 6464; asc dd;;

Gap Lock

• A gap lock is a lock on a gap between index records, or a lock on the gap before the first or after the last index record.
• Used in REPEATABLE READ and SERIALIZABLE isolation level. You can disable gap lock explicitly by changing the isolation level to READ COMMITTED
• Gap locking is not needed for statements that lock rows using a unique indexto search for a unique row. (This does not include the case that the search condition includes only some columns of a multiple-column unique index; in that case, gap locking does occur.)
• Gap locks in InnoDB are “purely inhibitive”, which means that their only purpose is to prevent other transactions from inserting to the gap.Gap locks can co-exist. A gap lock taken by one transaction does not prevent another transaction from taking a gap lock on the same gap. There is no difference between shared and exclusive gap locks. They do not conflict with each other, and they perform the same function.
• Gap locking can be disabled explicitly.
  • change the transaction isolation level to READ COMMITTED.
  • set innodb_locks_unsafe_for_binlog = 1(not in 8.0 doc)

Example:

create table ttp (a int, b varchar(10));
insert into ttp values (1, 'cc');
insert into ttp values (2, 'dd');
insert into ttp values (20, 'dd');
insert into ttp values (25, 'dd');
create index idx_a on ttp(a); -- 必不可少

delete from ttp where a = 23;

show engine innodb status 顯示如下：

TABLE LOCK table `test1`.`ttp` trx id A0640DCF lock mode IX
RECORD LOCKS space id 6789 page no 4 n bits 72 index `idx_a` of table `test1`.`ttp` trx id A0640DCF lock_mode X locks gap before rec
Record lock, heap no 5 PHYSICAL RECORD: n_fields 2; compact format; info bits 0
 0: len 4; hex 80000019; asc     ;;
 1: len 6; hex 000001ae5251; asc     RQ;;

執行insert into ttp values(22, '');``insert into ttp values(24, '');都不可以，但是insert into ttp values(25, '');是OK的，說明不是一個next-key lock.

Next-key Lock

• A next-key lock is a combination of a record lock on the index record and a gap lock on the gap before the index record.
• InnoDB performs row-level locking in such a way that when it searches or scans a table index(which prevents phantom rows), it sets shared or exclusive locks on the index records it encounters.
• A next-key lock is an index-record lock plus a gap lock on the gap preceding the index record.

next-key lock range example:
Suppose that an index contains the values 10, 11, 13, and 20.

(negative infinity, 10]
(10, 11]
(11, 13]
(13, 20]
(20, positive infinity)

For the last interval, the next-key lock locks the gap above the largest value in the index and the “supremum” pseudo-record having a value higher than any value actually in the index. The supremum is not a real index record, so, in effect, this next-key lock locks only the gap following the largest index value.

Example:

select * from ttp where a> 20 for update;

show engine innodb status shows:

TABLE LOCK table `test1`.`ttp` trx id A067CC23 lock mode IX
RECORD LOCKS space id 6789 page no 4 n bits 80 index `idx_a` of table `test1`.`ttp` trx id A067CC23 lock_mode X
Record lock, heap no 1 PHYSICAL RECORD: n_fields 1; compact format; info bits 0
 0: len 8; hex 73757072656d756d; asc supremum;;

Insert Intention Locks

• An insert intention lock is a type of gap lock set by INSERT operations prior to row insertion.
• This lock signals the intent to insert in such a way that multiple transactions inserting into the same index gap need not wait for each other if they are not inserting at the same position within the gap.

Example:

session1:
start transaction;
select * from tt where a > 100 for update;


session2:
start transaction;
insert into tt(a) values (91); --waiting

show engine innodb status;shows

insert into tt(a) values (91)
------- TRX HAS BEEN WAITING 4 SEC FOR THIS LOCK TO BE GRANTED:
RECORD LOCKS space id 6784 page no 3 n bits 80 index `PRIMARY` of table `test1`.`tt` trx id A068EAF9 lock_mode X locks gap before rec insert intention waiting
Record lock, heap no 4 PHYSICAL RECORD: n_fields 5; compact format; info bits 0
 0: len 4; hex 80000066; asc    f;;
 1: len 6; hex 0000a068e94a; asc    h J;;
 2: len 7; hex d600005a94011d; asc    Z   ;;
 3: SQL NULL;
 4: SQL NULL;

------------------
TABLE LOCK table `test1`.`tt` trx id A068EAF9 lock mode IX
RECORD LOCKS space id 6784 page no 3 n bits 80 index `PRIMARY` of table `test1`.`tt` trx id A068EAF9 lock_mode X locks gap before rec insert intention waiting
Record lock, heap no 4 PHYSICAL RECORD: n_fields 5; compact format; info bits 0
 0: len 4; hex 80000066; asc    f;;
 1: len 6; hex 0000a068e94a; asc    h J;;
 2: len 7; hex d600005a94011d; asc    Z   ;;
 3: SQL NULL;
 4: SQL NULL;

---TRANSACTION A068E9BF, ACTIVE 66 sec
2 lock struct(s), heap size 376, 2 row lock(s)
MySQL thread id 127963261, OS thread handle 0x7efbf5bb0700, query id 4122875322 10.16.163.157 mams_test
TABLE LOCK table `test1`.`tt` trx id A068E9BF lock mode IX
RECORD LOCKS space id 6784 page no 3 n bits 80 index `PRIMARY` of table `test1`.`tt` trx id A068E9BF lock_mode X
Record lock, heap no 1 PHYSICAL RECORD: n_fields 1; compact format; info bits 0
 0: len 8; hex 73757072656d756d; asc supremum;;

Record lock, heap no 4 PHYSICAL RECORD: n_fields 5; compact format; info bits 0
 0: len 4; hex 80000066; asc    f;;
 1: len 6; hex 0000a068e94a; asc    h J;;
 2: len 7; hex d600005a94011d; asc    Z   ;;
 3: SQL NULL;
 4: SQL NULL;

compatiblity table:

acquiring ↓/acquired→	Gap	Insert Intention	Record
Gap	Compatible	Compatible	Compatible
Insert Intention	Conflict	Compatible	Compatible
Record	Conflict	Compatible	Compatible

AUTO-INC Locks

• An AUTO-INC lock is a special table-level lock taken by transactions inserting into tables with AUTO_INCREMENT columns.

Extended Read

SELECT statements will not lock any rows while running which is generally correct, however there a notable exception – INSERT INTO table1 SELECT * FROM table2. This statement will perform locking read (shared locks) for table2 table. It also applies to similar tables with where clause and joins.

from INSERT INTO … SELECT Performance with Innodb tables.

If a non-INSERT write operation is more likely to not match any row because the INSERT part is yet to come on the transaction, don’t do it or use REPLACE INTO or use READ-COMMITTED transaction isolation.

from One more InnoDB gap lock to avoid

REPEATABLE READ
For locking reads (SELECT with FOR UPDATE or LOCK IN SHARE MODE), UPDATE, and DELETE statements, locking depends on whether the statement uses a unique index with a unique search condition, or a range-type search condition.

• For a unique index with a unique search condition, InnoDB locks only the index record found, not the gap before it.
• For other search conditions, InnoDB locks the index range scanned, using gap locks or next-key locks to block insertions by other sessions into the gaps covered by the range. For information about gap locks and next-key locks.

from Transaction Isolation Levels