[MySQL special] read lock and write lock

mysql There are four queues in , Respectively store the information of the current acquisition of read lock 、 Currently obtain the information of write lock 、 Wait for the lock reading message 、 Wait for the message of writing the lock ：

queue	meaning
current read-lock queue	Currently acquire read lock queue
current write-lock queue	Currently get write lock queue
pending read-lock queue	Waiting for lock reading queue
pending write-lock queue	Waiting for the write lock queue

 

Acquisition conditions of read lock

1. The current resource is not written and the lock is occupied .

2. There is currently no Waiting for the write lock queue There is no high priority write lock waiting in .

If the above conditions are met , The read lock can be obtained directly , If one cannot be satisfied , entering PRQ

 

Write lock acquisition conditions

1. The current resource is not occupied by write lock .

2. PWQ No waiting to write lock

3. RQ No read lock

lock , It's a computer .

Existing write lock , Repel any lock

as everyone knows , Write lock repels everything , For data security , And the accuracy of data reading , Write lock repels all other forms of lock .

Writing locks is like playing 【 Write 】 The computer game , Others must not play at the same time 【 Write 】 This computer , Want to play 【 Intention to write 】 I can't play . meanwhile , People who play games are also selfish , Afraid of losing and losing face , Don't let others watch 【 read 】, Want to see 【 Intentional reading 】 Don't let me see .

Existing lock reading , Exclude all forms of write lock , Including intentional write lock

When there is a read lock , No more write locks . But concurrent reads are allowed . Although intention lock is a kind of intention , The lock is not in the real sense , But there is also the truth that reading and writing are exclusive .

Reading a lock is like watching it on a computer 【 read 】 The movie , Others must not play at the same time 【 Write 】 The computer game , Want to play 【 Intention to write 】 I can't play . meanwhile , People who watch movies know how to share , Invite others to watch 【 read 】, Want to see 【 Intentional reading 】 Let's see .

 

 

When writing lock with existing intention , Exclude all entity locks

Intention writing lock , It means I want to write , The next step is to really get the write lock , Therefore, entity read / write locks will be excluded , But intention is intention after all , No practical significance has been obtained , You can't help but allow others to imagine .

Intentional lock writing is like waiting in line for the computer to play 【 Write 】 game , The next one is you , Waiting in line for so long , Others come to play 【 Write 】 You definitely don't agree with the game , Others come to see 【 read 】 You must not agree to the movie . Someone made a wish ：” I hope it's time for me to play games next 【 Intention to write 】“, You say? ：” If there is no accident, it will come to me right away , You go on YY Well , Allow you to think “. Someone made a wish ：” I hope it's my next movie 【 Intentional reading 】“, You say? ：” If there is no accident, it will come to me right away , You go on YY Well , Allow you to think “.

  When the existing intention reads the lock , Exclude entity write lock

Intention to write lock is an intention , Intend to read , The next step is to acquire the read lock . This will exclude the actual write lock . As mentioned earlier, concurrent reading is allowed , Therefore, read lock will not be excluded . Intention is intention after all , You can't help but allow others to imagine .

Intentional lock reading is like waiting in line 【 Write 】 The movie , The next one is you , Others come to play 【 Write 】 You definitely don't agree with the game , Others come to see 【 read 】 You readily agree to the movie （ After all, I haven't arrived yet , Besides, he may invite me to watch ）. Someone made a wish ：” I hope it's time for me to play games next 【 Intention to write 】“, You say? ：” If there is no accident, it will come to me right away , You go on YY Well , Allow you to think “. Someone made a wish ：” I hope it's my next movie 【 Intentional reading 】“, You say? ：” If there is no accident, it will come to me right away , Keep making wishes , Maybe we can watch it together “.

 

Update Use where When the conditions , Hit index , Lock a line , Row write lock use

 

Update Use where When the conditions , No hit index , Lock table

update Use where When the conditions , Hit index , But the scope of use , Locking clearance

 

update Use where Conditions , Some conditions use indexes , Lock all data rows covered by this index , Data row lock

 

Both transactions operate on two tables , Acquire different locks successively , Lead to a deadlock