![]() and Step 8(Transaction 8) is also doing a similar operation (insert into child, which is taking an `I nsert intension lock`) which is also blocked.(insert into child table, which is taking an `I nsert intension lock`) Which is making the Step 7 (Transaction 2) query to go into the wait state. As there is no record inserted into child table for parent_id=1 so this above given query will take a gap lock on parent_id greater than the last index record in our case it is, all ids greater than 1.In our case, Transaction 1(Step 3) is taking a Pessimistic lock on “p.id=1” but this query is taking a left join on the child table (parent_id) foreign key of parent.INSERT INTO child (id) VALUES (101) // Waiting. Transaction 2(Terminal 2)- client B will wait. The transaction takes an insert intention lock while it waits to obtain an exclusive lock. SELECT * FROM child WHERE id > 100 FOR UPDATE - | id | - | 102 | - Ĭlient B begins a transaction to insert a record into the gap. INSERT INTO child (id) values (90),(102) Insert Intention Locks:-Īn 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.Ĭlient A creates a table containing two index records (90 and 102) and then starts a transaction that places an exclusive lock on index records with an ID greater than 100. The exclusive lock includes a gap lock before record 102: Terminal 1: Client AĬREATE TABLE child (id int(11) NOT NULL, PRIMARY KEY(id)) ENGINE=InnoDB As a result, InnoDB can prevent Phantom-Read anomaly even if it’s transaction isolation level is Repeatable Read. This locking mechanism helps to prevent other transactions from inserting to the gap while the transaction reads the range. In addition to foreign-key constraint checking and duplicate key checking, gap locking is enabled for searches and an index scan if the transaction isolation level is above Repeatable Read.(Default one in MYSQL) We need to understand Gap Lock: Gap Lock: A gap lock is a lock on a gap between index records, or a lock on the gap before the first ![]() Select * from parent p left join child c on p.id=c.parent_id where p.id=2 for update - Step 6 // Pessimistic lock But, both queries are taking a lock on the different parent id? Strange isn’t it!!! select * from parent p left join child c on c.parent_id=p.id where p.id=1 for update - Step 3 // Pessimistic lock ? Main reason for Deadlock is Step 3 and Step 6. Let’s understand the main reason behind the deadlock. Now, you have re-produced the deadlock scenario. Insert child(`name`, `parent_id`, `created_at`) values('child 1', 2, now()) - Step 7 //Waiting for lock.īut, Successfully executed after Step 8. Select * from parent p left join child c on p.id=c.parent_id where p.id=2 for update - Step 6 Insert into parent values(2, 'example Order 2', now()) - step 5 Insert child(`name`, `parent_id`, `created_at`) values('child 1', 1, now()) - Step 8ĮRROR 1213 (40001): Deadlock found when trying to get lock try restarting transaction Terminal 2(Transaction 2) Select * from parent p left join child c on c.parent_id=p.id where p.id=1 for update - Step 3 // Pessimistic lock Insert into parent values(1, 'example Order 1', now()) - step 2 Open two MYSQL command prompt:- Terminal 1(Transaction 1) `parent_id` int(10) unsigned DEFAULT NULL,ĬONSTRAINT `child_ibfk_1` FOREIGN KEY (`parent_id`) REFERENCES `parent` (`id`) ) ENGINE=InnoDB AUTO_INCREMENT=2 DEFAULT CHARSET=utf8 `created_at` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP, `id` int(10) unsigned NOT NULL AUTO_INCREMENT, and MYSQL is doing a deadlock resolution by killing one of the transaction.įor better, understanding lets list down the steps to re-produce this kind of deadlock:įirst, create table schemas: CREATE TABLE `parent` (. ![]()
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