How would this deadlock happens? - mysql

Question

Recently I am handing a SQL issue, I have two transaction, the transaction A first got the Next-Key Locks, and the transaction B tried to get the same lock, so it was waiting, then the transaction A tried to get Insert Intention Locks, so the deadlock happened. But I am confuse that why would this happens?
Here is my table structure:
CREATE TABLE `changeset` (
`id` int(10) unsigned NOT NULL AUTO_INCREMENT',
`userId` int(10) NOT NULL COMMENT,
`documentId` varchar(20) NOT NULL,
`memberId` bigint(13) NOT NULL,
`createTime` bigint(13) NOT NULL,
`version` bigint(13) NOT NULL COMMENT,
`changesets` mediumtext,
PRIMARY KEY (`id`),
UNIQUE KEY `uniq_documentId_version` (`documentId`,`version`) USING BTREE
) ENGINE=InnoDB AUTO_INCREMENT=85771623 DEFAULT CHARSET=utf8
And here is my deadlock log:
(1) TRANSACTION:
TRANSACTION 22640, ACTIVE 66 sec starting index read
mysql tables in use 1, locked 1
LOCK WAIT 2 lock struct(s), heap size 1136, 1 row lock(s)
MySQL thread id 209, OS thread handle 123145559986176, query id 6204
localhost root Sending data
select * from changeset where documentId = '7oO5C_v' and version >=
13 for update
(1) WAITING FOR THIS LOCK TO BE GRANTED:
RECORD LOCKS space id 107 page no 15 n bits 704 index
uniq_documentId_version of table test.changeset trx id 22640
lock_mode X waiting
Record lock, heap no 2 PHYSICAL RECORD: n_fields 3; compact format;
info bits 0
0: len 7; hex 3976735431644a; asc 9vsT1dJ;;
1: len 8; hex 8000000000000000; asc ;;
2: len 4; hex 051cbef7; asc ;;
(2) TRANSACTION:
TRANSACTION 22639, ACTIVE 95 sec inserting
mysql tables in use 1, locked 1
6 lock struct(s), heap size 1136, 4 row lock(s), undo log entries 1
MySQL thread id 212, OS thread handle 123145561657344, query id 6210
localhost root update insert into changeset values (0, 9, '7oO5C_v',
814, 1, 13, 'x')
(2) HOLDS THE LOCK(S):
RECORD LOCKS space id 107 page no 15 n bits 704 index
uniq_documentId_version of table test.changeset trx id 22639
lock_mode X
Record lock, heap no 2 PHYSICAL RECORD: n_fields 3; compact format;
info bits 0
0: len 7; hex 3976735431644a; asc 9vsT1dJ;;
1: len 8; hex 8000000000000000; asc ;;
2: len 4; hex 051cbef7; asc ;;
(2) WAITING FOR THIS LOCK TO BE GRANTED:
RECORD LOCKS space id 107 page no 15 n bits 704 index
uniq_documentId_version of table test.changeset trx id 22639
lock_mode X locks gap before rec insert intention waiting
Record lock, heap no 2 PHYSICAL RECORD: n_fields 3; compact format;
info bits 0
0: len 7; hex 3976735431644a; asc 9vsT1dJ;;
1: len 8; hex 8000000000000000; asc ;;
2: len 4; hex 051cbef7; asc ;;
WE ROLL BACK TRANSACTION (1)

Answer 1

Have you used "(nolock) " in your selects?
Select * From Table with (nolock)