由select for update锁等待问题引发的深入思考
关于MySQL的加锁机制，其实十分复杂，不同的隔离级别，是否是主键或索引，锁的粒度等等。很多工作了很多年的MySQL DBA也不能把各种加锁场景一一讲清楚。有时候一个简单的锁等待场景都值得深入研究，大家更多的是知其然而不知其所以然。本文介绍的是一个很常见的锁等待问题，但很少有人知道其中的原理。一、实验场景
本文实验和研究的MySQL版本为8.0.31，数据库的隔离级别设置为RC，创建一张表，并在表中插入数据：
create table siri(
id int not null auto_increment,
a int not null,
b int not null,
c int not null,
primary key (id),
unique key uniq_a (a),
key idx_c (c)
)
insert into siri values (1,1,1,1),(2,2,2,2),(4,4,4,4),(6,6,6,4);好的，现在可以开始模拟实验场景了：
实验一：
Session1
Session2
mysql> begin;
Query OK, 0 rows affected (0.00 sec)
mysql> select * from siri where b=1 for update;
+----+---+---+---+
| id | a | b | c |
+----+---+---+---+
| 1 | 1 | 1 | 1 |
+----+---+---+---+
1 row in set (0.00 sec)
mysql> select * from siri where b=4 for update;
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
实验二：Session1
Session2
mysql> begin;
Query OK, 0 rows affected (0.00 sec)
mysql> select * from siri where id=1 for update;
+----+---+---+---+
| id | a | b | c |
+----+---+---+---+
| 1 | 1 | 1 | 1 |
+----+---+---+---+
1 row in set (0.00 sec)
mysql> select * from siri where b=4 for update;
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
实验三：
Session1
Session2
mysql> begin;
Query OK, 0 rows affected (0.00 sec)
mysql> select * from siri where b=1 for update;
+----+---+---+---+
| id | a | b | c |
+----+---+---+---+
| 1 | 1 | 1 | 1 |
+----+---+---+---+
1 row in set (0.00 sec)
mysql> select * from siri where id=4 for update;
+----+---+---+---+
| id | a | b | c |
+----+---+---+---+
| 4 | 4 | 4 | 4 |
+----+---+---+---+
1 row in set (0.00 sec)
从以上三个实验可以看出，session2是否被堵塞与session1中语句的条件字段是否是索引无关，而与session2中select for update语句的条件字段有关，session2中条件字段无索引则会被堵塞。mysql> select * from performance_schema.data_locks\G
*************************** 1. row ***************************
ENGINE: INNODB
ENGINE_LOCK_ID: 139907486244056:1220:139907418869440
ENGINE_TRANSACTION_ID: 3816000
THREAD_ID: 52900
EVENT_ID: 44
OBJECT_SCHEMA: test
OBJECT_NAME: siri
PARTITION_NAME: NULL
SUBPARTITION_NAME: NULL
INDEX_NAME: NULL
OBJECT_INSTANCE_BEGIN: 139907418869440
LOCK_TYPE: TABLE
LOCK_MODE: IX
LOCK_STATUS: GRANTED
LOCK_DATA: NULL
*************************** 2. row ***************************
ENGINE: INNODB
ENGINE_LOCK_ID: 139907486244056:59:4:2:139907418866384
ENGINE_TRANSACTION_ID: 3816000
THREAD_ID: 52900
EVENT_ID: 44
OBJECT_SCHEMA: test
OBJECT_NAME: siri
PARTITION_NAME: NULL
SUBPARTITION_NAME: NULL
INDEX_NAME: PRIMARY
OBJECT_INSTANCE_BEGIN: 139907418866384
LOCK_TYPE: RECORD
LOCK_MODE: X,REC_NOT_GAP
LOCK_STATUS: GRANTED
LOCK_DATA: 1
2 rows in set (0.00 sec)mysql> select * from sys.innodb_lock_waits\G
*************************** 1. row ***************************
wait_started: 2023-11-16 14:23:49
wait_age: 00:00:02
wait_age_secs: 2
locked_table: `test`.`siri`
locked_table_schema: test
locked_table_name: siri
locked_table_partition: NULL
locked_table_subpartition: NULL
locked_index: PRIMARY
locked_type: RECORD
waiting_trx_id: 3816028
waiting_trx_started: 2023-11-16 14:23:49
waiting_trx_age: 00:00:02
waiting_trx_rows_locked: 1
waiting_trx_rows_modified: 0
waiting_pid: 54820
waiting_query: select * from siri where b=4 for update
waiting_lock_id: 139907486245672:59:4:2:139907418878432
waiting_lock_mode: X,REC_NOT_GAP
blocking_trx_id: 3816020
blocking_pid: 54783
blocking_query: NULL
blocking_lock_id: 139907486244056:59:4:2:139907418866384
blocking_lock_mode: X,REC_NOT_GAP
blocking_trx_started: 2023-11-16 14:16:49
blocking_trx_age: 00:07:02
blocking_trx_rows_locked: 1
blocking_trx_rows_modified: 0
sql_kill_blocking_query: KILL QUERY 54783
sql_kill_blocking_connection: KILL 54783
1 row in set (0.01 sec)查询上面监控视图可以发现，在实验一和实验二中，session1所申请的锁资源也是一样的，一个是表级别的IX锁，一个是行级别的X锁。而造成锁等待的锁是行锁。所以这时候就有一个疑问了，行锁锁定的是b=1这一行，为啥session2中我们要申请b=4这一行的行锁会发生锁等待呢？其实原因也显而易见了：字段b无索引，申请b=4这一行的行锁会扫描全表，也就是说对表数据的每一行都会申请X锁。而在实验三中，可以走主键索引直接定位到b=4这一行，所以就不会造成锁等待了。
下面再看一个实验四：
Session1
Session2
mysql> begin;
Query OK, 0 rows affected (0.00 sec)
mysql> select * from siri where b=1 for update;
+----+---+---+---+
| id | a | b | c |
+----+---+---+---+
| 1 | 1 | 1 | 1 |
+----+---+---+---+
1 row in set (0.00 sec)
mysql> update siri set c=4 where b=4;
Query OK, 0 rows affected (0.00 sec)
Rows matched: 1 Changed: 0 Warnings: 0
可以发现，session2中直接对b=4这一行进行update是可以直接成功的，不会被阻塞。这说明update的加锁流程和select for update是不一样的。可以推测一下这两种加锁流程有什么区别：session2中update进行更新时也会扫描全表，但是遇到第一个锁等待时会做一个判断，发现锁住的行不是需要update的行时，则会跳过这个锁，这样就不会影响真正需要update的行，而select for update则不会做这个跳过，会一直等待锁。二、解读源码
为了验证我的猜想，深究背后的原理，还是得在实际场景下调试一下源码，阅读源码才能更好的了解为什么是这样的。
在mysql源码中，负责给行加锁的函数是sel_set_rec_lock，我们可以在该函数处打下断点，看看select for update和update这两种sql在申请锁的流程上面有什么区别。
/** Sets a lock on a record.
mostly due to we cannot reposition a record in R-Tree (with the
nature of splitting)
@param pcur cursor
@param rec record
@param index index
@param offsets rec_get_offsets(rec, index)
@param sel_mode select mode: SELECT_ORDINARY,
SELECT_SKIP_LOKCED, or SELECT_NO_WAIT
@param mode lock mode
@param type LOCK_ORDINARY, LOCK_GAP, or LOC_REC_NOT_GAP
@param thr query thread
@param mtr mtr
@return DB_SUCCESS, DB_SUCCESS_LOCKED_REC, or error code */
static inline dberr_t sel_set_rec_lock(btr_pcur_t *pcur, const rec_t *rec,
dict_index_t *index,
const ulint *offsets,
select_mode sel_mode, ulint mode,
ulint type, que_thr_t *thr, mtr_t *mtr) {
trx_t *trx;
dberr_t err = DB_SUCCESS;
const buf_block_t *block;
block = pcur->get_block();
trx = thr_get_trx(thr);
ut_ad(trx_can_be_handled_by_current_thread(trx));
if (UT_LIST_GET_LEN(trx->lock.trx_locks) > 10000) {
if (buf_LRU_buf_pool_running_out()) {
return (DB_LOCK_TABLE_FULL);
}
}
if (index->is_clustered()) {
err = lock_clust_rec_read_check_and_lock(
lock_duration_t::REGULAR, block, rec, index, offsets, sel_mode,
static_cast<lock_mode>(mode), type, thr);
} else {
if (dict_index_is_spatial(index)) {
if (type == LOCK_GAP || type == LOCK_ORDINARY) {
ib::error(ER_IB_MSG_1026) << "Incorrectly request GAP lock "
"on RTree";
ut_d(ut_error);
ut_o(return (DB_SUCCESS));
}
err = sel_set_rtr_rec_lock(pcur, rec, index, offsets, sel_mode, mode,
type, thr, mtr);
} else {
err = lock_sec_rec_read_check_and_lock(
lock_duration_t::REGULAR, block, rec, index, offsets, sel_mode,
static_cast<lock_mode>(mode), type, thr);
}
}
return (err);
}update语句是半一致性读，因此use_semi_consistent为true，select_mode为SELECT_SKIP_LOCKED，这表示会话不会浪费时间在创建锁等待上，可以跳过持有锁的行。而对于select for update语句，use_semi_consistent为false，select_mode为SELECT_ORDINARY，表示会话会创建一个锁等待，直到锁等待超时。
因此，对于实验四中的现象update不会被堵塞的原因已经比较清楚了，update在mysql内部被定义成了半一致性读（SELECT_SKIP_LOCKED），因此实验四的session2中update进行全表扫描读取主键时，读取到b=1这一列时，会跳过session1所持有的位于b=1行上的行锁，所以也就不会发生锁等待的现象。相反，实验二中select for update在mysql内部定义为普通读（SELECT_ORDINARY），读取到b=1这一列时，会被session1所持有的位于b=1行上的行锁堵塞，发生锁等待的现象。
来源:https://www.cnblogs.com/coygfly/p/18026342
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