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这篇文章主要介绍“Innodb 中为什么 lock in share mode 在 show engine 看不到行锁信息”，在日常操作中，相信很多人在 Innodb 中为什么 lock in share mode 在 show engine 看不到行锁信息问题上存在疑惑，丸趣 TV 小编查阅了各式资料，整理出简单好用的操作方法，希望对大家解答”Innodb 中为什么 lock in share mode 在 show engine 看不到行锁信息”的疑惑有所帮助！接下来，请跟着丸趣 TV 小编一起来学习吧！
一、问题提出
不知道有没有朋友和我一样用 lock in share mode 做加锁实验，但是却在 show engine innodb status 中看不到加锁信息，今天刚好有朋友在问 @在树枝上吹风，今天就做了一下简单的 debug，因为我也挺纳闷的。(我喜欢多问一个为什么也挺累的)
问题如下：
首先我开启了我的打印行锁参数，让加锁输出到日志中
mysql show variables like %gaopeng%
+--------------------------------+-------+| Variable_name | Value |
+--------------------------------+-------+
| gaopeng_mdl_detail | OFF || innodb_gaopeng_row_lock_detail | ON |
+--------------------------------+-------+然后跑如下语句
mysql show create table t \G
*************************** 1. row *************************** Table: t
Create Table: CREATE TABLE `t` ( `id` int(11) NOT NULL, `c` int(11) DEFAULT NULL, `d` int(11) DEFAULT NULL,
PRIMARY KEY (`id`),
KEY `c` (`c`)
) ENGINE=InnoDB DEFAULT CHARSET=utf81 row in set (0.00 sec)
mysql select * from t;
+----+------+------+| id | c | d |+----+------+------+| 0 | 0 | 0 || 5 | 5 | 5 || 10 | 10 | 10 || 15 | 15 | 15 || 20 | 20 | 20 || 25 | 25 | 25 |+----+------+------+6 rows in set (0.00 sec)
mysql begin;
Query OK, 0 rows affected (0.01 sec)
mysql select * from t where id=0 lock in share mode;
+----+------+------+| id | c | d |+----+------+------+| 0 | 0 | 0 |+----+------+------+1 row in set (4.21 sec)按理说这个时候应该在主键 ID= 0 这一行上了 LOCK_S，但是 show engine innodb 却看不到加锁信息如下：
------------
TRANSACTIONS
------------
Trx id counter 241482
Purge done for trx s n:o 241482 undo n:o 0 state: running but idle
History list length 182
Total number of lock structs in row lock hash table 1
LIST OF TRANSACTIONS FOR EACH SESSION:
---TRANSACTION 422211785606640, not started
0 lock struct(s), heap size 1160, 0 row lock(s)
---TRANSACTION 422211785605248, not started
0 lock struct(s), heap size 1160, 0 row lock(s)
--------
FILE I/O
--------根本看不到加锁信息。但是我的日志中却有输出如下：
2019-03-20T14:37:41.980845+08:00 10 [Note] InnoDB: TRX ID:(0) table:test/t index:PRIMARY space_id: 95 page_id:3 heap_no:2 row lock mode:LOCK_S|LOCK_NOT_GAP|
PHYSICAL RECORD: n_fields 5; compact format; info bits 0
0: len 4; hex 80000000; asc ;;
1: len 6; hex 00000003676a; asc gj;;
2: len 7; hex d8000000330110; asc 3 ;;
3: len 4; hex 80000000; asc ;;
4: len 4; hex 80000000; asc ;;因此我基本断定加锁肯定是做了的，但是为什么没有输出呢？
二、分析
我开始怀疑是否是提前释放了或者是打印的时候过滤掉了？后来发现都不是。看了到了一个 TRX_ID 为 422211785605248，这是只读事物的 TRX_ID 的形式，会不是因为应打印的时候只会打印读写的事物的锁结构信息，因为 Innodb 中读写事物有一个独立的链表，如果只打印这个链表上的信息就会出现这个问题。接着我做了一个事物先做了一个 delete 操作然后做 lock in share mode 语句可以看到 LOCK_S 结构就可以看到了，如下：
mysql begin;
Query OK, 0 rows affected (2.43 sec)
mysql delete from t2 limit 1; ## 这个语句我就显示的开始了一个读写事物 Query OK, 1 row affected (3.53 sec)
mysql select * from t where id=0 lock in share mode;
+----+------+------+| id | c | d |+----+------+------+| 0 | 0 | 0 |+----+------+------+1 row in set (2.98 sec)
mysql再来看看
---TRANSACTION 422211785606640, not started0 lock struct(s), heap size 1160, 0 row lock(s)
---TRANSACTION 422211785605248, not started0 lock struct(s), heap size 1160, 0 row lock(s)
---TRANSACTION 241482, ACTIVE 85 sec4 lock struct(s), heap size 1160, 2 row lock(s), undo log entries 1MySQL thread id 10, OS thread handle 140737153423104, query id 391 localhost root
TABLE LOCK table `test`.`t2` trx id 241482 lock mode IX
RECORD LOCKS space id 33 page no 19 n bits 624 index GEN_CLUST_INDEX of table `test`.`t2` trx id 241482 lock_mode X(LOCK_X) locks rec but not gap(LOCK_REC_NOT_GAP)
Record lock, heap no 447 PHYSICAL RECORD: n_fields 4; compact format; info bits 32
0: len 6; hex 00000000451d; asc E ;; 1: len 6; hex 00000003af4a; asc J;; 2: len 7; hex 3c000000453040; asc E0@;; 3: len 4; hex 80000001; asc ;;
TABLE LOCK table `test`.`t` trx id 241482 lock mode IS
RECORD LOCKS space id 95 page no 3 n bits 80 index PRIMARY of table `test`.`t` trx id 241482 lock mode S(LOCK_S) locks rec but not gap(LOCK_REC_NOT_GAP)
Record lock, heap no 2 PHYSICAL RECORD: n_fields 5; compact format; info bits 0
0: len 4; hex 80000000; asc ;; 1: len 6; hex 00000003676a; asc gj;; 2: len 7; hex d8000000330110; asc 3 ;; 3: len 4; hex 80000000; asc ;; 4: len 4; hex 80000000; asc ;;我们看到了 lock mode S(LOCK_S) locks rec but not gap(LOCK_REC_NOT_GAP) 的信息看来没有问题，猜测是实验是一样的，但是还是要源码验证一下。
三、源码验证 1、打印函数 lock_print_info_all_transactions
/*********************************************************************//**
Prints info of locks for each transaction. This function assumes that the
caller holds the lock mutex and more importantly it will release the lock
mutex on behalf of the caller. (This should be fixed in the future). */voidlock_print_info_all_transactions(/*=============================*/
FILE* file) /*! in/out: file where to print */{ ut_ad(lock_mutex_own()); fprintf(file, LIST OF TRANSACTIONS FOR EACH SESSION:\n
mutex_enter(trx_sys- mutex); /* First print info on non-active transactions */
/* NOTE: information of auto-commit non-locking read-only
transactions will be omitted here. The information will be
available from INFORMATION_SCHEMA.INNODB_TRX. */
PrintNotStarted print_not_started(file);// 建立一个结构体，目的是做 not start 事物的打印
ut_list_map(trx_sys- mysql_trx_list, print_not_started); // 这个地方打印出那些事物状态是 no start 的事物，但是这里存在一个问题，等会看代码在看。mysql_trx_list 是全事物。 const trx_t* trx;
TrxListIterator trx_iter; // 这个迭代器是 trx_sys- rw_trx_list 这个链表的迭代器
const trx_t* prev_trx = 0; /* Control whether a block should be fetched from the buffer pool. */
bool load_block = true; bool monitor = srv_print_innodb_lock_monitor (srv_show_locks_held != 0); while ((trx = trx_iter.current()) != 0) { // 通过迭代器进行迭代 ，显然这里不会有只读事物的信息。 check_trx_state(trx); if (trx != prev_trx) { lock_trx_print_wait_and_mvcc_state(file, trx);
prev_trx = trx; /* The transaction that read in the page is no
longer the one that read the page in. We need to
force a page read. */
load_block = true;
} /* If we need to print the locked record contents then we
need to fetch the containing block from the buffer pool. */
if (monitor) { /* Print the locks owned by the current transaction. */
TrxLockIterator lock_iter = trx_iter.lock_iter(); if (!lock_trx_print_locks( file, trx, lock_iter, load_block)) { /* Resync trx_iter, the trx_sys- mutex and
the lock mutex were released. A page was
successfully read in. We need to print its
contents on the next call to
lock_trx_print_locks(). On the next call to
lock_trx_print_locks() we should simply print
the contents of the page just read in.*/
load_block = false; continue;
}
}
load_block = true; /* All record lock details were printed without fetching
a page from disk, or we didn t need to print the detail. */
trx_iter.next();
}
lock_mutex_exit();
mutex_exit(trx_sys- mutex);
ut_ad(lock_validate());
}这个函数是调用的逻辑。
结构体 PrintNotStarted 括号重载
void operator()(const trx_t* trx)
{ ut_ad(trx- in_mysql_trx_list);
ut_ad(mutex_own( trx_sys- mutex)); /* See state transitions and locking rules in trx0trx.h */
if (trx_state_eq(trx, TRX_STATE_NOT_STARTED)) {// 这里我们发现只有状态为 TRX_STATE_NOT_STARTED 才会进行输出
fputs(--- , m_file);
trx_print_latched(m_file, trx, 600);
}
}我们这里可以看到只有状态为 TRX_STATE_NOT_STARTED 才会输出为 not start 状态。
TrxListIterator 迭代器初始化代码
TrxListIterator() : m_index()
{ /* We iterate over the RW trx list first. */
m_trx_list = trx_sys- rw_trx_list;
}我们这里可以看到只有读写事物才会进行锁结构的输出。
到此，关于“Innodb 中为什么 lock in share mode 在 show engine 看不到行锁信息”的学习就结束了，希望能够解决大家的疑惑。理论与实践的搭配能更好的帮助大家学习，快去试试吧！若想继续学习更多相关知识，请继续关注丸趣 TV 网站，丸趣 TV 小编会继续努力为大家带来更多实用的文章！
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