个人中心
文章发布
退出
作者热门文章
- mongodb - 在 MongoDB mapreduce 中，如何展平值对象？
- javascript - 对象传播与 Object.assign
- html - 输入类型 ="submit"Vs 按钮标签它们可以互换吗？
- sql - 使用 MongoDB 而不是 MS SQL Server 的优缺点
26
4
我有一个复杂的查询，它创建了一个可能需要 5 秒或更长时间才能运行的临时表。当同时在类似表上运行另一个事务时，这似乎会导致死锁。我无法在本地复制，但在生产中，我可以每隔几天让它发生 1 次。 (我记录了 mysql 错误)
查询非常复杂(显示在页面底部)；但你不需要理解逻辑；只是它从一堆表和连接中进行选择，并且可能需要一段时间才能运行。
我还有一个事务可以插入到许多相同的表中。我偶尔会收到 mysql 错误 1213: Deadlock found when trying to get lock;尝试重启事务。
这是交易的伪代码
START TRANSACTION
INSERT INTO phppos_sales
INSERT MANY RECORDS INTO phppos_sales_items
INSERT MANY RECORDS INTO phppos_sales_items_taxes
INSERT MANY RECORDS INTO phppos_sales_payments
END TRANSACTION
我该如何解决这个死锁？我尝试将 isolation level 更改为 READ UNCOMMITTED 但 mysql 设置不允许这样做；而且我需要在我无法控制服务器的各种环境中进行这项工作。
更改隔离级别时出错:
Cannot execute statement: impossible to write to binary log since BINLOG_FORMAT = STATEMENT and at least one table uses a storage engine limited to row-based logging. InnoDB is limited to row-logging when transaction isolation level is READ COMMITTED or READ UNCOMMITTED.
INNODB 引擎状态:
mysql> SHOW ENGINE INNODB STATUS;
| Type | Name | Status |
| InnoDB | |
=====================================
140520 12:00:17 INNODB MONITOR OUTPUT
=====================================
Per second averages calculated from the last 15 seconds
-----------------
BACKGROUND THREAD
-----------------
srv_master_thread loops: 1766819 1_second, 1766816 sleeps, 167043 10_second, 100947 background, 100945 flush
srv_master_thread log flush and writes: 1776023
----------
SEMAPHORES
----------
OS WAIT ARRAY INFO: reservation count 236559, signal count 288374
Mutex spin waits 546890, rounds 1796579, OS waits 33216
RW-shared spins 205374, rounds 5519210, OS waits 176937
RW-excl spins 5661, rounds 841678, OS waits 23933
Spin rounds per wait: 3.29 mutex, 26.87 RW-shared, 148.68 RW-excl
------------------------
LATEST FOREIGN KEY ERROR
------------------------
140520 11:27:44 Transaction:
TRANSACTION 86D125F, ACTIVE 0 sec inserting
mysql tables in use 1, locked 1
15 lock struct(s), heap size 3112, 6 row lock(s), undo log entries 2
MySQL thread id 1910245, OS thread handle 0x7fbf0042e700, query id 56114114 php-pos-web 10.181.16.33 phppoint update
INSERT INTO `phppos_sales_items_taxes` (`sale_id`, `item_id`, `line`, `name`, `percent`, `cumulative`) VALUES (11763, 1115, 3, 'PST', '8.000', '0')
Foreign key constraint fails for table `phppoint_fatpanda`.`phppos_sales_items_taxes`:
,
CONSTRAINT `phppos_sales_items_taxes_ibfk_1` FOREIGN KEY (`sale_id`) REFERENCES `phppos_sales_items` (`sale_id`)
Trying to add in child table, in index `PRIMARY` tuple:
DATA TUPLE: 8 fields;
0: len 4; hex 80002df3; asc - ;;
1: len 4; hex 8000045b; asc [;;
2: len 4; hex 80000003; asc ;;
3: len 3; hex 505354; asc PST;;
4: len 8; hex 8000000000080000; asc ;;
5: len 6; hex 0000086d125f; asc m _;;
6: len 7; hex 00000000000000; asc ;;
7: len 4; hex 80000000; asc ;;
But in parent table `phppoint_fatpanda`.`phppos_sales_items`, in index `PRIMARY`,
the closest match we can find is record:
PHYSICAL RECORD: n_fields 11; compact format; info bits 0
0: len 4; hex 80002df1; asc - ;;
1: len 4; hex 8000049a; asc ;;
2: len 4; hex 80000001; asc ;;
3: len 6; hex 0000086cfd29; asc l );;
4: len 7; hex f400000216012c; asc ,;;
5: len 0; hex ; asc ;;
6: len 0; hex ; asc ;;
7: len 11; hex 8000000000010000000000; asc ;;
8: len 11; hex 8000000000100000000000; asc ;;
9: len 11; hex 80000000002d0000000000; asc - ;;
10: len 4; hex 80000000; asc ;;
------------------------
LATEST DETECTED DEADLOCK
------------------------
140520 11:27:44
*** (1) TRANSACTION:
TRANSACTION 86D11A3, ACTIVE 2 sec fetching rows
mysql tables in use 9, locked 9
LOCK WAIT 364 lock struct(s), heap size 47544, 80177 row lock(s)
MySQL thread id 1910243, OS thread handle 0x7fbeb2090700, query id 56113840 10.181.26.42 phppoint Copying to tmp table
CREATE TEMPORARY TABLE phppos_sales_items_temp
(SELECT phppos_sales.deleted as deleted,phppos_sales.deleted_by as deleted_by, sale_time, date(sale_time) as sale_date, phppos_sales_items.sale_id, comment,payment_type, customer_id, employee_id,
phppos_items.item_id, NULL as item_kit_id, supplier_id, quantity_purchased, item_cost_price, item_unit_price, category,
discount_percent, (item_unit_price*quantity_purchased-item_unit_price*quantity_purchased*discount_percent/100) as subtotal,
phppos_sales_items.line as line, serialnumber, phppos_sales_items.description as description,
(item_unit_price*quantity_purchased-item_unit_price*quantity_purchased*discount_percent/100)+(item_unit_price*quantity_purchased-item_unit_price*quantity_purchased*discount_percent/100)*(SUM(CASE WHEN cumulative != 1 THEN percent ELSE 0 END)/100)
+(((item_unit_price*quantity_purchased-item_unit_price*quanti
*** (1) WAITING FOR THIS LOCK TO BE GRANTED:
RECORD LOCKS space id 454941 page no 114 n bits 408 index `location_id` of table `phppoint_fatpanda`.`phppos_sales` trx id 86D11A3 lock mode S waiting
Record lock, heap no 335 PHYSICAL RECORD: n_fields 2; compact format; info bits 0
0: len 4; hex 80000001; asc ;;
1: len 4; hex 80002df3; asc - ;;
*** (2) TRANSACTION:
TRANSACTION 86D125D, ACTIVE 0 sec inserting
mysql tables in use 1, locked 1
23 lock struct(s), heap size 3112, 12 row lock(s), undo log entries 10
MySQL thread id 1910245, OS thread handle 0x7fbf0042e700, query id 56114091 php-pos-web 10.181.16.33 phppoint update
INSERT INTO `phppos_sales_items_taxes` (`sale_id`, `item_id`, `line`, `name`, `percent`, `cumulative`) VALUES (11763, 1178, 2, 'GST', '5.000', '0')
*** (2) HOLDS THE LOCK(S):
RECORD LOCKS space id 454941 page no 114 n bits 408 index `location_id` of table `phppoint_fatpanda`.`phppos_sales` trx id 86D125D lock_mode X locks rec but not gap
Record lock, heap no 335 PHYSICAL RECORD: n_fields 2; compact format; info bits 0
0: len 4; hex 80000001; asc ;;
1: len 4; hex 80002df3; asc - ;;
*** (2) WAITING FOR THIS LOCK TO BE GRANTED:
RECORD LOCKS space id 454945 page no 386 n bits 288 index `PRIMARY` of table `phppoint_fatpanda`.`phppos_sales_items_taxes` trx id 86D125D lock_mode X insert intention waiting
Record lock, heap no 1 PHYSICAL RECORD: n_fields 1; compact format; info bits 0
0: len 8; hex 73757072656d756d; asc supremum;;
*** WE ROLL BACK TRANSACTION (2)
------------
TRANSACTIONS
------------
Trx id counter 86E47F7
Purge done for trx's n:o < 86E45C0 undo n:o < 0
History list length 1418
LIST OF TRANSACTIONS FOR EACH SESSION:
---TRANSACTION 86E47F6, not started
MySQL thread id 1913171, OS thread handle 0x7fbeb2090700, query id 56205829 10.181.26.42 phppoint
---TRANSACTION 0, not started
MySQL thread id 1913095, OS thread handle 0x7fbf005b4700, query id 56205830 localhost root
SHOW ENGINE INNODB STATUS
--------
FILE I/O
--------
I/O thread 0 state: waiting for completed aio requests (insert buffer thread)
I/O thread 1 state: waiting for completed aio requests (log thread)
I/O thread 2 state: waiting for completed aio requests (read thread)
I/O thread 3 state: waiting for completed aio requests (read thread)
I/O thread 4 state: waiting for completed aio requests (read thread)
I/O thread 5 state: waiting for completed aio requests (read thread)
I/O thread 6 state: waiting for completed aio requests (write thread)
I/O thread 7 state: waiting for completed aio requests (write thread)
I/O thread 8 state: waiting for completed aio requests (write thread)
I/O thread 9 state: waiting for completed aio requests (write thread)
Pending normal aio reads: 0 [0, 0, 0, 0] , aio writes: 0 [0, 0, 0, 0] ,
ibuf aio reads: 0, log i/o's: 0, sync i/o's: 0
Pending flushes (fsync) log: 0; buffer pool: 0
3599456 OS file reads, 9300371 OS file writes, 3988632 OS fsyncs
0.27 reads/s, 16384 avg bytes/read, 13.07 writes/s, 7.27 fsyncs/s
-------------------------------------
INSERT BUFFER AND ADAPTIVE HASH INDEX
-------------------------------------
Ibuf: size 1, free list len 84, seg size 86, 55915 merges
merged operations:
insert 68506, delete mark 4761, delete 38
discarded operations:
insert 0, delete mark 0, delete 0
Hash table size 2212699, node heap has 751 buffer(s)
5050.86 hash searches/s, 624.09 non-hash searches/s
---
LOG
---
Log sequence number 184365806376
Log flushed up to 184365806376
Last checkpoint at 184365791715
0 pending log writes, 0 pending chkp writes
2149282 log i/o's done, 3.47 log i/o's/second
----------------------
BUFFER POOL AND MEMORY
----------------------
Total memory allocated 1098907648; in additional pool allocated 0
Dictionary memory allocated 62951505
Buffer pool size 65536
Free buffers 1
Database pages 64784
Old database pages 23894
Modified db pages 88
Pending reads 0
Pending writes: LRU 0, flush list 0, single page 0
Pages made young 4073451, not young 0
0.07 youngs/s, 0.00 non-youngs/s
Pages read 3592230, created 1542046, written 6130789
0.27 reads/s, 18.00 creates/s, 6.40 writes/s
Buffer pool hit rate 1000 / 1000, young-making rate 0 / 1000 not 0 / 1000
Pages read ahead 0.00/s, evicted without access 0.00/s, Random read ahead 0.00/s
LRU len: 64784, unzip_LRU len: 0
I/O sum[382]:cur[0], unzip sum[0]:cur[0]
--------------
ROW OPERATIONS
--------------
0 queries inside InnoDB, 0 queries in queue
1 read views open inside InnoDB
Main thread process no. 15866, id 140457065543424, state: sleeping
Number of rows inserted 77431960, updated 1673031, deleted 160450, read 4825684197
1103.93 inserts/s, 2.53 updates/s, 0.00 deletes/s, 7772.15 reads/s
----------------------------
END OF INNODB MONITOR OUTPUT
============================
|
+-------
查询这是死锁的一部分:(连同上面的交易)
CREATE temporary TABLE phppos_sales_items_temp
(SELECT
phppos_sales.deleted AS deleted,
phppos_sales.deleted_by AS deleted_by,
sale_time,
Date(sale_time) AS sale_date,
phppos_sales_items.sale_id,
comment,
payment_type,
customer_id,
employee_id,
phppos_items.item_id,
NULL AS item_kit_id,
supplier_id,
quantity_purchased,
item_cost_price,
item_unit_price,
category,
discount_percent,
( item_unit_price * quantity_purchased -
item_unit_price * quantity_purchased *
discount_percent / 100 ) AS subtotal,
phppos_sales_items.line AS line,
serialnumber,
phppos_sales_items.description AS description,
( item_unit_price * quantity_purchased -
item_unit_price * quantity_purchased *
discount_percent / 100 ) + (
item_unit_price * quantity_purchased -
item_unit_price * quantity_purchased *
discount_percent
/ 100 ) * ( Sum(CASE
WHEN cumulative != 1 THEN percent
ELSE 0
end) / 100 ) + ( ( (
item_unit_price * quantity_purchased
-
item_unit_price * quantity_purchased
*
discount_percent / 100 ) * (
Sum(CASE
WHEN cumulative != 1 THEN
percent
ELSE 0
end) / 100 ) + (
item_unit_price * quantity_purchased
-
item_unit_price *
quantity_purchased
*
discount_percent /
100
)
)
* ( Sum(CASE
WHEN
cumulative = 1 THEN percent
ELSE
0
end)
)
/ 100 )
AS total,
( item_unit_price * quantity_purchased -
item_unit_price * quantity_purchased *
discount_percent / 100 ) * (
Sum(CASE
WHEN cumulative != 1 THEN percent
ELSE 0
end) / 100 ) + ( ( ( item_unit_price * quantity_purchased -
item_unit_price * quantity_purchased *
discount_percent / 100 ) * ( Sum(
CASE
WHEN cumulative != 1
THEN
percent
ELSE 0
end) / 100 ) + (
item_unit_price * quantity_purchased
-
item_unit_price * quantity_purchased
*
discount_percent / 100 ) ) * ( Sum(
CASE
WHEN cumulative = 1 THEN percent
ELSE 0
end) ) / 100 ) AS tax,
( item_unit_price * quantity_purchased -
item_unit_price * quantity_purchased *
discount_percent / 100 ) - (
item_cost_price * quantity_purchased ) AS profit
FROM phppos_sales_items
INNER JOIN phppos_sales
ON phppos_sales_items.sale_id = phppos_sales.sale_id
INNER JOIN phppos_items
ON phppos_sales_items.item_id = phppos_items.item_id
LEFT OUTER JOIN phppos_suppliers
ON phppos_items.supplier_id = phppos_suppliers.person_id
LEFT OUTER JOIN phppos_sales_items_taxes
ON phppos_sales_items.sale_id =
phppos_sales_items_taxes.sale_id
AND phppos_sales_items.item_id =
phppos_sales_items_taxes.item_id
AND phppos_sales_items.line =
phppos_sales_items_taxes.line
WHERE sale_time BETWEEN "2014-04-01 00:00:00" AND "2014-04-30 23:59:59"
AND phppos_sales.location_id = '1'
AND phppos_sales.store_account_payment = 0
GROUP BY sale_id,
item_id,
line)
UNION ALL
(SELECT phppos_sales.deleted
AS
deleted
,
phppos_sales.deleted_by
AS deleted_by,
sale_time,
Date(sale_time)
AS
sale_date,
phppos_sales_item_kits.sale_id,
comment,
payment_type,
customer_id,
employee_id,
NULL
AS
item_id,
phppos_item_kits.item_kit_id,
''
AS
supplier_id,
quantity_purchased,
item_kit_cost_price,
item_kit_unit_price,
category,
discount_percent,
( item_kit_unit_price * quantity_purchased -
item_kit_unit_price * quantity_purchased * discount_percent / 100 )
AS
subtotal,
phppos_sales_item_kits.line
AS
line,
''
AS
serialnumber,
phppos_sales_item_kits.description
AS
description,
( item_kit_unit_price * quantity_purchased -
item_kit_unit_price * quantity_purchased * discount_percent / 100 )
+
( item_kit_unit_price * quantity_purchased -
item_kit_unit_price * quantity_purchased * discount_percent / 100 ) *
( Sum( CASE WHEN cumulative != 1 THEN percent ELSE 0 end) / 100 )
+ ( ( ( item_kit_unit_price * quantity_purchased -
item_kit_unit_price * quantity_purchased * discount_percent / 100 ) *
( Sum(CASE
WHEN cumulative != 1 THEN percent
ELSE 0
end)
/
100 ) + ( item_kit_unit_price * quantity_purchased -
item_kit_unit_price * quantity_purchased *
discount_percent / 100 ) ) * (
Sum(
CASE
WHEN cumulative = 1 THEN percent
ELSE 0
end) ) / 100 ) AS total,
( item_kit_unit_price * quantity_purchased -
item_kit_unit_price * quantity_purchased * discount_percent / 100 ) * ( Sum(
CASE
WHEN cumulative != 1 THEN percent
ELSE 0
end) / 100 ) + ( ( ( item_kit_unit_price * quantity_purchased -
item_kit_unit_price * quantity_purchased
*
discount_percent
/ 100 ) * ( Sum(
CASE
WHEN cumulative != 1 THEN percent
ELSE 0
end) / 100 ) + (
item_kit_unit_price * quantity_purchased
-
item_kit_unit_price *
quantity_purchased
* discount_percent / 100 ) ) *
(
Sum(CASE
WHEN cumulative = 1 THEN percent
ELSE 0
end) ) / 100 ) AS tax,
( item_kit_unit_price * quantity_purchased -
item_kit_unit_price * quantity_purchased * discount_percent / 100 ) - (
item_kit_cost_price * quantity_purchased ) AS profit
FROM phppos_sales_item_kits
INNER JOIN phppos_sales
ON phppos_sales_item_kits.sale_id = phppos_sales.sale_id
INNER JOIN phppos_item_kits
ON phppos_sales_item_kits.item_kit_id =
phppos_item_kits.item_kit_id
LEFT OUTER JOIN phppos_sales_item_kits_taxes
ON phppos_sales_item_kits.sale_id =
phppos_sales_item_kits_taxes.sale_id
AND phppos_sales_item_kits.item_kit_id =
phppos_sales_item_kits_taxes.item_kit_id
AND phppos_sales_item_kits.line =
phppos_sales_item_kits_taxes.line
WHERE sale_time BETWEEN "2014-04-01 00:00:00" AND "2014-04-30 23:59:59"
AND phppos_sales.location_id = '1'
AND phppos_sales.store_account_payment = 0
GROUP BY sale_id,
item_kit_id,
line)
ORDER BY sale_id,
line;
最佳答案
当您将 SELECT 与写入语句(例如 INSERT INTO... 或 CREATE TABLE AS... 结合使用时，则 MySQL必须在 SELECT 中涉及的表上建立共享锁。
您有另一个并发事务 (2) 对表 phppos_sales 持有独占锁，因此事务 (1) 无法获得其 S 锁，并且事务 (1) 等待。
然后事务 (2) 请求对故事 phppos_sales_items_taxes 进行 X 锁定。但是事务 (1) 已经在队列中以获得对该表的 S 锁，事务 (2) 必须在队列中等待它。
因此事务 (2) 正在等待事务 (1)，而事务 (1) 正在等待事务 (2)。这是一个典型的僵局。
这只每隔几天发生一次，因为它取决于事务 (2) 在事务 (1) 开始其 SELECT 之前获取其对 phppos_sales 的第一个锁定。然后事务 (2) 尝试在事务 (1) 的 S 锁请求排队后获取它在 phppos_sales_items_taxes 上的第二个锁。
换句话说，这是一个竞争条件，而且很难重现。
如果事务 (2) 请求锁定它需要的所有表作为一个原子操作，那么事务 (1) 将无法在锁定请求之间潜入。
您可以通过显式使用 LOCK TABLES 实现此目的:
START TRANSACTION
LOCK TABLES phppos_sales WRITE, phppos_sales_items WRITE,
phppos_sales_items_taxes WRITE, ...other table(s)...
INSERT INTO phppos_sales
INSERT MANY RECORDS INTO phppos_sales_items
INSERT MANY RECORDS INTO phppos_sales_items_taxes
INSERT MANY RECORDS INTO phppos_sales_payments
UNLOCK TABLES;
COMMIT;
这确实意味着执行长时间运行的 SELECT 的事务 (1) 必须等待事务 (2) 完成其 INSERT 并解锁其表。
否则，如果 SELECT 先进行，则意味着事务 (2) 必须等待它完成。
如果避免使用 CREATE TABLE...SELECT 或 INSERT INTO...SELECT，则可以在没有锁争用的情况下填充临时表。也就是说，将 SELECT 的结果集取回您的应用程序，然后将这些行INSERT 到临时表中。这样 SELECT 就不需要任何 S 锁。
您可以在存储过程中使用游标来做同样的事情。
正如@BrendanF 评论的那样，您还可以将事务隔离级别更改为 READ-COMMITTED，而不是默认的 REPEATABLE-READ。您可以全局更改默认事务隔离级别，也可以逐个 session 更改隔离级别。这会稍微改变交易的语义，因此您应该了解这些差异。
但它确实消除了在插入/选择操作期间从表中读取时 SELECT 执行 S 锁的需要。
关于php - mysql CREATE临时表+事务导致死锁，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/23768456/
26
4
0
我正在尝试使用 Spark 从 Cassandra 读取数据。 DataFrame rdf = sqlContext.read().option("keyspace", "readypulse
这是代码: void i_log_ (int error, const char * file, int line, const char * fmt, ...) { /* Get erro
我必须调试一个严重依赖 Gtk 的程序。问题是由于某些原因，在使用 GtkWindow 对象时开始出现许多运行时警告。问题是，即使 Gtk 提示严重错误，它也不会因这些错误而中止。我没有代码库的更改历
我正在尝试从已有效编译和链接的程序中检索二进制文件。我已经通过 GL_PROGRAM_BINARY_LENGTH 收到了它的长度。该文档说有两个实例可能会发生 GL_INVALID_OPERATION
我有一个托管在 Azure 环境中的服务。我正在使用控制台应用程序使用该服务。这样做时，我得到了异常: "The requested service, 'http://xxxx-d.yyyy.be/S
我有以下代码，它被 SEGV 信号杀死。使用调试器表明它被 main() 中的第一个 sem_init() 杀死。如果我注释掉第一个 sem_init() ，第二个会导致同样的问题。我试图弄清楚是什么
目前我正在编写一个应用程序(目标 iOS 6，启用 ARC)，它使用 JSON 进行数据传输，使用核心数据进行持久存储。 JSON 数据由 PHP 脚本通过 json_encode 从 MySQL 数
我对 Xamarin.Forms 还是很陌生。我在出现的主页上有一个非常简单的功能 async public Task BaseAppearing() { if (UserID
这是我的代码的简化版本。 public class MainActivity extends ActionBarActivity { private ArrayList entry = new Arr
我想弄明白为什么我的两个 Java 库很难很好地协同工作。这是场景: 库 1 有一个类 A，其构造函数如下: public A(Object obj) { /* boilerplate */ } 在以
如果网站不需要身份验证，我的代码可以正常工作，如果需要，则在打印“已创建凭据”后会立即出现 EXC_BAD_ACCESS 错误。我不会发布任何内容，并且此代码是直接从文档中复制的 - 知道出了什么问题
我在使用 NSArray 填充 UITableView 时遇到问题。我确信我正在做一些愚蠢的事情，但我无法弄清楚。当我尝试进行简单的计数时，我得到了 EXC_BAD_ACCESS，我知道这是因为我试图
我在 UITableViewCell 上有一个 UITextField，在另一个单元格上有一个按钮。 我单击 UITextField(出现键盘)。 UITextField 调用了以下方法: - (BO
我有一个应用程序出现间歇性崩溃。崩溃日志显示了一个堆栈跟踪，这对我来说很难破译，因此希望其他人看到了这一点并能为我指出正确的方向。 基本上，应用程序在启动时执行反向地理编码请求，以在标签中显示用户的位
我开发了一个 CGImage，当程序使用以下命令将其显示在屏幕上时它工作正常: [output_view.layer performSelectorOnMainThread:@selector(set
我正在使用新的 EncryptedSharedPreferences以谷歌推荐的方式上课: private fun securePrefs(context: Context): SharedPrefe
我有一个中继器，里面有一些控件，其中一个是文本框。我正在尝试使用 jquery 获取文本框，我的代码如下所示: $("#").click(function (event) {}); 但我总是得到 nu
在以下场景中观察到 TTS 初始化错误，太随机了。 已安装 TTS 引擎，存在语音集，并且可以从辅助功能选项中播放示例 tts。 TTS 初始化在之前初始化和播放的同一设备上随机失败。 在不同的设备(
maven pom.xml org.openjdk.jol jol-core 0.10 Java 类: public class MyObjectData { pr
在不担心冲突的情况下，可以使用 MD5 作为哈希值，字符串长度最多为多少？ 这可能是通过为特定字符集中的每个可能的字符串生成 MD5 哈希来计算的，长度不断增加，直到哈希第二次出现(冲突)。没有冲突的
我是一名优秀的程序员,十分优秀！