一、问题起源
在MySQL的官方文档中有明确的说明不支持嵌套事务:
代码如下:
Transactions cannot be nested. This is a consequence of the implicit commit performed for any current transaction when you issue a START TRANSACTION statement or one of its synonyms.
但是在我们开发一个复杂的系统时难免会无意中在事务中嵌套了事务,比如A函数调用了B函数,A函数使用了事务,并且是在事务中调用了B函数,B函数也有一个事务,这样就出现了事务嵌套。这时候其实A的事务就意义不大了,为什么呢?上面的文档中就有提到,简单的翻译过来就是:
代码如下:
当执行一个START TRANSACTION指令时,会隐式的执行一个commit操作。
所以我们就要在系统架构层面来支持事务的嵌套。所幸的是在一些成熟的ORM框架中都做了对嵌套的支持,比如doctrine或者laravel。接下来我们就一起来看下这两个框架是怎样来实现的。
友情提示,这两个框架的函数和变量的命名都比较的直观,虽然看起来很长,但是都是通过命名就能直接得知这个函数或者变量的意思,所以不要一看到那么一大坨就被吓到了 :)
二、doctrine的解决方案
首先来看下在doctrine中创建事务的代码(干掉了不相关的代码):
代码如下:
public function beginTransaction()
{
++$this->_transactionNestingLevel;
if ($this->_transactionNestingLevel == 1) {
$this->_conn->beginTransaction();
} else if ($this->_nestTransactionsWithSavepoints) {
$this->createSavepoint($this->_getNestedTransactionSavePointName());
}
}
这个函数的第一行用一个_transactionNestingLevel来标识当前嵌套的级别,如果是1,也就是还没有嵌套,那就用默认的方法执行一下START TRANSACTION就ok了,如果大于1,也就是有嵌套的时候,她会帮我们创建一个savepoint,这个savepoint可以理解为一个事务记录点,当需要回滚时可以只回滚到这个点。
然后看下rollBack函数:
代码如下:
public function rollBack()
{
if ($this->_transactionNestingLevel == 0) {
throw ConnectionException::noActiveTransaction();
}
if ($this->_transactionNestingLevel == 1) {
$this->_transactionNestingLevel = 0;
$this->_conn->rollback();
$this->_isRollbackOnly = false;
} else if ($this->_nestTransactionsWithSavepoints) {
$this->rollbackSavepoint($this->_getNestedTransactionSavePointName());
--$this->_transactionNestingLevel;
} else {
$this->_isRollbackOnly = true;
--$this->_transactionNestingLevel;
}
}
可以看到处理的方式也很简单,如果level是1,直接rollback,否则就回滚到前面的savepoint。
然后我们继续看下commit函数:
代码如下:
public function commit()
{
if ($this->_transactionNestingLevel == 0) {
throw ConnectionException::noActiveTransaction();
}
if ($this->_isRollbackOnly) {
throw ConnectionException::commitFailedRollbackOnly();
}
if ($this->_transactionNestingLevel == 1) {
$this->_conn->commit();
} else if ($this->_nestTransactionsWithSavepoints) {
$this->releaseSavepoint($this->_getNestedTransactionSavePointName());
}
--$this->_transactionNestingLevel;
}
算了,不费口舌解释这段了吧 :)
三、laravel的解决方案
laravel的处理方式相对简单粗暴一些,我们先来看下创建事务的操作:
代码如下:
public function beginTransaction()
{
++$this->transactions;
if ($this->transactions == 1)
{
$this->pdo->beginTransaction();
}
}
感觉如何?so easy吧?先判断当前有几个事务,如果是第一个,ok,事务开始,否则就啥都不做,那么为啥是啥都不做呢?继续往下看rollBack的操作:
代码如下:
public function rollBack()
{
if ($this->transactions == 1)
{
$this->transactions = 0;
$this->pdo->rollBack();
}
else
{
--$this->transactions;
}
}
明白了吧?只有当当前事务只有一个的时候才会真正的rollback,否则只是将计数做减一操作。这也就是为啥刚才说laravel的处理比较简单粗暴一些,在嵌套的内层里面实际上是木有真正的事务的,只有最外层一个整体的事务,虽然简单粗暴,但是也解决了在内层新建一个事务时会造成commit的问题。原理就是这个样子了,为了保持完整起见,把commit的代码也copy过来吧!
代码如下:
public function commit()
{
if ($this->transactions == 1) $this->pdo->commit();
--$this->transactions;
}

MySQL is an open source relational database management system, mainly used to store and retrieve data quickly and reliably. Its working principle includes client requests, query resolution, execution of queries and return results. Examples of usage include creating tables, inserting and querying data, and advanced features such as JOIN operations. Common errors involve SQL syntax, data types, and permissions, and optimization suggestions include the use of indexes, optimized queries, and partitioning of tables.

MySQL is an open source relational database management system suitable for data storage, management, query and security. 1. It supports a variety of operating systems and is widely used in Web applications and other fields. 2. Through the client-server architecture and different storage engines, MySQL processes data efficiently. 3. Basic usage includes creating databases and tables, inserting, querying and updating data. 4. Advanced usage involves complex queries and stored procedures. 5. Common errors can be debugged through the EXPLAIN statement. 6. Performance optimization includes the rational use of indexes and optimized query statements.

MySQL is chosen for its performance, reliability, ease of use, and community support. 1.MySQL provides efficient data storage and retrieval functions, supporting multiple data types and advanced query operations. 2. Adopt client-server architecture and multiple storage engines to support transaction and query optimization. 3. Easy to use, supports a variety of operating systems and programming languages. 4. Have strong community support and provide rich resources and solutions.

InnoDB's lock mechanisms include shared locks, exclusive locks, intention locks, record locks, gap locks and next key locks. 1. Shared lock allows transactions to read data without preventing other transactions from reading. 2. Exclusive lock prevents other transactions from reading and modifying data. 3. Intention lock optimizes lock efficiency. 4. Record lock lock index record. 5. Gap lock locks index recording gap. 6. The next key lock is a combination of record lock and gap lock to ensure data consistency.

The main reasons for poor MySQL query performance include not using indexes, wrong execution plan selection by the query optimizer, unreasonable table design, excessive data volume and lock competition. 1. No index causes slow querying, and adding indexes can significantly improve performance. 2. Use the EXPLAIN command to analyze the query plan and find out the optimizer error. 3. Reconstructing the table structure and optimizing JOIN conditions can improve table design problems. 4. When the data volume is large, partitioning and table division strategies are adopted. 5. In a high concurrency environment, optimizing transactions and locking strategies can reduce lock competition.

In database optimization, indexing strategies should be selected according to query requirements: 1. When the query involves multiple columns and the order of conditions is fixed, use composite indexes; 2. When the query involves multiple columns but the order of conditions is not fixed, use multiple single-column indexes. Composite indexes are suitable for optimizing multi-column queries, while single-column indexes are suitable for single-column queries.

To optimize MySQL slow query, slowquerylog and performance_schema need to be used: 1. Enable slowquerylog and set thresholds to record slow query; 2. Use performance_schema to analyze query execution details, find out performance bottlenecks and optimize.

MySQL and SQL are essential skills for developers. 1.MySQL is an open source relational database management system, and SQL is the standard language used to manage and operate databases. 2.MySQL supports multiple storage engines through efficient data storage and retrieval functions, and SQL completes complex data operations through simple statements. 3. Examples of usage include basic queries and advanced queries, such as filtering and sorting by condition. 4. Common errors include syntax errors and performance issues, which can be optimized by checking SQL statements and using EXPLAIN commands. 5. Performance optimization techniques include using indexes, avoiding full table scanning, optimizing JOIN operations and improving code readability.


Hot AI Tools

Undresser.AI Undress
AI-powered app for creating realistic nude photos

AI Clothes Remover
Online AI tool for removing clothes from photos.

Undress AI Tool
Undress images for free

Clothoff.io
AI clothes remover

AI Hentai Generator
Generate AI Hentai for free.

Hot Article

Hot Tools

MantisBT
Mantis is an easy-to-deploy web-based defect tracking tool designed to aid in product defect tracking. It requires PHP, MySQL and a web server. Check out our demo and hosting services.

Dreamweaver Mac version
Visual web development tools

ZendStudio 13.5.1 Mac
Powerful PHP integrated development environment

MinGW - Minimalist GNU for Windows
This project is in the process of being migrated to osdn.net/projects/mingw, you can continue to follow us there. MinGW: A native Windows port of the GNU Compiler Collection (GCC), freely distributable import libraries and header files for building native Windows applications; includes extensions to the MSVC runtime to support C99 functionality. All MinGW software can run on 64-bit Windows platforms.

SublimeText3 Chinese version
Chinese version, very easy to use