Deadlock occurs because multiple transactions access the same resource in different orders and form loop dependencies. A typical scenario is transactions A and B cross-wait for the lock held by the other party. For troubleshooting, you can view the LATEST DETECTED DEADLOCK section through the SHOW ENGINE INNODB STATUS command to analyze the locks held by the transaction, waiting locks and the SQL involved. Solutions include: 1. Unified access order; 2. Reduce transaction granularity; 3. Use index reasonably; 4. Use lower isolation levels; 5. Implement the retry mechanism. In addition, implicit lock conflict, self-increase field competition and confusing batch update order are also common causes. When encountering deadlocks, you should check the log first, and then optimize the SQL order and index design.
MySQL error "Deadlock found when trying to get lock" usually occurs in concurrent operations, where multiple transactions are waiting for each other's lock resources held by the other party, resulting in a dead loop. This type of problem is more common in high concurrency scenarios, especially when it involves multi-table updates, long transactions, or unreasonable index design.
The following analyzes this problem from several common perspectives and gives some practical troubleshooting and optimization suggestions.
1. Why does a deadlock occur?
The fundamental reason for deadlocks is that multiple transactions access the same resource (such as tables or rows) in different orders, and each transaction holds part of the resource while trying to obtain resources occupied by other transactions, thus forming a loop dependency.
Typical examples:
- Transaction A updates a row in Table 1 and then tries to update Table 2;
- At the same time, transaction B updates a row in Table 2 and tries to update Table 1;
- If both perform the second step at the same time, they may wait for each other to release the lock, causing a deadlock.
MySQL's InnoDB engine automatically detects this deadlock and selects a "victim" to roll back its transactions to break the deadlock.
2. How to view deadlock information?
When a deadlock occurs, you can view the detailed deadlock log through the following command:
SHOW ENGINE INNODB STATUS;
This command outputs a lot of content, and focuses on the LATEST DETECTED DEADLOCK section, which shows the information such as which statements the two transactions have executed, which locks are held, and which locks are waiting for.
Key information includes:
- The type of lock currently held by each transaction (such as record locks, gap locks)
- Lock resource waiting
- SQL statements involved
- Table structure and index usage
This information can be used to locate which SQLs cause deadlocks and how they cross-wait.
3. Common solutions and avoidance methods
1. Unified access order
Ensure that all transactions access the database objects in the same order. For example, if multiple transactions need to operate orders and order_items tables, then orders are operated first and then order_items .
2. Reduce transaction granularity
Try to shorten the transaction execution time and reduce the amount of data operations in the transaction. Don't do too much in a transaction, especially logic that doesn't involve database operations.
3. Use index rationally
Without a suitable index, InnoDB may be upgraded to a table-level lock or even a full table scan lock, increasing the probability of deadlock. Check if your WHERE condition hits the index, especially the primary key or unique index.
4. Use lower isolation levels (depending on business)
In some scenarios, changing the transaction isolation level from the default REPEATABLE READ READ COMMITTED can reduce the use of gap locks, thereby reducing the probability of deadlocks. But data consistency should be weighed based on business needs.
5. Retry mechanism delays processing
Implementing the retry mechanism after a transaction fails at the application layer. For example, after catching a deadlock exception, delay the transaction again. Although the problem cannot be completely solved, it can alleviate the impact to a certain extent.
4. Details that are easy to ignore
- Implicit lock conflict : Sometimes SQL that seems to have no direct conflict may also result in lock competition due to index structures (such as unique constraint checks).
- Self-increase field competition : In high concurrent insertion scenario, if the table has a self-increase primary key, inserting multiple transactions simultaneously may lead to lock competition.
- Cluster batch update order : If your application code generates SQL dynamically and updates data in different orders, it is likely to become a potential source of deadlocks.
Basically that's it. When encountering a deadlock, don't panic. Check the log first, then look at the SQL execution order and index status, and then gradually optimize. Although the problem is complex, as long as you grasp the core reasons, it can be solved in most cases.
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