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Explain MySQL Query Cache (and why it's often disabled/deprecated).

MySQL query cache is often disabled or even marked as deprecated because it performs poorly in environments with high concurrency and frequent data updates. 1) Query cache improves performance by storing the results of SELECT statements, but depends on data stability. 2) In modern MySQL versions, query cache has been abandoned, and alternatives such as InnoDB buffer pooling, query rewriting and index optimization are recommended.

Apr 07, 2025 am 12:13 AM

Explain explicit table locking (LOCK TABLES) versus InnoDB row-level locking.

The difference between explicit table locking in MySQL and InnoDB row-level locking is the lock granularity and applicable scenarios. Explicit table locking locks the entire table through the LOCKTABLES statement, suitable for backup or batch updates; InnoDB row-level locking locks affected rows through transactions and indexes, suitable for high concurrency environments.

Apr 07, 2025 am 12:12 AM

行锁MySql锁

How do you analyze a MySQL query execution plan using EXPLAIN?

The EXPLAIN command is used to show how MySQL executes queries and helps optimize performance. 1) EXPLAIN displays the query execution plan, including access type, index usage, etc. 2) By analyzing the EXPLAIN output, bottlenecks such as full table scanning can be found. 3) Optimization suggestions include selecting the appropriate index, avoiding full table scanning, optimizing join query and using overlay indexes.

Apr 07, 2025 am 12:10 AM

查询执行计划

What are prefix indexes in MySQL and when are they useful/problematic?

Prefix indexing is a tool in MySQL used to optimize query performance, reducing the index size by indexing the first N characters of a string field. When using prefix indexes, you need to pay attention to: 1. Select the appropriate prefix length, 2. Avoid query conditions involving the middle or back characters of the string, 3. Use in combination with other index types, 4. Regularly monitor and adjust the index strategy.

Apr 07, 2025 am 12:08 AM

MySQL索引前缀索引

How can MySQL Query Optimizer Hints be used (e.g., USE INDEX, FORCE INDEX)?

The methods for using MySQL query optimizer tips are: 1. Use USEINDEX prompt optimizer to give priority to the specified index; 2. Use FORCEINDEX to force the optimizer to use the specified index. By adding these prompts to SQL queries, query performance can be significantly improved, but you need to avoid selecting wrong indexes and overuse of FORCEINDEX, and debugging through EXPLAIN statements.

Apr 07, 2025 am 12:06 AM

MySQL优化查询提示

Strategies for optimizing COUNT(*) queries on large InnoDB tables.

Optimizing COUNT(*) queries for InnoDB tables can be done by the following methods: 1. Using approximation values to estimate the total number of rows through random sampling; 2. Creating indexes to reduce the scan range; 3. Using materialized views, pre-calculate the results and refresh them regularly to improve query performance.

Apr 06, 2025 am 12:10 AM

MySQL优化

How does innodb_flush_log_at_trx_commit affect performance and durability?

The value of innodb_flush_log_at_trx_commit determines how InnoDB handles redolog's flush operation: 1. When the value is 1, the disk is flushed every transaction commit to ensure the highest data durability, but may affect performance. 2. When the value is 0, refresh it once every second to improve performance but may lose data for the last second. 3. When the value is 2, it is written to the operating system cache. The performance is between the first two, but there is still a risk of data loss.

Apr 06, 2025 am 12:07 AM

数据库性能

What are Global Transaction Identifiers (GTIDs) in MySQL replication?

GTIDs are used in MySQL replication to ensure that each transaction is executed uniquely. 1) GTIDs are composed of UUID and incremental transaction IDs, which simplifies data synchronization. 2) To enable GTID replication, you must set gtid_mode and enforce_gtid_consistency to ON on the master server, and use MASTER_AUTO_POSITION=1 on the slave server. 3) GTID supports multi-source replication, but you need to be careful to manage transaction order. 4) Avoid non-transactional statements and GTID conflicts, and when optimizing performance, you can reduce transaction size and use parallel replication.

Apr 06, 2025 am 12:05 AM

gtidMySQL复制

How does indexing work with NULL values in MySQL?

In MySQL, NULL values are not indexed by default, but can be processed through function indexing. 1.NULL values are not usually used by B-Tree index for search. 2. Use function indexes such as IFNULL (discount, 0) to convert NULL values into indexable values. 3. Consider using NOTNULL constraints to simplify index design.

Apr 06, 2025 am 12:04 AM

MySQL索引NULL值

Explain the purpose and usage of the MySQL Performance Schema.

MySQLPerformanceSchema is a tool for monitoring and optimizing database performance. Its functions include identifying performance bottlenecks and optimizing queries. 1) Enable PerformanceSchema to achieve by updating the setup_instruments table. 2) The basic usage includes viewing the current query and waiting for events. 3) Advanced usage involves analyzing the most frequent queries to optimize performance.

Apr 06, 2025 am 12:02 AM

性能监控

What is the Using filesort status in EXPLAIN and how to avoid it?

"Usingfilesort" means that MySQL does not use indexes when sorting, but uses file sorting, which will degrade query performance. Methods to avoid include: 1. Create appropriate indexes, such as CREATEINDEXidx_last_nameONusers(last_name); 2. Adjust the query to use index fields or overwrite the index.

Apr 05, 2025 am 12:05 AM

filesortMySQL优化

What is the InnoDB Adaptive Hash Index?

InnoDBAdaptiveHashIndex accelerates equivalence query by dynamically generating hash indexes. 1) Monitor the query mode, 2) Create hash index, 3) Perform hash search to reduce B-tree traversal and improve query efficiency.

Apr 05, 2025 am 12:03 AM

innodb

Describe strategies for optimizing SELECT COUNT(*) queries on large tables.

Methods to optimize SELECTCOUNT(*) queries include: 1. Use indexes, such as COUNT(1) or COUNT(primary_key); 2. Maintain counter tables and update row counts in real time; 3. Use approximate counting algorithms, such as HyperLogLog, which are suitable for scenarios where accurate counting is not required.

Apr 05, 2025 am 12:02 AM

数据库优化SQL优化

Common causes of replication lag in MySQL and how to troubleshoot.

The reasons for MySQL replication delay include insufficient hardware resources, network problems, large transactions, and lock contention. Solutions include: 1. Monitoring and log analysis, 2. Optimizing hardware resources, 3. Network optimization, 4. Transaction optimization, 5. Lock contention management. Through these measures, replication delays can be effectively reduced, data consistency and system stability can be ensured.

Apr 04, 2025 am 12:13 AM

故障排除