MySQL查询执行流程
查询流程:
客户端发送一条查询给服务器;
服务器先检查查询缓存,如果命中了缓存,则立即返回存储在缓存中的结果;否则,进入下一阶段;
服务器进行SQL解析、预处理,再由优化器生成对应的执行计划;
MySQL根据优化器生成的执行计划,调用存储引擎的API来执行查询;
将结果返回给客户端;
查询缓存
用于保存MySQL查询语句返回的完整结果,被命中时,MySQL会立即返回结果,省去解析、优化和执行等阶段;
MySQL保存结果于缓存中,把select语句本身做hash计算,计算的结果作为key,查询结果作为value;
查询语句的大小写会影响缓存的存储和命中,故需保持查询语句的大小写一致性;
何种语句不会被缓存
查询语句中有一些不确定数据时,不会缓存,如now(),current_time()等
若查询中包含用户自定义函数,存储函数,用户变量,临时表,mysql库中系统表,或者任何包含权限的表,一般都不会缓存
缓存会带来额外开销,因为:
读查询在开始之前必须先检查是否命中缓存;
若某个读查询可以被缓存且未被缓存,那么当完成执行后,MySQL会将其结果存入查询缓存;
对写操作也有影响,因为当写入数据时,MySQL必须将对应表的所有缓存都设置失效,这在缓存内存较大时将导致很大的系统消耗;
故查询缓存并非必需,其效率取决于全部查询中开销较大的查询是否能被缓存命中;
缓存相关变量查询
如何判断缓存命中率
缓存命中率相关的变量
计算缓存命中率
缓存优化的思路
批量写入而非多次单个写入;
缓存空间不宜过大,因为大量缓存同时失效会导致服务器假死;
必要时,使用sql_cache和sql_no_cache手动控制缓存;
对写密集型的应用场景来说,禁用缓存反而能提高性能

ACID attributes include atomicity, consistency, isolation and durability, and are the cornerstone of database design. 1. Atomicity ensures that the transaction is either completely successful or completely failed. 2. Consistency ensures that the database remains consistent before and after a transaction. 3. Isolation ensures that transactions do not interfere with each other. 4. Persistence ensures that data is permanently saved after transaction submission.

MySQL is not only a database management system (DBMS) but also closely related to programming languages. 1) As a DBMS, MySQL is used to store, organize and retrieve data, and optimizing indexes can improve query performance. 2) Combining SQL with programming languages, embedded in Python, using ORM tools such as SQLAlchemy can simplify operations. 3) Performance optimization includes indexing, querying, caching, library and table division and transaction management.

MySQL uses SQL commands to manage data. 1. Basic commands include SELECT, INSERT, UPDATE and DELETE. 2. Advanced usage involves JOIN, subquery and aggregate functions. 3. Common errors include syntax, logic and performance issues. 4. Optimization tips include using indexes, avoiding SELECT* and using LIMIT.

MySQL is an efficient relational database management system suitable for storing and managing data. Its advantages include high-performance queries, flexible transaction processing and rich data types. In practical applications, MySQL is often used in e-commerce platforms, social networks and content management systems, but attention should be paid to performance optimization, data security and scalability.

The relationship between SQL and MySQL is the relationship between standard languages and specific implementations. 1.SQL is a standard language used to manage and operate relational databases, allowing data addition, deletion, modification and query. 2.MySQL is a specific database management system that uses SQL as its operating language and provides efficient data storage and management.

InnoDB uses redologs and undologs to ensure data consistency and reliability. 1.redologs record data page modification to ensure crash recovery and transaction persistence. 2.undologs records the original data value and supports transaction rollback and MVCC.

Key metrics for EXPLAIN commands include type, key, rows, and Extra. 1) The type reflects the access type of the query. The higher the value, the higher the efficiency, such as const is better than ALL. 2) The key displays the index used, and NULL indicates no index. 3) rows estimates the number of scanned rows, affecting query performance. 4) Extra provides additional information, such as Usingfilesort prompts that it needs to be optimized.

Usingtemporary indicates that the need to create temporary tables in MySQL queries, which are commonly found in ORDERBY using DISTINCT, GROUPBY, or non-indexed columns. You can avoid the occurrence of indexes and rewrite queries and improve query performance. Specifically, when Usingtemporary appears in EXPLAIN output, it means that MySQL needs to create temporary tables to handle queries. This usually occurs when: 1) deduplication or grouping when using DISTINCT or GROUPBY; 2) sort when ORDERBY contains non-index columns; 3) use complex subquery or join operations. Optimization methods include: 1) ORDERBY and GROUPB


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