欢迎进入Oracle社区论坛,与200万技术人员互动交流 >>进入 什么是执行计划 所谓执行计划,顾名思义,就是对一个查询任务,做出一份怎样去完成任务的详细方案。举个生活中的例子,我从珠海要去英国,我可以选择先去香港然后转机,也可以先去北京转机,或者去
欢迎进入Oracle社区论坛,与200万技术人员互动交流 >>进入
什么是执行计划
所谓执行计划,顾名思义,就是对一个查询任务,做出一份怎样去完成任务的详细方案。举个生活中的例子,我从珠海要去英国,我可以选择先去香港然后转机,也可以先去北京转机,或者去广州也可以。但是到底怎样去英国划算,也就是我的费用最少,这是一件值得考究的事情。同样对于查询而言,我们提交的SQL仅仅是描述出了我们的目的地是英国,但至于怎么去,通常我们的SQL中是没有给出提示信息的,是由数据库来决定的。
我们先简单的看一个执行计划的对比:
SQL> set autotrace traceonly
执行计划一:
SQL> select count(*) from t;
COUNT(*)
----------
24815
Execution Plan
0 SELECT STATEMENT Optimizer=CHOOSE
1 0 SORT (AGGREGATE)
2 1 TABLE ACCESS (FULL) OF 'T'
执行计划二:
SQL> select count(*) from t;
COUNT(*)
24815
Execution Plan
0 SELECT STATEMENT Optimizer=CHOOSE (Cost=26 Card=1)
1 0 SORT (AGGREGATE)
2 1 INDEX (FULL SCAN) OF 'T_INDEX' (NON-UNIQUE) (Cost=26 Card=28180)
这两个执行计划中,第一个表示求和是通过进行全表扫描来做的,把整个表中数据读入内存来逐条累加;第二个表示根据表中索引,把整个索引读进内存来逐条累加,而不用去读表中的数据。但是这两种方式到底哪种快呢?通常来说可能二比一快,但也不是绝对的。这是一个很简单的例子演示执行计划的差异。对于复杂的SQL(表连接、嵌套子查询等),执行计划可能几十种甚至上百种,但是到底那种最好呢?我们事前并不知道,数据库本身也不知道,但是数据库会根据一定的规则或者统计信息(statistics)去选择一个执行计划,通常来说选择的是比较优的,但也有选择失误的时候,这就是这次讨论的价值所在。
Oracle优化器模式
Oracle优化器有两大类,基于规则的和基于代价的,在SQLPLUS中我们可以查看init文件中定义的缺省的优化器模式。
SQL> show parameters optimizer_mode
NAME TYPE VALUE
optimizer_mode string CHOOSE
SQL>
这是Oracle8.1.7 企业版,我们可以看出,默认安装后数据库优化器模式为CHOOSE,我们还可以设置为 RULE、FIRST_ROWS,ALL_ROWS。可以在init文件中对整个instance的所有会话设置,也可以单独对某个会话设置:
SQL> ALTER SESSION SET optimizer_mode = RULE;
会话已更改。
SQL> ALTER SESSION SET optimizer_mode = FIRST_ROWS;
会话已更改。
SQL> ALTER SESSION SET optimizer_mode = ALL_ROWS;
会话已更改。
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MySQL is suitable for beginners to learn database skills. 1. Install MySQL server and client tools. 2. Understand basic SQL queries, such as SELECT. 3. Master data operations: create tables, insert, update, and delete data. 4. Learn advanced skills: subquery and window functions. 5. Debugging and optimization: Check syntax, use indexes, avoid SELECT*, and use LIMIT.

MySQL efficiently manages structured data through table structure and SQL query, and implements inter-table relationships through foreign keys. 1. Define the data format and type when creating a table. 2. Use foreign keys to establish relationships between tables. 3. Improve performance through indexing and query optimization. 4. Regularly backup and monitor databases to ensure data security and performance optimization.

MySQL is an open source relational database management system that is widely used in Web development. Its key features include: 1. Supports multiple storage engines, such as InnoDB and MyISAM, suitable for different scenarios; 2. Provides master-slave replication functions to facilitate load balancing and data backup; 3. Improve query efficiency through query optimization and index use.

SQL is used to interact with MySQL database to realize data addition, deletion, modification, inspection and database design. 1) SQL performs data operations through SELECT, INSERT, UPDATE, DELETE statements; 2) Use CREATE, ALTER, DROP statements for database design and management; 3) Complex queries and data analysis are implemented through SQL to improve business decision-making efficiency.

The basic operations of MySQL include creating databases, tables, and using SQL to perform CRUD operations on data. 1. Create a database: CREATEDATABASEmy_first_db; 2. Create a table: CREATETABLEbooks(idINTAUTO_INCREMENTPRIMARYKEY, titleVARCHAR(100)NOTNULL, authorVARCHAR(100)NOTNULL, published_yearINT); 3. Insert data: INSERTINTObooks(title, author, published_year)VA

The main role of MySQL in web applications is to store and manage data. 1.MySQL efficiently processes user information, product catalogs, transaction records and other data. 2. Through SQL query, developers can extract information from the database to generate dynamic content. 3.MySQL works based on the client-server model to ensure acceptable query speed.

The steps to build a MySQL database include: 1. Create a database and table, 2. Insert data, and 3. Conduct queries. First, use the CREATEDATABASE and CREATETABLE statements to create the database and table, then use the INSERTINTO statement to insert the data, and finally use the SELECT statement to query the data.

MySQL is suitable for beginners because it is easy to use and powerful. 1.MySQL is a relational database, and uses SQL for CRUD operations. 2. It is simple to install and requires the root user password to be configured. 3. Use INSERT, UPDATE, DELETE, and SELECT to perform data operations. 4. ORDERBY, WHERE and JOIN can be used for complex queries. 5. Debugging requires checking the syntax and use EXPLAIN to analyze the query. 6. Optimization suggestions include using indexes, choosing the right data type and good programming habits.


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