如何个性化设置自己的sql*plus环境,sql*plus简单,强大和无处不在。用sql*plus主要做什么呢?sql*plus会自动运行glogin.sql,该
sql*plus简单,,强大和无处不在。用sql*plus主要做什么呢?
1)autotrace
2)编程
sql*plus会自动运行glogin.sql,该脚本包含全局默认设置。我们可按需自定义自己的sql*plus环境。
1)、首先在系统的环境设置中定义一个环境变量SQLPATH,把你自己的环境设置脚本login.sql和connect.sql
分别放在SQLPATH目录下。
2)、定义一个login.sql脚本,此脚本是SQL*PLUS打开的时候自动执行的脚本。
3)、定义一个脚本connect.sql,此脚本是在身份切换时,调用的脚本。
在实现这login.sql这个脚本,我们需要先掌握些基本的sql*plus知识。
友情提醒:命令太多不必记住。
a]help index
b]help 命令
这两步合起来,友善的sql*plus就会告诉你了。
1)执行一个sql脚本
@
@@:执行嵌套脚本。说明嵌套脚本和宿主在同一个目录下。
练习:
在$Oracle_HOME/dbs下有脚本think.sql和sub_think.sql。
think.sql的内容是:@@sub_think.sql
则我们可以这样执行:
@$ORACLE_HOME/dbs/think.sql
2)对当前的输入进行编辑
edit
通常我们说的DML,DDL,DCL等都是sql*plus语句,他们执行完会被保存在sql buffer的内存区域,并且只能保存一条最近执行的sql语句。
3)重新运行在sql buffer中的语句
/
4)将显示的内容输出到指定文件
spool filename
/*你在sql*plus中所做的任何事*/
spool off
然后你就可以到file所在路径查看。
5)col命令
a] 改变缺省的列标题
COL 默认列标题 HEADING 自定义列标题

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

MySQL/InnoDB supports four transaction isolation levels: ReadUncommitted, ReadCommitted, RepeatableRead and Serializable. 1.ReadUncommitted allows reading of uncommitted data, which may cause dirty reading. 2. ReadCommitted avoids dirty reading, but non-repeatable reading may occur. 3.RepeatableRead is the default level, avoiding dirty reading and non-repeatable reading, but phantom reading may occur. 4. Serializable avoids all concurrency problems but reduces concurrency. Choosing the appropriate isolation level requires balancing data consistency and performance requirements.

MySQL is suitable for web applications and content management systems and is popular for its open source, high performance and ease of use. 1) Compared with PostgreSQL, MySQL performs better in simple queries and high concurrent read operations. 2) Compared with Oracle, MySQL is more popular among small and medium-sized enterprises because of its open source and low cost. 3) Compared with Microsoft SQL Server, MySQL is more suitable for cross-platform applications. 4) Unlike MongoDB, MySQL is more suitable for structured data and transaction processing.

MySQL index cardinality has a significant impact on query performance: 1. High cardinality index can more effectively narrow the data range and improve query efficiency; 2. Low cardinality index may lead to full table scanning and reduce query performance; 3. In joint index, high cardinality sequences should be placed in front to optimize query.

The MySQL learning path includes basic knowledge, core concepts, usage examples, and optimization techniques. 1) Understand basic concepts such as tables, rows, columns, and SQL queries. 2) Learn the definition, working principles and advantages of MySQL. 3) Master basic CRUD operations and advanced usage, such as indexes and stored procedures. 4) Familiar with common error debugging and performance optimization suggestions, such as rational use of indexes and optimization queries. Through these steps, you will have a full grasp of the use and optimization of MySQL.

MySQL's real-world applications include basic database design and complex query optimization. 1) Basic usage: used to store and manage user data, such as inserting, querying, updating and deleting user information. 2) Advanced usage: Handle complex business logic, such as order and inventory management of e-commerce platforms. 3) Performance optimization: Improve performance by rationally using indexes, partition tables and query caches.


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