刚解决了一个异步io的问题,还是专门描述一下Oracle中设置异步IO的方式。首先要检查操作系统上,异步io的相关包是否已经安装,如
刚解决了一个异步io的问题,还是专门描述一下Oracle中设置异步IO的方式。
首先要检查操作系统上,,异步io的相关包是否已经安装,如果没有的话,通过rpm进行安装,并检查系统上异步io的参数设置:
[oracle@localhost ~]$ rpm-qa|grep aio
libaio-0.3.107-10.el6.x86_64
libaio-devel-0.3.107-10.el6.x86_64[oracle@localhost ~]$ more/proc/sys/fs/aio-max-nr1048576然后检查数据库在链接时是否已经加载了aio的包:
[oracle@localhost ~]$/usr/bin/ldd $ORACLE_HOME/bin/oracle|grep libaio
libaio.so.1=>/lib64/libaio.so.1(0x0000003e13000000)如果没有看到libaio的包,说明Oracle没有链接aio,那么这时就需要重新make oracle的可执行文件,9i的方法为:
make-f ins_rdbms.mk async_on
make-f ins_rdbms.mk ioracle10g以后的方法为:
make PL_ORALIBS=-laio-f ins_rdbms.mk async_on编译完成后,操作系统的设置完成,还需要设置数据库的初始化参数:
SQL>SHOWparameter disk_asynch_io
NAMETYPEVALUE------------------------------------ -------------------------------- --------------------disk_asynch_ioBOOLEANTRUESQL>SHOWparameter filesystemio_options
NAMETYPEVALUE------------------------------------ -------------------------------- ------------------filesystemio_options stringNONE对于裸设备而言,这样的设置就足够了,但是对于文件系统而言,还需要讲filesystemio_options设置为ASYNCH并重启数据库:
SQL>ALTERSYSTEMSETFILESYSTEMIO_OPTIONS=ASYNCH SCOPE=SPFILE;
System altered.由于编译oracle可执行文件的时候也要关闭数据库,合理的安排设置参数的顺序,可减少数据库的重启次数。

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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