How to optimize MySQL

Regarding database optimization, there is a lot of information and methods on the Internet, but the quality of many of them is uneven, some of them are not summarized well enough, and the content is redundant. I accidentally found this article. It is very classic and has a lot of traffic. I hope it will be helpful to everyone in future development

1. Select the most applicable field attributes

MySQL can well support the access of large amounts of data, but generally speaking, the smaller the table in the database, the faster the queries executed on it will be. Therefore, when creating a table, in order to obtain better performance, we can set the width of the fields in the table as small as possible.

For example, when defining the postal code field, if you set it to CHAR(255), it will obviously add unnecessary space to the database. Even using VARCHAR is redundant, because CHAR(6 ) can complete the task very well. Likewise, if possible, we should use MEDIUMINT instead of BIGIN to define integer fields.

Another way to improve efficiency is to set fields to NOT NULL when possible, so that the database does not need to compare NULL values ​​when executing queries in the future.

For some text fields, such as "province" or "gender", we can define them as ENUM type. Because in MySQL, the ENUM type is treated as numeric data, and numeric data is processed much faster than text types. In this way, we can improve the performance of the database.

2. Use joins (JOIN) instead of sub-queries (Sub-Queries)

MySQL supports SQL subqueries starting from 4.1. This technique allows you to use a SELECT statement to create a single column of query results, and then use this result as a filter condition in another query. For example, if we want to delete customers who do not have any orders in the basic customer information table, we can use a subquery to first retrieve the IDs of all customers who issued orders from the sales information table, and then pass the results to the main query, as shown below :

DELETE  FROM  customerinfo
WHERE  CustomerID  NOT  in  (SELECT customerid  FROM  salesinfo)

Using subqueries can complete many SQL operations that logically require multiple steps to complete at one time. It can also avoid transaction or table locks, and it is also easy to write. However, there are situations where subqueries can be replaced by more efficient joins (JOINs). For example, if we want to retrieve all users who do not have order records, we can use the following query to complete it:

SELECT  *  FROM  customerinfo
WHERE  customerid  NOT IN (SELECT customerid   FROM   salesinfo)

If we use a connection (JOIN)... to complete this query, the speed will be much faster. Especially when there is an index on CustomerID in the salesinfo table, the performance will be better. The query is as follows:

SELECT  *  FROM  customerinfo
LEFT  JOIN  salesinfo  ON   customerinfo.customerid =salesinfo.customerid
WHERE  salesinfo.customerid   IS NULL

Connection (JOIN).. The reason why it is more efficient is that MySQL does not need to be in memory. Creating a temporary table to complete this logical query requires two steps.

3. Use union (UNION) to replace manually created temporary tables

MySQL supports union queries starting from version 4.0, which can use temporary tables to Two or more select queries are combined into one query. When the client's query session ends, the temporary table will be automatically deleted to ensure that the database is tidy and efficient. When using union to create a query, we only need to use UNION as the keyword to connect multiple select statements. It should be noted that the number of fields in all select statements must be the same. The following example demonstrates a query using UNION.

SELECT   name,phone  FROM  client UNION
SELECT  name,birthdate  FROM  author  UNION
SELECT  name,supplier FROM product

4. Transaction

Although we can use sub-queries (Sub-Queries), connections (JOIN) and unions (UNION) to create a variety of Query, but not all database operations can be completed with only one or a few SQL statements. More often, a series of statements are needed to complete a certain kind of work. But in this case, when a certain statement in this statement block runs incorrectly, the operation of the entire statement block will become uncertain. Imagine that you want to insert certain data into two related tables at the same time. This may happen: after the first table is successfully updated, an unexpected situation occurs in the database, causing the operation in the second table to not be completed. , In this way, the data will be incomplete and even the data in the database will be destroyed. To avoid this situation, you should use transactions. Its function is: either every statement in the statement block succeeds or fails. In other words, the consistency and integrity of the data in the database can be maintained. Things start with the BEGIN keyword and end with the COMMIT keyword. If a SQL operation fails during this period, the ROLLBACK command can restore the database to the state before BEGIN started.

BEGIN;
  INSERT   INTO   salesinfo   SET   customerid=14;
  UPDATE   inventory   SET   quantity =11   WHERE   item='book';
COMMIT;

Another important role of transactions is that when multiple users use the same data source at the same time, it can use the method of locking the database to provide users with a safe access method, which can ensure the user's operation Not to be disturbed by other users.

5. Lock table

尽管事务是维护数据库完整性的一个非常好的方法，但却因为它的独占性，有时会影响数据库的性能，尤其是在很大的应用系统中。由于在事务执行的过程中，数据库将会被锁定，因此其它的用户请求只能暂时等待直到该事务结束。如果一个数据库系统只有少数几个用户来使用，事务造成的影响不会成为一个太大的问题；但假设有成千上万的用户同时访问一个数据库系统，例如访问一个电子商务网站，就会产生比较严重的响应延迟。

其实，有些情况下我们可以通过锁定表的方法来获得更好的性能。下面的例子就用锁定表的方法来完成前面一个例子中事务的功能。

LOCK TABLE inventory WRITE SELECT quantity  FROM   inventory   WHERE Item='book';
...
UPDATE   inventory   SET   Quantity=11   WHERE  Item='book';UNLOCKTABLES

这里，我们用一个select语句取出初始数据，通过一些计算，用update语句将新值更新到表中。包含有WRITE关键字的LOCKTABLE语句可以保证在UNLOCKTABLES命令被执行之前，不会有其它的访问来对inventory进行插入、更新或者删除的操作。

6、使用外键

锁定表的方法可以维护数据的完整性，但是它却不能保证数据的关联性。这个时候我们就可以使用外键。

例如，外键可以保证每一条销售记录都指向某一个存在的客户。在这里，外键可以把customerinfo表中的customerid映射到salesinfo表中customerid，任何一条没有合法customerid的记录都不会被更新或插入到salesinfo中。

CREATE  TABLE   customerinfo( customerid   int primary key) engine = innodb;

CREATE  TABLE   salesinfo( salesid int not null,customerid  int not null, primary key(customerid,salesid),foreign key(customerid)  references  customerinfo(customerid) on delete cascade)engine = innodb;

注意例子中的参数“on delete cascade”。该参数保证当customerinfo表中的一条客户记录被删除的时候，salesinfo表中所有与该客户相关的记录也会被自动删除。如果要在MySQL中使用外键，一定要记住在创建表的时候将表的类型定义为事务安全表InnoDB类型。该类型不是MySQL表的默认类型。定义的方法是在CREATE TABLE语句中加上engine=INNODB。如例中所示。

7、使用索引

索引是提高数据库性能的常用方法，它可以令数据库服务器以比没有索引快得多的速度检索特定的行，尤其是在查询语句当中包含有MAX(),MIN()和ORDERBY这些命令的时候，性能提高更为明显。

那该对哪些字段建立索引呢？

一般说来，索引应建立在那些将用于JOIN,WHERE判断和ORDERBY排序的字段上。尽量不要对数据库中某个含有大量重复的值的字段建立索引。对于一个ENUM类型的字段来说，出现大量重复值是很有可能的情况

例如customerinfo中的“province”..字段，在这样的字段上建立索引将不会有什么帮助；相反，还有可能降低数据库的性能。我们在创建表的时候可以同时创建合适的索引，也可以使用ALTERTABLE或CREATEINDEX在以后创建索引。此外，MySQL从版本3.23.23开始支持全文索引和搜索。全文索引在MySQL中是一个FULLTEXT类型索引，但仅能用于MyISAM类型的表。对于一个大的数据库，将数据装载到一个没有FULLTEXT索引的表中，然后再使用ALTERTABLE或CREATEINDEX创建索引，将是非常快的。但如果将数据装载到一个已经有FULLTEXT索引的表中，执行过程将会非常慢。

8、优化的查询语句

绝大多数情况下，使用索引可以提高查询的速度，但如果SQL语句使用不恰当的话，索引将无法发挥它应有的作用。

下面是应该注意的几个方面。

a、 首先，最好是在相同类型的字段间进行比较的操作

在MySQL3.23版之前，这甚至是一个必须的条件。例如不能将一个建有索引的INT字段和BIGINT字段进行比较；但是作为特殊的情况，在CHAR类型的字段和VARCHAR类型字段的字段大小相同的时候，可以将它们进行比较。

b、 其次，在建有索引的字段上尽量不要使用函数进行操作

例如，在一个DATE类型的字段上使用YEAE()函数时，将会使索引不能发挥应有的作用。所以，下面的两个查询虽然返回的结果一样，但后者要比前者快得多。

ｃ、第三，在搜索字符型字段时，我们有时会使用LIKE关键字和通配符，这种做法虽然简单，但却也是以牺牲系统性能为代价的

例如下面的查询将会比较表中的每一条记录。

SELECT  *  FROM  books  WHERE  name  like   "MySQL%"

但是如果换用下面的查询，返回的结果一样，但速度就要快上很多：

SELECT  *  FROM  books  WHERE  name ＞=  "MySQL"  and  name  ＜"MySQM"

最后，应该注意避免在查询中让MySQL进行自动类型转换，因为转换过程也会使索引变得不起作用。

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