subquery vs join performance mysql

response speed. 1. is null and IS NOT NULL Null cannot be used as an index. any column containing null values will not be included in the index. Even if there are multiple columns in the index, as long as one of these columns contains null, this column will be excluded from the index. That is to say, if a column has a null value, even if the column is indexed, the performance will not be improved. Any statement optimizer that uses is null or is not n

Limitations of Union:MySQL cannot push the limit condition from the outer to the inner layer.Such as:Can be optimized to:Parallel execution:MySQL cannot use multi-core features to execute queries in parallel.Hash Association:MySQL does not support hash associations.Skip Index ScanNot supported. Tested, supported in version 5.6.Restrictions on updating on the same table:MySQL queries and updates to the same table are not allowed at the same time. This is not actually the optimizer's limit, and

be defined as ENUM. In MySQL, The ENUM type is processed as the numeric data, and the numeric data is processed much faster than the text type. In this way, we can improve the performance of the MySQL database. 2. Use JOIN to replace subqueries (Sub-Queries) MySQL supports

1. select the most suitable field attribute MySQL can support access to large data volumes, but generally, the smaller the table in the database, the faster the query will be executed on it. Therefore, when creating a table, we can set the field width in the table as small as possible to achieve better performance. For example, if you set it to Char (255) when defining the zip code field, it is obvious that

the index and data rows, and the distribution of the index.Due to inaccurate statistics, or the implementation mechanism of MySQL itself, in some cases, the cost of calculation is not accurate.The optimizations that MySQL can handle are: To redefine the order of associated tables Convert an outer join to an inner

High-Performance MySql evolution (10): Limitations of the query optimizer bitsCN.com The article "common methods of the query optimizer" lists some common optimization methods of the optimizer. The query optimizer also has some limitations while providing these features. these limitations are often improved with the upgrade of the MYSQL version, so this article w

fields, such as "province" or "gender", can be defined as ENUM. In MySQL, the ENUM type is processed as the numeric data, and the numeric data is processed much faster than the text type. In this way, we can improve the database performance. 2. use JOIN instead of Sub-Queries) MySQL supports SQL subqueries from 4.1.

1. select the most suitable field attribute MySQL can support access to large data volumes, but generally, the smaller the table in the database, the faster the query will be executed on it. Therefore, when creating a table, we can set the field width in the table as small as possible to achieve better performance. For example, if you set it to Char (255) when defining the zip code field, it is obvious th

negative impact on performance, but in addition to this method, there are basically not many other better solutions. Moreover, since the database through a good extension, the load of each MySQL Server can be better controlled, simply for a single Query, its response time may be higher than the non-segmentation, so the performance of the negative impact is not t

table structure. The EXPLAIN query results also show you how your index primary key is used and how your data tables are searched and sorted ...... And so on. SELECT one of your SELECT statements (we recommend that you SELECT the most complex one with multi-table join) and add the keyword "EXPLAIN" to the front. You can use phpmyadmin to do this. Then, you will see a table. In the following example, we forget to add the group_id index and have tabl

1. select the most suitable field attribute MySQL can support access to large data volumes, but generally, the smaller the table in the database, the faster the query will be executed on it. Therefore, when creating a table, we can set the field width in the table as small as possible to achieve better performance. For example, if you set it to CHAR (255) when defining the zip code field, it is obvious that

When I entered Pinterest, my first three weeks were spent in this part, where the latest projects applied the results of production problems to the entire software stack. In this part, we learn how Pinterest is built by building Pinterest, and it's not uncommon to submit code and make meaningful contributions in just a few days. In Pinterest, new engineers have the flexibility to choose which group to join, and as part of their work experience in this

| NULL | Select A.* from article as a INNER join (select A.aid from article as a INNER join (select CID from || 3226 | Root | % | T_girl | Query | 0 | NULL | Select A.* from article as a INNER join (select A.aid from article as a INNER join (select CID from |+------+------+--------------------+--------------------+---

("SELECT username FROM user WHERE signup_date> = CURDATE ()"); // Enable query cache $ Today = date ("Y-m-d "); $ R = mysql_query ("SELECT username FROM user WHERE signup_date> = '$ today '"); The difference between the preceding two SQL statements is CURDATE (). The query cache of MySQL does not work for this function. Therefore, SQL functions such as NOW (), RAND (), and other such functions do not enable the query cache, because the returned re

this. Then, you'll see a table. In the following example, we forget to add the group_id index and have a table join: When we index the group_id field: As we can see, the previous result shows a search of 7883 rows, and the second one searches only 9 and 16 rows of two tables. Looking at the rows column allows us to find potential performance issues.3. Use LIMIT 1 when only one row of data is us

1. select the most suitable field attributeMySQL can support access to large data volumes, but generally, the smaller the table in the database, the faster the query will be executed on it. Therefore, when creating a table, we can set the field width in the table as small as possible to achieve better performance. For example, if you set it to CHAR (255) when defining the zip code field, it is obvious that unnecessary space is added to the database, a

Mysql directly uses limit for paging operations. If we do not add any processing, the data may be too big, and it will be very difficult, next I will introduce the mysql paging instance and paging performance optimization. 1. The most common paging Method for MYSQL: The Code is as follows: Copy code

join performance is not good, it has a great performance advantage compared with MySQL subqueries. MySQL's subquery Execution Plan has been a big problem. Although this problem has existed for many years, it has been common in all the stable versions that have been released

this. Then, you'll see a table. In the following example, we forget to add the group_id index and have a table join: When we index the group_id field: As we can see, the previous result shows a search of 7883 rows, and the second one searches only 9 and 16 rows of two tables. Looking at the rows column allows us to find potential performance issues.3. Use LIMIT 1 when only one row of data is us

task. Similarly, if possible, we should use MEDIUMINT instead of BIGIN to define integer fields. Another way to improve efficiency is to set the field to not null whenever possible, so that the database does NOT need to compare NULL values during future queries. Some text fields, such as "Province" or "gender", can be defined as ENUM. In MySQL, The ENUM type is processed as the numeric data, and the numeric data is processed much faster than the text