subquery vs join performance mysql

Tags: subquery digital compilation full table scan large efficient list over technology1, do not use sequential lookup, because the order lookup is slow, through the characteristics of the specific data structure to improve the query speed, this data structure is can be understood as an index. 2, the index is generally stored on disk in the form of files, index retrieval requires disk I/O operations, in order to minimize disk I/O. The disk is often

query results will also tell you how your index primary key is being leveraged, how your data tables are searched and sorted ... Wait, wait. Pick one of your SELECT statements (it is recommended to pick one of the most complex, multi-table joins) and add the keyword explain to the front. You can use phpMyAdmin to do 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 g

article system pagination display, the general query is this: Copy Code code as follows: SELECT ID, title, content from article order by created DESC LIMIT 10000, 10; Usually such queries build the index in the Created field (where the ID is the primary key), but when the limit is very large, query efficiency is still very low, change the query: Copy Code code as follows: SELECT ID, title, content from article INNER

Mysql Performance Monitoring related bitsCN.com Mysql Performance Monitoring 1. obtain the total number of processes under the mysql User II. host performance status III. CPU usage IV. disk I/O volume 5. swap inbound/outbound [m

, join paging mode The code below copies the code select * from ' content ' as T1 JOIN (SELECT ID from ' content ' ORDER by id desc LIMIT.) ( $page-1) * $pagesize. ", 1) as T2 WHERE t1.id After my testing, the efficiency of join paging and sub-query paging is basically on one level, and the time consumed is basically the same. Explain SQL statements: The code be

information data of active users; Personalized and customized information data for active users; Quasi-real-time statistical information data; Some other frequently accessed but less-changed data; 3 Query The effect of statements on system performance Requirements: Take out a group (assuming ID 1) under the user number (ID), user nickname (Nick_name), and follow the time to join the group (user_group.gmt_c

the most complex, multiple table joins) and add the keyword explain to the front. You can use phpMyAdmin to do 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 last one searches only 9 and 16 rows of two tables. Viewing the Rows column allows us to identify potential

information data for active users; Quasi-real-time statistical information data; Some other frequently accessed but less-changed data; 3 Query The effect of statements on system performance Requirements: Take out a group (assuming ID 1) under the user number (ID), user nickname (Nick_name), and follow the time to join the group (user_group.gmt_create) in reverse order, remove the first 20. Solution One: SE

Abstract: This article describes how to optimize SQL queries. You can manually use the EXPLAIN statement to check the efficiency of SQL queries. In addition, some principles for optimizing SQL statements are described, mainly about how to optimize SQL statements when retrieving records and loading data. Use the EXPLAIN statement to check SQL statements When you put the keyword "EXPLAIN" in front of a SELECT statement, MySQL explains how it will proce

follows: Mysql> explain select store_id, film_id FROM sakila. inventory \ G (2) for example, when the page is displayed in the article system, the general query is as follows: The code is as follows: SELECT id, title, content FROM article order by created desc limit 10000, 10; In general, the index will be created in the created field (where id is the primary key), but when the LIMIT offset is large, the query efficiency is still very low, change th

tell you how your index primary key is being leveraged, how your data tables are searched and sorted ... Wait, wait. Pick one of your SELECT statements (it is recommended to pick one of the most complex, multi-table joins) and add the keyword explain to the front. You can use it phpmyadmin to do it. Then, you'll see a table. In the following example, we forget to add an group_id index and have a table join: When we group_id index a field: As we can

files after all the associations have finished. If so, the extra field in the explain result will have a using temporary; Using Filesort. If there is a limit in the query, the limit is also applied after the sort. So even if you need to return fewer rows, the temporary table and the amount of data that needs to be sorted are still very large.mysql5.6 has made a number of important improvements here. When you only need to return partial sorting results, for example, using the limit clause,

MySQL performance optimization and mysql Optimization How to extract a random entry from a data table in MySQL, while ensuring the highest efficiency. Method 1This is the most primitive and intuitive syntax, as shown below:SELECT * FROM foo order by rand () LIMIT 1This method is feasible when the data volume in the dat

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 last one searches only 9 and 16 rows of two tables. Viewing the Rows column allows us to identify potential performance issues. 3. Use LIMIT 1 when only one row of data is used There are times when you query the table and you already know that there will on

-text index or make an index yourself (say, search for keywords or tags, etc.)5. Use a fairly typed example in the Join table and index itIf your application has many join queries, you should confirm that the fields of join in two tables are indexed. In this way, MySQL internally initiates the mechanism for you to opti

for keywords or tags, etc.)5. Use a fairly typed example in the Join table and index itIf your application has many join queries, you should confirm that the fields of join in two tables are indexed. In this way, MySQL internally initiates the mechanism for you to optimize the SQL statement for join.Also, the fields t

the Join table and index itIf your application has many join queries, you should confirm that the fields of join in two tables are indexed. In this way, MySQL internally initiates the mechanism for you to optimize the SQL statement for join.Also, the fields that are used for join