Akiko: Latte guy
Link: https://dbaplus.cn/news-11-2413-1.html
In this article, we will talk about the common MySQL optimization methods in the project, a total of 19, as follows:
1, the EXPLAIN
To do MySQL optimization, we should use EXPLAIN to view the SQL execution plan.
Here is a simple example, annotating (1, 2, 3, 4, 5) the data we want to focus on:
- Type column, connection type. A good SQL statement must be at least range level. Prevent the occurrence of all levels.
- Key column, the name of the index used. If no index is selected, the value is NULL. Mandatory indexes can be used.
- Key_len column, index length.
- Rows, the number of rows scanned. This value is an estimate.
- Extra column, details. Note that common unfriendly values are: Using filesort, Using temporary.
2. SQL statements should not contain too many values for IN
MySQL optimizes IN to store all constants IN an array that is sorted. However, if the number is large, the consumption is also relatively large. Select id from t where num in(1,2,3); Or use connections instead.
3, the SELECT statement must specify the field name
SELECT* adds a lot of unnecessary consumption (CPU, IO, memory, network bandwidth); Increased the possibility of using overridden indexes; When the table structure changes, the front break also needs to be updated. Therefore, the field name is required to be followed directly by select.
4. Use limit 1 when only one piece of data is needed
This is to make the type column in EXPLAIN const
5. If the sort field does not use an index, sort as little as possible
6. If the other fields in the constraint do not have indexes, use or as little as possible
If one of the fields on both sides of or is not an index field and the other conditions are not index fields, the query does not remove the index. Many times it is better to use union all or union (if necessary) instead of “or”.
7, Try to use union all instead of union
The main difference between union and Union all is that the former requires the combination of result sets and then carries out the unique filtering operation, which involves sorting, increases a large number of CPU operations, and increases resource consumption and delay. Of course, the premise of union all is that the two result sets have no duplicate data.
8, do not use ORDER BY RAND()
select id from `dynamic` order by rand() limit 1000;
The above SQL statement can be optimized to:
select id from `dynamic` t1 join (select rand() * (select max(id) from `dynamic`) as nid) t2 on t1.id > t2.nidlimit 1000;
9. Distinguish between in and exists and not in and not exists
Select * from table A where id in (select id from table B)
The SQL statement above is equivalent to
Select * from A where exists(select * from B where b.id = a.id)
The distinction between in and EXISTS mainly results in a change in driver order (which is key to performance changes). In the case of exists, the outer layer table is the driver table and is accessed first. In, the subquery is executed first. So IN is suitable for the situation where the outside is large and the inside is small; EXISTS applies to a situation where the outer surface is small and the inner surface is large.
For not in and NOT EXISTS, it is recommended to use not EXISTS, not in May have logic problems, not in is not just a matter of efficiency. How to efficiently write a SQL statement that replaces not EXISTS?
Original SQL statement:
The select colname… From A where a.id not in (select B. id from B)
Efficient SQL statements:
The select colname… From A Left join B on a.id = b.id where B.id is null
The extracted result set is shown in the following figure, and the data in Table A is not in Table B:
10. Use proper paging methods to increase the efficiency of paging
select id,name from product limit 866613, 20
When you do paging with the SQL statement above, you may find that using limit paging queries directly gets slower and slower as the amount of table data increases.
The optimization method is as follows: You can take the ID of the maximum number of rows on the previous page, and then limit the start of the next page based on that maximum ID. For example, in this column, the maximum ID of the previous page is 866612. SQL can be written as follows:
select id,name from product where id> 866612 limit 20
11, segmented query
In some user selection pages, the time range selected by some users may be too large, resulting in slow query. The main reason is that too many lines are scanned. This time can be through the program, segmented query, circular traversal, the results of the combined processing for display.
The following SQL statement can be used when the number of rows scanned exceeds millions:
12. Avoid null values for fields in the WHERE clause
The determination of NULL causes the engine to abandon the index and perform a full table scan instead.
It is not recommended to use % prefix fuzzy query
For example, LIKE “%name” or “%name%”, this query will cause index invalidation and perform full table scan. But you can use LIKE “name%”.
How to query %name%
As shown in the figure below, the secret field is indexed but not used in explain results:
So how to solve this problem? Answer: use full-text indexes.
Select id,fnum, FDST from dynamic_201606 where user_name like ‘%zhangsan%’; . Such statements, ordinary index is not sufficient to meet the query requirements. Fortunately, in MySQL, there are full-text indexes to help us.
SQL syntax for creating a full-text index is:
ALTER TABLE `dynamic_201606` ADD FULLTEXT INDEX `idx_user_name` (`user_name`);
SQL statements that use full-text indexes are:
select id,fnum,fdst from dynamic_201606 where match(user_name) against(‘zhangsan’ in boolean mode);
Note: Before you need to create a full-text index, contact your DBA to confirm that it can be created. It is also important to note that the query statement is written differently from ordinary indexes.
14. Avoid expressing fields in where clauses
Such as:
select user_id,user_project from user_base where age*2=36;
Mysql > alter table index (); alter table index ();
select user_id,user_project from user_base where age=36/2;
Avoid implicit type conversions
The type conversion occurs when the type of the column column in the WHERE clause is inconsistent with the type of the parameter passed in. It is recommended to determine the type of the parameter in the WHERE clause first.
16. For union indexes, follow the leftmost prefix rule
For example, the index contains the fields ID, name, school, can directly use the ID field, or the order of ID, name, but name; No school can use this index. Therefore, when creating a federated index, you must pay attention to the order of the index fields, and put the commonly used query fields first.
17, if necessary, you can use force index to force a query to go to an index
Sometimes the MySQL optimizer takes the index it sees fit to retrieve SQL statements, but it may not use the index we want. Forceindex can then be used to force the optimizer to use our specified index.
18. Note the range query statement
For a federated index, a range query, such as between, >, or <, invalidates the following index fields.
19. About JOIN optimization
Select * from LEFT JOIN A; select * from RIGHT JOIN B; select * from LEFT JOIN A; select * from LEFT JOIN B;
Note:
1) MySQL does not have full join.
select * from A left join B on B.name = A.namewhere B.name is nullunion allselect * from B;
2) Try to use inner join to avoid left join:
There are at least two tables participating in the joint query, which are generally of different sizes. If the join mode is inner JOIN, MySQL will automatically select the small table as the driver table without other filtering conditions. However, left JOIN follows the principle of left drive right in the selection of the driver table, that is, the left join table is called the driver table.
3) Rational use of indexes:
The index field of the driven table serves as the limit field of ON.
4) Use small tables to drive large tables:
From the schematic diagram can intuitively see that if you can reduce the drive table, reduce the number of cycles in the nested loop, to reduce the total IO and CPU operation times.
5) STRAIGHT_JOIN:
Inner join is selected by MySQL as the driver table, but some special cases need to select another table as the driver table, such as group by, order by, etc. “Using filesort”, “Using temporary”. By STRAIGHT_JOIN to enforce the join order, the table name on the left of STRAIGHT_JOIN would be the driven table and the table name on the right would be the driven table. One prerequisite for using STRAIGHT_JOIN was that the query was an inner join, that is, an inner join. STRAIGHT_JOIN is not recommended for other links because it could result in inaccurate query results.
This method can sometimes reduce the time by three times.
Above 19 MySQL optimization methods hope to help you!