Learn some common Sql statement tuning techniques to successfully reach others overtime you pay.

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 restriction condition do not have an index, use the fields on both sides of the OR or as little as possible. If one of the fields is not an index field and the other conditions are not index fields, the query will not move 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;
Copy the code

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;
Copy the code

9. Distinguish between in and exists and not in and not exists

Select * from table Awhere id in(select id from table B)Copy the code

The SQL statement above is equivalent to

Select * from table AwhereExists (select * from table BwhereB.i d = table Anderson, d)Copy the code

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 tablewhere a.id not in(select b.id from B)Copy the code

Efficient SQL statements:

The select colname... From table A Left join table B onwhere a.id = b.id where b.id is null
Copy the code

10. Use proper paging methods to increase the efficiency of paging

select id,name from product limit 866613, 20
Copy the code

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
Copy the code

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.

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%”.

14. Avoid expressing fields in where clauses

Such as:

select user_id,user_project from user_base where age*2=36;
Copy the code

Mysql > alter table index (); alter table index ();

select user_id,user_project from user_base where age=36/2;
Copy the code

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;
Copy the code

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.