SQL optimization skills and precautions

    When you finish writing a sql, If you are not sure about the execution of this statement , It is recommended to use EXPLAIN keyword , According to the results of the execution , Conduct optimization analysis .EXPLAIN Keyword details

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1. in Don't include too many values
   
   ​​​​​​​Oracle in ,in The maximum number of parameters that can be placed in a statement is 1000 individual ,mysql in ,in The number of parameters in the statement is Don't limit Of . But for the whole paragraph sql The length of the statement is limited （max_allowed_packet Default bit 4M）, however Better not to exceed 200 It's worth （ about in Performance tests that contain the number of values ）.

2. select Statement indicates the field name
   
   If used select * Unnecessary io Or back to the table （ Why not recommend select *）
3. Index the sorting field
   
   The records in the database are organized in a set way , The records in a record set are often out of order （ Or there is a default order , For example, in the order of being stored in the database , But this order cannot be absolutely guaranteed ）, And the index is ordered , So using index can greatly speed up the query
4. To avoid the where Field in the clause null Value judgement
   
   about null The judgment of may cause the engine to give up using the index and perform a full table scan , Why is it possible ？ It depends. mysql Is the cost of deciding whether to execute an index small enough . Found by testing , Suppose the number of data records remains unchanged , If most of the data is null value , that is null No index , A full scan will be performed , and is not null Will use the index , conversely , If most of them are not empty ,is null Use index ,is not null No index .（Mysql in null Whether the value uses the index ）
5. Don't use % Prefix fuzzy query
   
   Such as like %name perhaps like %name%, This can also lead to index invalidation , and like name% I can walk the index
6. Avoid implicit type conversion
   
   where Clause column Type conversion when the type of the field is inconsistent with the type of the parameter passed in , It is suggested that where Parameter types in .
7. Joint index , Follow the leftmost prefix rule
   
   For example, a federated index contains fields id、name、school, It can be used directly id Field , It's fine too id、name In this order , They'll go through the index , however name;school Can't use this index . So when you create a union index, you must pay attention to the order of index fields , Common query fields are placed at the top .（ The leftmost prefix rule ）
   
   hypothesis key1,key2,key3 Index for Federation  select key1,key2,key3 from person where key2 = 'boss'​​​​​​​, In this case, do you think you don't go to the index ？ In fact, it will also go through the index , Because it doesn't need to go back to the table , Directly in this secondary index （ Joint index ） Get value inside .
8. Don't operate on Columns
   
   If the query conditions contain functions or expressions , It can also cause indexes to fail
   for example select * from user where YEAR(birthday) < 2000
9. You can use force index To force an index
   
   sometimes MySQL The optimizer takes the index it thinks appropriate to retrieve SQL sentence , But maybe the index it uses is not what we want . At this point, it can be used forceindex To force the optimizer to use the index we specify .