N methods of SQL optimization

SQL Optimization of the N Methods

1.SQL In the sentence IN Should not contain too many values :

for example ：select id from handsome where number in(1,2,3) For continuous values , It works between Don't use in 了 .

2.SELECT The statement must indicate the field name :

No use * To query , No use * To query , No use * To query , Important things are to be repeated for 3 times , Find which field , Just write specific fields .

3. When only one piece of data is queried , Use limit 1

【 This is very useful 】

4. To avoid the where Field in the clause null Value judgement

5. To avoid the where The clause performs an expression operation on the field :

select  id from user  WHERE id*100=500;
 above sql Just do the arithmetic operation on the field , This will cause the engine to give up using indexes , It is suggested to change it to ：
select  id from user  WHERE id=500/100;

6. For federated indexes , Follow the leftmost prefix rule :

For example, composite index (id,name,sex) When you use it , Sure id perhaps id,name . No direct name, perhaps sex Will cause the federated index to fail

Be careful : id, name,sex The order of filling in these three fields will not affect , mysql It will be automatically optimized to the leftmost matching order

The first three sql Can hit the index , The middle two do not conform to the leftmost matching principle , Index failure

The last item sql Due to the leftmost index id So the index is partially successful , Partial failure . id Field index used successfully

explain select * from  `user_test` where uid=10  ; 
explain select * from  `user_test` where uid=10 and name=' A man of current affairs '; 
explain select * from  `user_test` where uid=10 and name=' A man of current affairs ' and phone='13047967256'; 

explain select * from  `user_test` where name=' A man of current affairs ' and phone='13047967256'; 
explain select * from  `user_test` where name=' A man of current affairs '; 

explain select * from  `user_test` where uid=10 and phone='13047967256'; 

7. Use as much as possible inner join, avoid left join：

If the connection is inner join, In the absence of other filter conditions MySQL Will automatically select the small table as the driving table , however left join In the selection of driving table, we follow the principle of left driving right , namely left join The table on the left is called the drive table .

8. Note the range query statement :

For federated indexes , If there are scope queries , such as between、>、< Under equal conditions , It will invalidate the following index fields .

terms of settlement : Where business permits , Use >= perhaps <= This does not affect the use of indexes

9. Not recommended % Prefix fuzzy query :

for example : LIKE“%name” perhaps LIKE“%name%”, This kind of query will lead to index failure and full table scanning . But you can use LIKE “name%”.

10. stay where Used in clauses or To join the conditions , If or One of the connection conditions has no index , Will cause the engine to discard the index and scan the whole table

terms of settlement : take or Both sides of the connection are indexed , You can use

11. Should try to avoid in where Clause to function fields , This causes the engine to abandon the use of indexes for a full table scan

12. String type fields If you don't use quotation marks when querying ’’ , Will result in automatic implicit conversion , Then the index fails

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