Talk about SQL optimization

## One . Index failure

1. If there is... In the condition or, Even if there is a conditional index, it will not be used （or Conditions are indexed independently , use UNION Replace or）

2. For multi-column indexes , Not the first part of use , Index will not be used

3.like The query is based on % start ( With % The ending is OK )

4. If the column type is string , Be sure to quote the data in the condition , Otherwise, the index is not used

5. If mysql It is estimated that full table scanning is faster than indexing , Index is not used

6. Should try to avoid in where Field in the clause null Value judgement , Failing to do so will cause the engine to abandon the index for a full table scan , Such as

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        select id from t where num is null
       
       

        
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（ It's better not to leave it in the database NULL, Use as much as possible NOT NULL Fill the database .）

7. Should try to avoid in where Used in clauses != or <> The operator , Otherwise, the engine will discard the index and scan the whole table .

8.in and not in Be careful with , Otherwise, it will result in a full table scan （ For continuous values , It works between Don't use in 了 ）（ use exists Instead of in）

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        select num from a where exists(select 1 from b where num=a.num)
       
       

        
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9. If in where Use parameters in Clause , It can also lead to a full table scan . because SQL Local variables are resolved only at runtime , But the optimizer can't delay the choice of access plan to run time ; It has to be selected at compile time . however and , If an access plan is established at compile time , The value of the variable is still unknown , Therefore, it cannot be selected as an input item of index . As the following statement will scan the whole table ：

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        select id from t where num = @num
       
       

        
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You can force queries to use indexes instead ：

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        select id from t with(index( Index name )) where num = @num
       
       

        
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10. Should try to avoid in where The clause performs an expression operation on the field , This causes the engine to abandon the use of indexes for a full table scan . Such as ：

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        select id from t where num/2 = 100
       
       

        
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Should be changed to

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        select id from t where num = 100*2
       
       

        
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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 . Such as

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        select id from t where substring(name,1,3) = ’abc’       -–name With abc At the beginning id
        
select id from t where datediff(day,createdate,’2005-11-30′) = 0    -–‘2005-11-30’    -- Generated id
       
       

        
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Should be changed to

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        select id from t where name like 'abc%'
        
select id from t where createdate >= '2005-11-30' and createdate < '2005-12-1'
       
       

        
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12. When using index fields as conditions , If the index is a composite index , Then the first field in the index must be used as a condition to ensure that the system uses the index , Otherwise the index will not be used , And try to make the order of the fields consistent with the order of the index .

## Other optimization

1. Try to use numeric fields , If only contains the numerical value information the field as far as possible does not design for the character type , This reduces the performance of queries and connections , And it increases storage overhead . This is because the engine is processing queries and joins Then it compares each character in the string one by one , For digital models, only one comparison is enough .

2. Use as much as possible varchar/nvarchar Instead of char/nchar , Because first of all the longer fields have less storage space , You can save storage space , Second, for queries , Searching in a relatively small field is obviously more efficient .

3. Don't use... Anywhere select * from t , Replace... With a specific list of fields “*”, Do not return any fields that are not available .

4. Try to avoid using cursors , Because the efficiency of cursors is poor

5. Break up the big DELETE or INSERT sentence , Submit in bulk SQL sentence

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        while(1){
        

        
 　　// Do it every time 1000 strip 
        

        
　　 mysql_query(“delete from logs where log_date <= ’2012-11-01’ limit 1000”);
        

        
 　　if(mysql_affected_rows() == 0){
        

        
　　 　　// Delete complete , sign out ！
        
　　 　　break;
        
　　}
        

        
// Pause for a while at a time , Release the table so that other processes / Thread access .
        
usleep(50000)
        

        
}
       
       

        
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Reference article ：

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