MySQL -- Index Optimization -- order by

brief introduction

explain

In this paper, MySQL Of ORDER BY Optimization scheme of index .

ORDER BY Two implementation methods of

1. 1. 
      
2. Automatic implementation with ordered index .（ Use the orderliness of the ordered index instead of sorting （ Fast ））

1. That is to say explain The results of Using index

2. Select the results and then sort .（ Slow speed ）

1. That is to say explain The results of Using filesort

3. 
   

Whether to go to the index or not

Query and sort are indexed

1. 1. 
      
2. SELECT Fields and ORDER BY The fields are exactly the same （ Or form a joint index ）.

1. example ：SELECT col1 FROM tb1 ORDER BY col1;

2. WHERE Fields and ORDER BY The fields are exactly the same （ Or form a joint index ）.

1. example ：SELECT col1 FROM tb1 WHERE col2=2 ORDER BY col2;

3. 
   

Query without index

1. 1. 
      
2. SELECT Field does not contain ORDER BY Field .

1. example ：SELECT col1 FROM tb1 ORDER BY col2

2. SELECT Field contains ORDER BY Field + Other fields .

1. example ：SELECT col1,col3 FROM tb1 ORDER BY col2

3. WHERE Field does not contain ORDER BY Field .

1. example ：SELECT col1 FROM tb1 WHERE col2=2 ORDER BY col3

4. 
   

example

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        DROP TABLE IF EXISTS test;
        
CREATE TABLE test(
        
    id int primary key auto_increment,
        
    c1 varchar(10),
        
    c2 varchar(10),
        
    c3 varchar(10),
        
    c4 varchar(10),
        
    c5 varchar(10)
        
) ENGINE=INNODB default CHARSET=utf8;
        

        
INSERT INTO test(c1,c2,c3,c4,c5) VALUES('a1','a2','a3','a4','a5');
        
INSERT INTO test(c1,c2,c3,c4,c5) VALUES('b1','b2','b3','b4','b5');
        
INSERT INTO test(c1,c2,c3,c4,c5) VALUES('c1','c2','c3','c4','c5');
        
INSERT INTO test(c1,c2,c3,c4,c5) VALUES('d1','d2','d3','d4','d5');
        
INSERT INTO test(c1,c2,c3,c4,c5) VALUES('e1','e2','e3','e4','e5');
        

        
CREATE INDEX idx_c1234 ON test(c1,c2,c3,c4);
       
       

        
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result ：

Equivalent query

Case study 1：SELECT *,WHERE Follow the leftmost prefix ,WHERE And ORDER BY Follow the leftmost prefix

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        EXPLAIN SELECT * FROM test WHERE c1 = 'b1' AND c2 = 'b2' ORDER BY c3;
       
       

        
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result ： Query index ; Sort and index

Case study 2：SELECT *,WHERE Do not follow the leftmost prefix ,ORDER BY Do not follow the leftmost prefix ,WHERE And ORDER BY Do not follow the leftmost prefix

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        EXPLAIN SELECT * FROM test WHERE c2 = 'b2' AND c3 = 'b3' ORDER BY c4;
       
       

        
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result ： Queries don't index ; Sort without index

Case study 3：SELECT *,WHERE Do not follow the leftmost prefix ,ORDER BY Follow the leftmost prefix ,WHERE And ORDER BY Do not follow the leftmost prefix

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        EXPLAIN SELECT * FROM test WHERE c2 = 'b2' AND c3 = 'b3' ORDER BY c1;
       
       

        
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result ： Queries don't index ; Sort without index （ Same as above ）

Case study 4：SELECT And ORDER BY identical ,WHERE Do not follow the leftmost prefix ,ORDER BY Follow the leftmost prefix ,WHERE And ORDER BY Do not follow the leftmost prefix

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        EXPLAIN SELECT c1 FROM test WHERE c2 = 'b2' AND c3 = 'b3' ORDER BY c1;
       
       

        
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result ： Queries don't index ; Sort and index （ Overlay index ）

Range queries

Case study 1：SELECT *, And WHERE And ORDER BY Dissimilarity

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        EXPLAIN SELECT * FROM test WHERE c2 > 'b1' ORDER BY c1;
       
       

        
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result ： Queries don't index ; Sort without index

Case study 2：SELECT No *,WHERE And ORDER BY Dissimilarity ,ORDER BY Left most prefix

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        EXPLAIN SELECT c3 FROM test WHERE c2 > 'b1' ORDER BY c1;
       
       

        
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result ： Queries don't index , Sort and index . This situation is equivalent to using indexes only for sorting

（ Be careful ： take c3 Switch to c1,c2 etc. , The results are the same ）

Case study 3：SELECT *,WHERE And ORDER BY Same and follow the leftmost

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        EXPLAIN SELECT * FROM test WHERE c1 > 'b1' ORDER BY c1;
       
       

        
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result ： Query index ; There is no operation for sorting , Directly use the query results

Case study 4：SELECT *,WHERE And ORDER BY Different and follow the leftmost

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        EXPLAIN SELECT * FROM test WHERE c1 > 'b1' ORDER BY c2;
       
       

        
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result ： Query index ; Sort without index

Other websites

​ ​Mysql stay order by The use mechanism of time index - Programmer base ​​​