提高mysql中的查询性能

Question

I have used this query

SELECT  COUNT(CASE WHEN C <= 1 THEN 1 END) AS nooffamiliesHavingcount1,
        COUNT(CASE WHEN C BETWEEN 2 AND 4 THEN 1 END) AS nooffamiliesHavingcountbetween2And4,
        COUNT(CASE WHEN C > 4 THEN 1 END) AS nooffamiliesHavingcountgreaterthan3
FROM    (   SELECT  COUNT(*) AS C
            FROM    user where user_id = (select user_id from location where location_id in(select location_id from country where state_name='STATE'))
            GROUP BY House_No
        ) t

Here sub query returning approximately 10000 records . The user table has 10,00,000 records. It is taking too much time.Then the error it is saying is server gone away. I am using mysql.

I searched from google.But no luck for me.

What changes i need to do for my tables.How i can execute this query successfully by increasing the query performance.Please suggest me.Thanks in advance....

Answer 1

Try this query

SELECT 
  COUNT(CASE WHEN C <= 1 THEN 1 END) AS nooffamiliesHavingcount1,
  COUNT(CASE WHEN C BETWEEN 2 AND 4 THEN 1 END) AS nooffamiliesHavingcountbetween2And4,
  COUNT(CASE WHEN C > 4 THEN 1 END) AS nooffamiliesHavingcountgreaterthan3
FROM  
  (SELECT 
    COUNT(*) AS C
  FROM 
    user u,  
    location l, 
    country c 
  where 
    l.state_name='STATE' AND 
    l.some_other_column_id= 4 AND  <------- Add your condition
    c.location_id = l.location_id AND 
    u.user_id = l.user_id 
  GROUP BY 
    u.House_No) t

Use proper joins as it will be easy to understand..

SELECT 
  COUNT(CASE WHEN C <= 1 THEN 1 END) AS nooffamiliesHavingcount1,
  COUNT(CASE WHEN C BETWEEN 2 AND 4 THEN 1 END) AS nooffamiliesHavingcountbetween2And4,
  COUNT(CASE WHEN C > 4 THEN 1 END) AS nooffamiliesHavingcountgreaterthan3
FROM  
  (SELECT 
    COUNT(*) AS C
  FROM 
    user u
  INNER JOIN  
    location l
  ON 
    l.state_name='STATE' AND 
    l.some_other_column_id= 4   <------- Add your condition
    u.user_id = l.user_id 
  INNER JOIN
    country c 
  ON 
    c.location_id = l.location_id 
  GROUP BY 
    u.House_No) t

EDITED

In most cases JOINs are faster than sub-queries and it is very rare for a sub-query to be faster. I accept using subquery is more logical and easy to understand but when it comes about performance it is not as good as joins. If you are using joins your db will optimize your query on its own which is not in the case of subquery. Try using explain for both of your query and you will get clear idea how the query executes.

Hope this helps...

Answer 2

Can you try below:

SELECT COUNT(CASE WHEN COUNT( ) <= 1 THEN 1 END) AS nooffamiliesHavingcount1, COUNT(CASE WHEN COUNT( ) BETWEEN 2 AND 4 THEN 1 END) AS nooffamiliesHavingcountbetween2And4, COUNT(CASE WHEN COUNT(*) > 4 THEN 1 END) AS nooffamiliesHavingcountgreaterthan3 FROM User user Inner JOIN (select user_id from location loc Inner Join country con on loc.location_id =con.location_id where state_name='STATE' )as temp

on user.user_id =temp.user_id group by House_No