MySQL GROUP BY花费很多时间来获取记录

Question

I want to query the database to fetch the last visit time of every user here is the query:

SELECT 
u.user_id,
u.firstname,
u.lastname,
u.email,
pv.visit_time 
FROM
  users u 
LEFT OUTER JOIN pageviews pv 
    ON u.user_id = pv.user_id 
   GROUP BY pv.user_id 
LIMIT 0, 12 

This query is taking 30 to 40 seconds to execute on live server, however if i remove the GROUP BY clause then it is taking 3 to 6 seconds but with duplicate records. Any idea what's wrong with this query?

Also i have tried DISTINCT but found same issue. Thanks, any help would be appreciated.

Answer 1

The group by clause and distinct requires a full scan of the table.

Maybe the query without the group by clause can be faster in returning the first rows, have you checked how long it takes to retrieve the whole result set?

If it takes only 3-6 seconds, I would refresh the statistics, maybe the optimiser is not doing the best choices for the join (I imagine that the table pageviews is a large one).

Answer 2

What are your indexes?

Do you really want a left join, as that would seem irrelevant? Using a LEFT OUTER JOIN it would just seem that you are going to get a row for user_id of NULL, but with nulls also in the other columns.

Further you are using GROUP BY to return a single row for each user. However which row is returned is not defined, so it could be any page views visit_time that is brought back for a user.

Also you have only a single column in the GROUP BY clause but other non aggregate columns in the select. With default options in MySQL this will work but will not work in most flavours of SQL and will also not work when MySQL is performing the group by in strict mode (see this manual page ).

Add a index on u.user_id and a compound index on pv.user_id AND pv.visit_time. Then assuming you want the latest visit time for each user try the query as:-

SELECT u.user_id,
    u.firstname,
    u.lastname,
    u.email,
    MAX(pv.visit_time)
FROM users u 
INNER JOIN pageviews pv 
ON u.user_id = pv.user_id 
GROUP BY u.user_id, u.firstname, u.lastname, u.email
ORDER BY u.user_id
LIMIT 0, 12

(strictly speaking the ORDER BY clause is not required as it is implicitly done by the GROUP BY clause, but it does make it more explicit what is expected to anyone reading the code in future).

Answer 3

Select t1.x, t1.y, t1.z from table1 t1 Group by t1.x,t1.y,t1.z.... 

It will give better performance dude...

Group by fields (x,y,z) should be appended with select statement to get better performance..

Try it ...(group by operation will happen with in result set for above query)