MySql内部联接需要10秒钟以上

Question

I have two tables posts and followings

posts (id,userid,post,timestamp) 30 000 rows 

and

followings(id_me,userid) 90 000 rows

I want to get lattest 10 posts form posts table based on the people i follow and my posts

SELECT p.*
 FROM posts as p INNER JOIN
     followings as f
     ON (f.id_me=(my user id) AND p.userid=f.userid )
        OR 
        p.userid=(my user id)
ORDER BY id DESC LIMIT 10

But it takes about 10-15 seconds to return. Thanks in advance!

Answer 1

Using an OR in SQL is a performance killer, try this:

SELEC p.*
FROM posts as p INNER JOIN
 followings as f
 ON (f.id_me=(my user id) AND p.userid IN (f.userid,(my user id)))
ORDER BY id DESC LIMIT 10

Answer 2

Do this query using union :

(SELECT p.*
 FROM posts p INNER JOIN
      followings f
      ON (f.id_me=(my user id) AND p.userid=f.userid
)
union
(select p.*
 from posts p
 where p.userid=(my user id)
)
ORDER BY id DESC
LIMIT 10

If the two conditions never overlap, then use union all instead.

Answer 3

An OR condition like that prevents the query optimizer from making use of indexes. Use a UNION instead:

SELECT *
FROM (SELECT p.*
      FROM posts as p 
      INNER JOIN followings as f
      ON f.id_me=(my user id) AND p.userid=f.userid

      UNION

      SELECT *
      FROM posts
      WHERE userid = (my user id)) u
ORDER BY id DESC
LIMIT 10

Answer 4

It might be just me but I think your WHERE clause is in an inefficient location:

SELECT
    p.*
FROM
    posts p
    INNER JOIN
        followings f
        ON p.userid=f.userid
WHERE
    MyUserID IN (p.userid, f.id_me)
ORDER BY
    id DESC
LIMIT
    10

Answer 5

First, remove the filter from the join clause, let the join just correlate the joining tables.

(
SELECT p.*
 FROM posts as p
  INNER JOIN followings as f ON p.userid=f.userid
  where f.id_me=(my user id)
UNION
SELECT p.*
 FROM posts as p
  where p.userid=(my user id)
)
ORDER BY id DESC LIMIT 10

second, verify your indexes if that ids got no indexes it ill perform a full table scan for each cartesian product of both tables (30k x 90k =~ 3700k pairs being compared)

third, if you don't follow yourself you need a union from post you are following and your posts

Answer 6

I read in comments that you have the required indexes. The problem is the query. Combining OR with a JOIN confuses the poor and (often) dumb optimizer. The LIMIT 10 should be helpful but the optimizer is not (yet) smart enough to make the best plan.

Try this query:

  ( SELECT p.*
    FROM posts AS p
      JOIN followings AS f
         ON  f.id_me = (my_user_id) 
         AND p.userid = f.userid
    ORDER BY p.id DESC 
    LIMIT 10
  ) 
UNION ALL
  ( SELECT p.*
    FROM posts AS p
    WHERE p.userid = (my_user_id) 
    ORDER BY p.id DESC 
    LIMIT 10
  ) AS x
ORDER BY id DESC
LIMIT 10 ;