如何优化mysql查询

Question

how to increase the performance of this mysql query

SELECT ''                                           AS sharedid,
       hubber_posts.userID                          AS postowner,
       hubber_posts.*,
       ''                                           AS sharedby,
       hubber_posts.userID                          AS userID,
       hubber_posts.posted_date                     AS DATE,
       ''                                           AS sharebyusr,
       ''                                           AS sharebyusrimg,
       Concat_ws(' ', firstname, lastname)          AS fullname,
       username                                     AS postedBy,
       hubber_user.image,
       hubber_user.gender                           AS gender,
       (SELECT accounttype
        FROM   hubber_user_security us
        WHERE  hubber_user.ID = us.userID
               AND hubber_posts.userID = us.userID) AS accounttype,
       ''                                           AS sharebyusrtype
FROM   hubber_posts
       INNER JOIN hubber_user
               ON hubber_posts.userID = hubber_user.ID
WHERE  hubber_posts.status = 1 

Answer 1

My suggestion is to support a join where hubber_posts is the base table and the 2 other tables are joined using nested loops.

• No need to index hubber_posts for the join.
• hubber_user.ID should be a PK.
• hubber_user_security.userID should be indexed (and defined as a FK references hubber_user.ID).

As for the WHERE clause - only if you have relatively few rows where hubber_posts.status = 1 then you should add an index on hubber_posts.status

Ps

since the join contain the condition -

ON hubber_posts.userID = hubber_user.ID 

There is no need to compare both hubber_posts.userID and hubber_user.ID to us.userID

Answer 2

Your example code has a correlated subquery. MySQL performs poorly with those, as of late 2016.

Try converting it to a JOINed table.

   SELECT all that stuff,
          us.accounttype
     FROM   hubber_posts
     JOIN hubber_user ON hubber_posts.userID = hubber_user.ID
     LEFT JOIN hubber_user_security us ON hubber_user.ID = us.userID
    WHERE  hubber_posts.status = 1 

I used LEFT JOIN . Without the LEFT , any rows without a matching entry in that table will be suppressed from the result set.

Answer 3

You query is essentially this:

SELECT . . .
       (SELECT accounttype
        FROM   hubber_user_security us
        WHERE  u.ID = us.userID AND
               p.userID = us.userID
       ) AS accounttype,
       . . .
FROM hubber_posts p INNER JOIN
     hubber_user u
     ON p.userID = u.ID
WHERE p.status = 1 ;

For this query, the optimal indexes are:

• hubber_posts(status, userId)
• hubber_user(Id)
• hubber_user_security(userId)

I would note that the subquery has an extra correlation condition that is not necessary -- the user ids are already equal. And, you run the risk of getting an error if there are multiple account types.

You may intend:

   (SELECT GROUP_CONCAT(accounttype)
    FROM   hubber_user_security us
    WHERE  u.ID = us.userID
   ) as accounttypes,