[Solved] Optimize MySQL random when using ordering by two parameters
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Optimize MySQL random when using ordering by two parameters

Database type:

I need to get some data from tables and order them by two parameters. One of the parameters are RAND() because records needs to be random. I have large database and using RAND() slows down performance alot and I want to avoid it. Is there any chance to optimize performance of random ordering ? I need to order my result based on count (lowest to highest) and then I need to perform ordering by RANDOM.

This is query that I am using

SELECT c.username, IF(s.Broj IS NULL,0,s.Broj) as Ukupno
FROM user AS c
LEFT JOIN (
  SELECT username, COUNT(*) AS Broj
  FROM odis
  GROUP BY username
) AS s
ON c.username = s.username
ORDER BY s.broj ASC, RAND()

How to optimize this SQL query?

The following recommendations will help you in your SQL tuning process.
You'll find 3 sections below:

  1. Description of the steps you can take to speed up the query.
  2. The optimal indexes for this query, which you can copy and create in your database.
  3. An automatically re-written query you can copy and execute in your database.
The optimization process and recommendations:
  1. Avoid Subqueries (query line: 10): We advise against using subqueries as they are not optimized well by the optimizer. Therefore, it's recommended to join a newly created temporary table that holds the data, which also includes the relevant search index.
  2. Create Optimal Indexes (modified query below): The recommended indexes are an integral part of this optimization effort and should be created before testing the execution duration of the optimized query.
  3. Explicitly ORDER BY After GROUP BY (modified query below): By default, the database sorts all 'GROUP BY col1, col2, ...' queries as if you specified 'ORDER BY col1, col2, ...' in the query as well. If a query includes a GROUP BY clause but you want to avoid the overhead of sorting the result, you can suppress sorting by specifying 'ORDER BY NULL'.
Optimal indexes for this query:
ALTER TABLE `odis` ADD INDEX `odis_idx_username` (`username`);
The optimized query:
SELECT
        c.username,
        IF(s.Broj IS NULL,
        0,
        s.Broj) AS Ukupno 
    FROM
        user AS c 
    LEFT JOIN
        (
            SELECT
                odis.username,
                COUNT(*) AS Broj 
            FROM
                odis 
            GROUP BY
                odis.username 
            ORDER BY
                NULL
        ) AS s 
            ON c.username = s.username 
    ORDER BY
        s.broj ASC,
        RAND()

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* original question posted on StackOverflow here.