[Solved] MySQL sum last X rows per group

How to optimize this SQL query?

In case you have your own slow SQL query, you can optimize it automatically here.

For the query above, the following recommendations will be helpful as part of the 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 Correlated Subqueries (query line: 4): A correlated subquery is a subquery that contains a reference (column: records_round_id) to a table that also appears in the outer query. Usually correlated queries can be rewritten with a join clause, which is the best practice. The database optimizer handles joins much better than correlated subqueries. Therefore, rephrasing the query with a join will allow the optimizer to use the most efficient execution plan for the query.
  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. Sort and Limit Before Joining (modified query below): In cases where the joins aren't filtering any rows, it's possible to sort and limit the amount of rows using a subquery in the FROM clause, before applying the joins to all other tables.
Optimal indexes for this query:
ALTER TABLE `records` ADD INDEX `records_idx_player_id_id` (`player_id`,`id`);
ALTER TABLE `records` ADD INDEX `records_idx_id` (`id`);
The optimized query:
        SUM(records_kills) / SUM(t1.duration) * 60 * 60 kills_hour_last_10 
                records.kills AS records_kills,
                records.round_id AS records_round_id 
                records.player_id = 1 
            ORDER BY
                records.id DESC LIMIT 10) AS records 
                ON rounds.id = records.records_round_id 
            1 = 1 LIMIT 10
        ) t1

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* original question posted on StackOverflow here.