[Solved] MySQL Complex Query, user conversation

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 Calling Functions With Indexed Columns (query line: 13): When a function is used directly on an indexed column, the database's optimizer won’t be able to use the index. For example, if the column `sender` is indexed, the index won’t be used as it’s wrapped with the function `LEAST`. If you can’t find an alternative condition that won’t use a function call, a possible solution is to store the required value in a new indexed column.
  2. Avoid Calling Functions With Indexed Columns (query line: 13): When a function is used directly on an indexed column, the database's optimizer won’t be able to use the index. For example, if the column `receiver` is indexed, the index won’t be used as it’s wrapped with the function `LEAST`. If you can’t find an alternative condition that won’t use a function call, a possible solution is to store the required value in a new indexed column.
  3. Avoid Calling Functions With Indexed Columns (query line: 13): When a function is used directly on an indexed column, the database's optimizer won’t be able to use the index. For example, if the column `sender` is indexed, the index won’t be used as it’s wrapped with the function `GREATEST`. If you can’t find an alternative condition that won’t use a function call, a possible solution is to store the required value in a new indexed column.
  4. Avoid Calling Functions With Indexed Columns (query line: 13): When a function is used directly on an indexed column, the database's optimizer won’t be able to use the index. For example, if the column `receiver` is indexed, the index won’t be used as it’s wrapped with the function `GREATEST`. If you can’t find an alternative condition that won’t use a function call, a possible solution is to store the required value in a new indexed column.
  5. Avoid Selecting Unnecessary Columns (query line: 2): Avoid selecting all columns with the '*' wildcard, unless you intend to use them all. Selecting redundant columns may result in unnecessary performance degradation.
  6. Avoid Subqueries (query line: 15): 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.
  7. 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.
  8. 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'.
  9. Prefer Direct Join Over Joined Subquery (query line: 4): We advise against using subqueries as they are not optimized well by the optimizer. Therefore, we recommend to replace subqueries with JOIN clauses.
Optimal indexes for this query:
ALTER TABLE `messages` ADD INDEX `messages_idx_timestamp` (`timestamp`);
The optimized query:
SELECT
        f.* 
    FROM
        messages f 
    WHERE
        (
            :user_id IN (
                f.sender, f.receiver
            )
        ) 
        AND (
            (
                LEAST(f.sender, f.receiver), GREATEST(f.sender, f.receiver), f.timestamp
            ) IN (
                SELECT
                    LEAST(b.sender,
                    b.receiver) AS x,
                    GREATEST(b.sender,
                    b.receiver) AS y,
                    MAX(b.timestamp) AS msg_time 
                FROM
                    messages b 
                GROUP BY
                    x,
                    y 
                ORDER BY
                    NULL
            )
        ) 
    ORDER BY
        f.timestamp DESC

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* original question posted on StackOverflow here.