EverSQL Database Performance Knowledge Base

The optimization process and recommendations:

1. Avoid Selecting Unnecessary Columns (query line: 17): Avoid selecting all columns with the '*' wildcard, unless you intend to use them all. Selecting redundant columns may result in unnecessary performance degradation.
2. Avoid Subqueries (query line: 4): 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.
3. Avoid Subqueries (query line: 19): 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.
4. 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.
5. 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'.
6. Use UNION ALL instead of UNION (query line: 16): Always use UNION ALL unless you need to eliminate duplicate records. By using UNION ALL, you'll avoid the expensive distinct operation the database applies when using a UNION clause.

Optimal indexes for this query:

ALTER TABLE `clienti_con_rinnovo` ADD INDEX `clienti_rinnovo_idx_id` (`id`);
ALTER TABLE `transactions` ADD INDEX `transactions_idx_renew_id` (`renew`,`id`);
ALTER TABLE `transactions` ADD INDEX `transactions_idx_id_datetransaction` (`id`,`dateTransaction`);

The optimized query:

SELECT
        * 
    FROM
        (SELECT
            transactions.id,
            MAX(transactions.dateTransaction) AS last_transaction_renew 
        FROM
            transactions 
        WHERE
            transactions.renew IS NOT NULL 
        GROUP BY
            transactions.id 
        ORDER BY
            NULL) AS clienti_con_rinnovo 
    UNION
    SELECT
        * 
    FROM
        (SELECT
            transactions.id,
            MAX(transactions.dateTransaction) AS last_transaction_renew 
        FROM
            transactions 
        WHERE
            transactions.renew IS NULL 
        GROUP BY
            transactions.id 
        ORDER BY
            NULL) AS clienti_senza_rinnovo 
    LEFT JOIN
        clienti_con_rinnovo 
            ON clienti_con_rinnovo.id = clienti_senza_rinnovo.id 
    WHERE
        clienti_con_rinnovo.id IS NULL