[Solved] aggregate functions with joined tables. Why do these project different customer ids?

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. 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.
Optimal indexes for this query:
ALTER TABLE `customer` ADD INDEX `customer_idx_customer_id` (`customer_id`);
ALTER TABLE `payment` ADD INDEX `payment_idx_customer_id` (`customer_id`);
The optimized query:
SELECT
        c.customer_id,
        c.first_name,
        SUM(p.amount) total,
        COUNT(p.amount) n_payments 
    FROM
        customer c 
    INNER JOIN
        payment p 
            ON c.customer_id = p.customer_id 
    GROUP BY
        c.customer_id 
    ORDER BY
        n_payments DESC

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* original question posted on StackOverflow here.