[Solved] How can I make this query run faster in SQL?

EverSQL Database Performance Knowledge Base

How can I make this query run faster in SQL?

How can I make this query run faster:

SELECT *
FROM ( SELECT DISTINCT l.year,
              (select count(laws.id) from laws as laws WHERE laws.year = l.year  ) as count
      FROM laws as l
      ORDER BY l.year DESC
    ) s
WHERE count > 0

It runs about 23 seconds for 10000 records.

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 Correlated Subqueries In Select Clause (modified query below): The aggregation function located in a subquery inside the SELECT clause, is executed once for every matched row. Extracting this subquery to a temporary table will improve performance significantly.
  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. 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 `es_temp1` ADD INDEX `es_temp1_idx_year` (`year`);
ALTER TABLE `laws` ADD INDEX `laws_idx_year` (`year`);
The optimized query:
SELECT
        * 
    FROM
        (SELECT
            DISTINCT l.year,
            es_temp1.count 
        FROM
            laws AS l 
        LEFT JOIN
            es_temp1 
                ON es_temp1.year = l.year 
        ORDER BY
            l.year DESC) s 
    WHERE
        count > 0

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* original question posted on StackOverflow here.