How to optimize MySQL that uses IN

Question

I got 3 tables:

procedures (some kind of information about different internal procedures)

procedure_department (link table, for which department each procedure is valid for)

permissions (each user can add/edit/delete procedure he got permissions to)

Im willing to select all the procedures that the current user got access to or added himself.

$sql = "SELECT p.id, p.name, p.type, p.status
        FROM procedures AS p
            LEFT JOIN procedure_department AS pd ON p.id = pd.procedureID
        WHERE p.addedBy = 47 OR pd.departmentID IN (SELECT `option` FROM permissions WHERE user = 47 AND module = 'info' AND permission = 'add') ";

However it takes years to execute and I would like to see any suggestions for optimization from structure or query point of view.

Answer 1

Although this has been fixed in more recent versions, some versions of MySQL do a poor job of optimizing in with a subquery. You can replace it with exists:

SELECT p.id, p.name, p.type, p.status
FROM procedures p LEFT JOIN
     procedure_department pd
     ON p.id = pd.procedureID
WHERE p.addedBy = 47 OR
      exists (SELECT 1
              FROM permissions
              WHERE user = 47 AND module = 'info' AND permission = 'add' and
                    option = pd.departmentID
             );

For best performance, add an index on permissions(option, user, module, permission).