How to Optimize MySQL Queries with \"Using index; Using temporary; Using filesort\" Error?

Dealing with "Using index; Using temporary; Using filesort" Error in MySQL Queries

A common concern when working with MySQL queries is encountering the warning, "Using index; Using temporary; Using filesort." This message indicates that MySQL is relying on temporary tables and filesort operations, which can impact performance.

In this particular case, the query suffers from two performance issues:

1. Using Temporary Table: The query contains an ORDER BY clause with a different GROUP BY clause and columns from multiple tables. This triggers MySQL to create a temporary table.
2. Using Filesort: The ORDER BY clause sorts by a computed field, which cannot be performed using an index. MySQL therefore resorts to using filesort.

To resolve these issues:

• For Temporary Table Issue: Avoid having different ORDER BY and GROUP BY clauses or use columns from the same table in these clauses.
• For Filesort Issue: Try creating an index on the computed field used in the ORDER BY clause.

Here is an optimized version of the query:

    EXPLAIN SELECT 
            el.object_id, 
            leo.object_desc, 
            COUNT(el.object_id) as count_rows
        FROM 
            event_log el 
            LEFT JOIN lookup_event_objects leo ON leo.object_id = el.object_id
        GROUP BY 
            el.object_id
        ORDER BY 
            count_rows DESC,
            leo.object_desc ASC

By eliminating the use of temporary tables and filesort operations, this revised query should improve performance.

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