Why is My MySQL Query Using \'Using Index; Using Temporary; Using Filesort\'?

Using Index, Using Temporary, Using Filesort: Does Your MySQL Query Need Optimization?

When retrieving data from database tables, it's crucial to write efficient queries that minimize processing time and resource consumption. MySQL provides various optimization techniques to facilitate faster query execution, but sometimes, queries can still exhibit suboptimal behavior. In this case, we'll explore a query that returns "Using index; Using temporary; Using filesort" in the EXPLAIN output.

Let's examine the provided 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

Understanding "Using Temporary"

MySQL documentation states that a temporary table can be created when there's a discrepancy between the ORDER BY and GROUP BY clauses, or when the ORDER BY or GROUP BY contains columns from tables other than the first table in the join queue. In this query, we have both conditions met. The ORDER BY clause sorts by count_rows, which is a computed field, and the GROUP BY clause groups by el.object_id from a different table. Hence, the use of a temporary table to facilitate the sorting and grouping process.

Understanding "Using Filesort"

Since the sorting is performed on a computed field count_rows, an index cannot be utilized for this operation. Therefore, MySQL resorts to using filesort, which involves reading the data from disk and sorting it in memory before returning the results.

Optimizing the Query

To improve the performance of this query and eliminate the use of temporary tables and filesort, we can consider creating an appropriate index, such as:

CREATE INDEX idx_event_log_object_count ON event_log(object_id, COUNT(event_id));

This index will allow MySQL to directly retrieve the count of events for each object_id, eliminating the need for a temporary table and filesort.

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