Why is my MySQL SELECT statement with ORDER BY and LIMIT slow on a large dataset, and how can I speed it up?

MySQL Statement Executes Slowly with a Large Dataset

When working with large databases, query execution time can become a concern. This article discusses a specific scenario where a query on a table with over one million records takes over a minute to execute.

The Problem

The following query was taking over a minute to execute, even with an indexed "id" column:

SELECT *
FROM `ratings`
ORDER BY id
LIMIT 499500, 500;

The table contained around one million records, and the query involved ordering by the "id" column and limiting the results to a specific range.

Analysis and Solution

After investigating the query and table structure, it was determined that the slow execution time was due to the use of a "filesort" operation. This occurs when MySQL cannot find an appropriate index to use for the ORDER BY clause and has to sort the table rows in memory.

To solve this issue, it was recommended to use a more selective WHERE clause instead of the ORDER BY clause. In this case, the following query executed much faster:

SELECT *
FROM `ratings`
WHERE id >= 499501
LIMIT 500;

This query used the indexed "id" column to narrow down the rows to be sorted, significantly reducing the time required to execute the query.

Additional Considerations

It is important to note that using a WHERE clause instead of an ORDER BY clause may not always be possible or practical. In such cases, other optimizations such as creating additional indexes or optimizing the database structure may be necessary. Additionally, potential deadlocks should also be ruled out as a possible cause of slow performance.

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