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mysqlperformancesql-order-bylimit

Why does MYSQL higher LIMIT offset slow the query down?

Scenario in short: A table with more than 16 million records [2GB in size]. The higher LIMIT offset with SELECT, the slower the query becomes, when using ORDER BY *primary_key*

So 

SELECT * FROM large ORDER BY `id`  LIMIT 0, 30

takes far less than 

SELECT * FROM large ORDER BY `id` LIMIT 10000, 30

That only orders 30 records and same eitherway. So it's not the overhead from ORDER BY.
Now when fetching the latest 30 rows it takes around 180 seconds. How can I optimize that simple query?

Solution

  • It's normal that higher offsets slow the query down, since the query needs to count off the first OFFSET + LIMIT records (and take only LIMIT of them). The higher is this value, the longer the query runs.

    The query cannot go right to OFFSET because, first, the records can be of different length, and, second, there can be gaps from deleted records. It needs to check and count each record on its way.

    Assuming that id is the primary key of a MyISAM table, or a unique non-primary key field on an InnoDB table, you can speed it up by using this trick:

    SELECT  t.* 
FROM    (
        SELECT  id
        FROM    mytable
        ORDER BY
                id
        LIMIT 10000, 30
        ) q
JOIN    mytable t
ON      t.id = q.id

    See this article: