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How to tune “Order by Limit N” SQL statement?

Discussion in 'MySQL' started by Richard To, Aug 17, 2020.

  1. Richard To

    Richard To New Member

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    There may be some business requirements that need to retrieve the first N number of rows from a join tables. Some people may encounter unexpected performance problem.

    Here is an example SQL that retrieves first 1000 row from a join tables of Employee and Department in the order of dpt_id. Where emp_dept and dpt_id columns are both indexed.

    select *
    from employee ,department
    where emp_dept=dpt_id
    order by dpt_id
    limit 1000;

    Here the following are the query plans of this SQL, it takes 1 mins and 8 seconds to finish. The query shows a nested loop from “Full Table Scan Employee” to “Unique Key Lookup Department” table to extract all records. An “Order by” operation of dpt_id of Department table is executed followed from this join result.
    [​IMG]
    You can see the most expensive step is to extract all Employee data with Department data and then an Order by dpt_id of Department table. Let’s see if we rewrite the original SQL text into the syntax that “order by dpt_id” is changed to “order by emp_dept”. It is a semantically eqvialent rewrite of original SQL statement since “emp_dept = dpt_id”.

    select *
    from employee,
    department
    where emp_dept = dpt_id
    order by emp_dept
    limit 1000;

    Here is the query plan of the rewritten SQL with less cost and run much faster. The new query plan shows that no “Use temporary; Using filesort” in Tabular Explain, it means the Nested Loop operation will be stop at the Limit 1000 records is done.

    [​IMG]
    This kind of rewrite can be archived by Tosska SQL Tuning Expert for MySQL automatically, it shows that the rewrite is almost 70 times faster than the original SQL.

    https://tosska.com/tosska-sql-tuning-expert-tse-for-mysql-2/
    [​IMG]
     
    shabbir likes this.

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