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Explicitly Disable Sorting of Result Set

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Problem Definition​

In earlier versions of MySQL, GROUP BY would default to sorting by the grouping fields even without an ORDER BY clause. This could cause unnecessary file sorting which impacts SQL query performance. Adding ORDER BY NULL forces disabling sorting on the result set.

For example in the following:

SELECT l_orderkey, sum(l_quantity) 
FROM lineitem
GROUP BY l_orderkey;

In MySQL 5.x, GROUP BY l_orderkey would cause default sorting, which can be avoided by adding ORDER BY NULL:

SELECT l_orderkey, sum(l_quantity)
FROM lineitem
GROUP BY l_orderkey
ORDER BY NULL;

PawSQL Rewrite​

Conditions​

  • MySQL database, version below 8.0

  • GROUP clause without SORT clause

    Database Types and Versions​

  • MySQL, versions below 8.0

    In MySQL 8.0, GROUP BY itself does not sort on the grouping fields, so GROUP BY l_orderkey can be used directly without needing ORDER BY NULL.

About PawSQL​

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