The Ultimate Guide to High Availability with MariaDB

Why is ORDER BY in a FROM Subquery Ignored?

You are viewing an old version of this article. View the current version here.

Query with ORDER BY in a FROM subquery produces unordered result. Is this a bug? Below is an example of this:

SELECT field1, field2 FROM ( SELECT field1, field2 FROM table1 ORDER BY field2 ) alias

returns a result set that is not necessarily ordered by field2. This is not a bug.

A "table" (and subquery in the FROM clause too) is - according to the SQL standard - an unordered set of rows. Rows in a table (or in a subquery in the FROM clause) do not come in any specific order. That's why the optimizer can ignore the ORDER BY clause that you have specified. In fact, the SQL standard does not even allow the ORDER BY clause to appear in this subquery (we allow it, because ORDER BY ... LIMIT ... changes the result, the set of rows, not only their order).

You need to treat the subquery in the FROM clause, as a set of rows in some unspecified and undefined order, and put the ORDER BY on the top-level SELECT.

Source: MDEV-3926, Comment by Sergei Golubchik

See also

Comments

Comments loading...
Content reproduced on this site is the property of its respective owners, and this content is not reviewed in advance by MariaDB. The views, information and opinions expressed by this content do not necessarily represent those of MariaDB or any other party.