Why is ORDER BY in a FROM Subquery Ignored?

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, 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


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