Does 'Select' always order by primary key?

Question

A basic simple question for all of you DBA.

When I do a select, is it always guaranteed that my result will be ordered by the primary key, or should I specify it wit

Answer 1

When I do a select, is it always guaranteed that my result will be ordered by the primary key, or should I specify it with an 'order by'?

No, it's by far not guaranteed.

SELECT  *
FROM    table

most probably will use TABLE SCAN which does not use primary key at all.

You can use a hint:

SELECT  /*+ INDEX(pk_index_name) */
        *
FROM    table

, but even in this case the ordering is not guaranteed: if you use Enterprise Edition, the query may be parallelized.

This is a problem, since ORDER BY cannot be used in a SELECT clause subquery and you cannot write something like this:

SELECT  (
        SELECT  column
        FROM    table
        WHERE   rownum = 1
        ORDER BY
                other_column
        )
FROM    other_table