subselect, order by and left join

[Morten K. Poulsen]

(re-post)

Dear list,

Please let me know if this is not the list to ask this kind of question.

I am trying to optimize a query that joins two relatively large (750000 rows in
each) tables. If I do it using a subselect, I can "force" the planner to choose
the fastest path. Now, my question is:

If I have a subselect with an ORDER BY, and I LEFT JOIN the result with the
other table, is the order maintained? Or is PostgreSQL free to return the rows
in any order, after the join?

My query is the following:

SELECT a.*
FROM (SELECT * FROM tree WHERE parent_id=1363405 ORDER BY order_index DESC) AS a
LEFT JOIN content AS b ON a.object_id=b.id
WHERE (b.onair = 't') LIMIT 1;

Thanks,
Morten

--
Morten K. Poulsen <mo***************@afdelingp.dk>
http://www.afdelingp.dk/

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[Stephan Szabo]

On Mon, 8 Nov 2004, Morten K. Poulsen wrote:

Please let me know if this is not the list to ask this kind of question.

I am trying to optimize a query that joins two relatively large (750000 rows in
each) tables. If I do it using a subselect, I can "force" the planner to choose
the fastest path. Now, my question is:

If I have a subselect with an ORDER BY, and I LEFT JOIN the result with the
other table, is the order maintained? Or is PostgreSQL free to return the rows
in any order, after the join?

AFAIK, you have no guarantees as to the output order unless you have
another order by. The join may destroy the ordering, so even if you get
the ordering you want right now, you shouldn't rely on it.

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[Morten K. Poulsen]

On Mon, Nov 08, 2004 at 04:54:40AM -0800, Stephan Szabo wrote:

If I have a subselect with an ORDER BY, and I LEFT JOIN the result with the
other table, is the order maintained? Or is PostgreSQL free to return the
rows in any order, after the join?

AFAIK, you have no guarantees as to the output order unless you have another
order by. The join may destroy the ordering, so even if you get the ordering
you want right now, you shouldn't rely on it.

OK. Thanks for the reply.

Morten

--
Morten K. Poulsen <mo***************@afdelingp.dk>
http://www.afdelingp.dk/

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