Re: Preserving order through an inner join

OK, then.

The sub-select needs to go.

Thanks for helpful advice.

Kevin

--- On Sun, 9/26/10, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:

> From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
> Subject: Re: [GENERAL] Preserving order through an inner join
> To: "Kevin Jardine" <kevinjardine(at)yahoo(dot)com>
> Cc: pgsql-general(at)postgresql(dot)org
> Date: Sunday, September 26, 2010, 9:37 PM
> Kevin Jardine <kevinjardine(at)yahoo(dot)com>
> writes:
> > I have a query structured like this:
> > SELECT stuff FROM
> > (SELECT more stuff FROM
> > table1
> > ORDER BY field1) AS q1
> > INNER JOIN table2 ON ( ... )
>
> > and have found that the INNER JOIN is ignoring the
> order set for q1.
>
> > The final results are not ordered by field1.
>
> Indeed.  Many of the possible join techniques won't
> preserve that ordering.
>
> > This works for other databases (eg. MySQL and
> Sqllite3) but not PostgreSQL.
>
> It might sometimes accidentally fail to fail, but I think
> you'll find
> that there are *no* SQL databases where this is guaranteed
> to work the
> way you expect.  The SQL standard explicitly disavows
> any particular
> output row order unless there is a top-level ORDER
> BY.  (In fact,
> unless things have changed recently an ORDER BY in a
> sub-select isn't
> even legal per spec.)
>
> > I can make some small changes to the query structure
> as long as it works for the other DBs as well. Moving the
> ORDER BY outside q1 would be a large amount of work, however
> (these queries are generated by a program), so I am hoping
> that there is a simpler solution.
>
> Nope, that's what you need to do.
>
>            
> regards, tom lane
>
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