On Thu, Jul 01, 2004 at 15:19:32 +0200,
Dennis Bjorklund <[EMAIL PROTECTED]> wrote:
>
> Looking again at the doc and the example I now know why it can't parse
> it. The example when simplified is:
>
> SELECT *
> FROM (select 1 ORDER BY 1
> UNION ALL
> select 2) AS x;
>
> and it does not parse since the there is an ORDER BY in the first query.
> If we look at the doc page then the UNION comes before the ORDER BY, so it
> is in fact an invalid query (I've not checke the standard, just the select
> doc page).
>
> If you put a () around the first (inner) select it all works. But why
> is the order by there at all? The order of the rows from the UNION ALL can
> (in theory) be random anyway, right?
If DISTINCT ON or LIMIT was used in inner select, then the ORDER BY would
be relevant; so you can't just blindly remove ORDER BY when it is part of
a union.
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TIP 9: the planner will ignore your desire to choose an index scan if your
joining column's datatypes do not match
