Dotazovaci plan PostgreSQL

[Karel Zak]

On Tue, Apr 06, 2004 at 02:02:50PM +0200, Ing. Pavel Janousek wrote:
> > -----Original Message-----
> > From: Karel Zak [mailto:zakkr na zf.jcu.cz] 
> > Subject: Re: Dotazovaci plan PostgreSQL
> >     select data from addr union select data from addr2 order by data
> 
> 	Jsem neduvěřivý...:-)
> 
> 	Skutečně je vždy zaručeno, že "Query1 union Query2 order by
> data" splňuje:
> 
> a) distinct(data) = data
> b) vsechna data jsou sesortovana (kde a jak pozna, kde je konec DQL pro
> Query2? - zrovna v mem pripade "data" je zaroven sloupec tabulky v
> Query2 - proto ty zavorky a proto ten select * (union query) order
> by...)
> ?

 Citace z kodu:

         SQL92 requires
                      SELECT foo UNION SELECT bar ORDER BY baz
         to be parsed as
                      (SELECT foo UNION SELECT bar) ORDER BY baz
         not
                      SELECT foo UNION (SELECT bar ORDER BY baz)

         Likewise FOR UPDATE and LIMIT.


 Jak to pozna bych nechal na gramtice parseru :-)

    Karel

-- 
 Karel Zak  <zakkr na zf.jcu.cz>
 http://home.zf.jcu.cz/~zakkr/