Re: ORDER BY <field not in return list>

On Mon, Jul 25, 2005 at 07:06:46PM -0300, Marc G. Fournier wrote:
>
>
> On Mon, 25 Jul 2005, Jim C. Nasby wrote:
>
> >On Mon, Jul 25, 2005 at 06:11:08PM -0300, Marc G. Fournier wrote:
> >>
> >>Just curious as to whether or not a warning or something should be issued
> >>in a case like:
> >>
> >> SELECT c.*
> >> FROM company c, company_summary cs
> >> WHERE c.id = cs.id
> >> AND cs.detail = 'test'
> >>ORDER BY cs.fullname;
> >>
> >>Unless I'm missing something, the ORDER BY clause has no effect, but an
> >>EXPLAIN shows it does take extra time, obviously ...
> >
> >Uh, I'd hope it had an effect. Note that RDBMSes have been moving
> >towards allowing fields in ORDER BY that aren't in the SELECT list,
> >though in the past it was common that anything in ORDER BY had to also
> >be in SELECT.
>
> 'k, in the test case I've been working with, the query always returns 1
> row, so my test case wouldn't have shown a difference ... but, if it does
> have an affect, how? The ORDER BY is on the final result set, and if
> there is no cs.fullname in that result, what exactly is it ordering?

decibel=# select usename, usesysid from pg_user;
usename | usesysid
----------+----------
postgres | 1
decibel | 100
(2 rows)

decibel=# select usesysid from pg_user order by usename;
usesysid
----------
100
1
(2 rows)

decibel=# explain analyze select usesysid from pg_user order by usename;
QUERY PLAN
----------------------------------------------------------------------------------------------------------
Sort (cost=1.02..1.02 rows=1 width=68) (actual time=0.182..0.183 rows=2 loops=1)
Sort Key: pg_shadow.usename
-> Seq Scan on pg_shadow (cost=0.00..1.01 rows=1 width=68) (actual time=0.060..0.077 rows=2 loops=1)
Total runtime: 0.518 ms
(4 rows)

decibel=#
--
Jim C. Nasby, Database Consultant decibel(at)decibel(dot)org
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