
> I experienced that UNIONs in 7.1.1 are rather slow:
> 
> tir=# explain (select nev from cikk) union (select 
> tevekenyseg from log);
> NOTICE:  QUERY PLAN:
> 
> Unique  (cost=667.63..687.18 rows=782 width=12)
>   ->  Sort  (cost=667.63..667.63 rows=7817 width=12)
>         ->  Append  (cost=0.00..162.17 rows=7817 width=12)
>               ->  Subquery Scan *SELECT* 1  (cost=0.00..28.16 
> rows=1316 width=12)
>                     ->  Seq Scan on cikk  (cost=0.00..28.16 
> rows=1316 width=12)
>               ->  Subquery Scan *SELECT* 2  
> (cost=0.00..134.01 rows=6501 width=12)
>                     ->  Seq Scan on log  (cost=0.00..134.01 
> rows=6501 width=12)
> 
> Of course a simple SELECT is fast:
> 
> tir=# explain select nev from cikk;
> NOTICE:  QUERY PLAN:
> 
> Seq Scan on cikk  (cost=0.00..28.16 rows=1316 width=12)
> 
> 
> For me it seems to be slow due to the sorting. Is this right?
> Is this normal at all? Is it possible to make it faster?

If you know, that your result does not produce duplicates
(which are filtered away with "union") you can use a 
"union all" which should be substantially faster, since it does 
not need to sort.

Andreas

Wed, 19 Sep 2001 09:15:52 +0200
"Zeugswetter Andreas SB SD" <ZeugswetterA@spardat.at>
Re: slow UNIONing
> > For me it seems to be slow due to the sorting. Is this right?
> > Is this normal at all? Is it possible to make it faster?
> 
> If you know, that your result does not produce duplicates
> (which are filtered away with "union") you can use a 
> "union all" which should be substantially faster, since it does 
> not need to sort.

Thank you to all who helped. I knew nothing about UNION ALL, but now it's
OK. Regards, Zoltan


Fri, 21 Sep 2001 08:50:57 +0200 (CEST)
Kovacs Zoltan <kovacsz@pc10.radnoti-szeged.sulinet.hu>
Re: slow UNIONing