> -----Original Message-----
> From: owner-pgsql-hackers@postgreSQL.org
> [mailto:owner-pgsql-hackers@postgreSQL.org]On Behalf Of Zeugswetter
> Andreas IZ5
> Sent: Tuesday, August 24, 1999 6:48 PM
> To: pgsql-hackers
> Subject: AW: [HACKERS] Caution: tonight's commits force initdb
>
>
>
> > Hmm,Index scan is chosen to select all rows.
> > AFAIK,sequential scan + sort is much faster than index scan in
> > most cases.
> >
> > cost of index scan < cost of sequential scan + cost of sort
> >
> This is usually true. It might need resources though that are not
> available,
Without taking SORT into account
[From my example]
cost of sequential scan = 1716.32 andcost of index scan = 2284.55
cost of sequential scan > cost of index scan * 0.7
It's unbelievable for me.
> e.g. 8 GB sort space. It also depends on whether the application is
> interested in
> first row (interactive), or all row performance (batch). Other DB's can
> switch modes
> to decide on the wanted behavior. So I think there is no yes/no
> decision on
> this.
>
We could use LIMIT clause to get first rows now and optimizer
should take LIMIT/OFFSET into account(TODO item).
Regards.
Hiroshi Inoue
Inoue@tpf.co.jp