Re: Slow SQL?

On Tue, Jul 12, 2016 at 10:23:24AM +0200, Bjørn T Johansen wrote:
> I am trying to move a small system from Oracle to PostgreSQL and
> I have come upon a sql that runs really slow compared to on the Oracle
> database and I am not able to interpret why this is slow.

I loaded your explain analyze to https://explain.depesz.com/, as:
https://explain.depesz.com/s/iXK

as you can see there, the problem is that you made 280 thousand checks
for "sed_uttak y", which seems to be related to this part:


> Select a.status, a.plass, a.navn, a.avlsnr,
>            date_part('day',(now() - s.dato)) dato_diff, v.tekst, COALESCE(a.avlsverdi,0)
> From   sed_uttak s, sem_avlsverdi a, semin_vare v
> where a.aktiv = 1
> And    s.dato  = (Select Max(y.dato)
>                           From sed_uttak y
>                           Where y.avlsnr = s.avlsnr)

from what I understand, you're doing it to work on newest record from
sed_uttak, for each avlsnr.

What is rowcount in the table, and how many different avlsnr are there?

You might want to do something like:

with s as (
    select distinct on (avlsnr) *
    from sed_uttak
    order by avlsnr desc, dato desc
)

and then use "s" instead of set_uttak, and get rid of the s.dato
= (select max....) checks.

Best regards,

depesz

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