Re: delete taking long time

On Tuesday, March 15, 2016, Andreas Kretschmer <andreas(at)a-kretschmer(dot)de>
wrote:

>
> >
> > I am trying to delete the connections with date 2016-03-10 by using the
> > following:
> >
> >
> > delete from connection where uid in (select uid from connection where ts
> >
> > '2016-03-10 00:30:00');
>
> try to rewrite that to :
>
> delete from connection where ts > '2016-03-10 00:30:00';
>
> It's simpler - and (maybe) faster.
>
>
>
It also gives a different answer...

>
> > There are around 800.000 records matching this rule, and seems to be
> taking
> > an awful lot of time - 4 hours and counting. What could be the reason for
> > such a performance hit and how could I optimise this for future cases?
> >
> > Regards.
>
> the db has to touch such many rows, and has to write the transaction log.
> And
> update every index. And it has to check the referenced tables for the
> constraints. Do you have proper indexes?

Given the lack of indexes on the one table that is shown I suspect this is
the most likely cause (FK + indexes)

>
>
David J.