Re: Why does the number of rows are different in actual and estimated.

Does FK Constraint help to improve performance? Or it is only
for maintaining data integrity?

On Thu, Dec 13, 2012 at 7:38 PM, Claudio Freire <klaussfreire(at)gmail(dot)com>wrote:

> On Thu, Dec 13, 2012 at 8:50 PM, Evgeny Shishkin <itparanoia(at)gmail(dot)com>
> wrote:
> >>> OP joins 8 tables, and i suppose join collapse limit is set to default
> 8. I thought postgresql's optimiser is not mysql's.
> >>
> >> It's not obvious to me that there's anything very wrong with the plan.
> >> An 8-way join that produces 150K rows is unlikely to run in milliseconds
> >> no matter what the plan. The planner would possibly have done the last
> >> join step differently if it had had a better rowcount estimate, but even
> >> if that were free the query would still have been 7 seconds (vs 8.5).
> >>
> >
> > May be in this case it is. I once wrote to this list regarding similar
> problem - joining 4 tables, result set are off by 2257 times - 750ms vs
> less then 1ms. Unfortunately the question was not accepted to the list.
> >
> > I spoke to Bruce Momjian about that problem on one local conference, he
> said shit happens :)
>
> I think it's more likely a missing FK constraint.
>