Re: FETCH FIRST clause PERCENT option

On Thu, Jan 3, 2019 at 4:51 PM Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com>
wrote:

>
> On 1/3/19 1:00 PM, Surafel Temesgen wrote:
> > Hi
> >
> > On Tue, Jan 1, 2019 at 10:08 PM Tomas Vondra
> > <tomas(dot)vondra(at)2ndquadrant(dot)com <mailto:tomas(dot)vondra(at)2ndquadrant(dot)com>>
> wrote:
>
> > The execution part of the patch seems to be working correctly, but I
> > think there's an improvement - we don't need to execute the outer
> plan
> > to completion before emitting the first row. For example, let's say
> the
> > outer plan produces 10000 rows in total and we're supposed to return
> the
> > first 1% of those rows. We can emit the first row after fetching the
> > first 100 rows, we don't have to wait for fetching all 10k rows.
> >
> >
> > but total rows count is not given how can we determine safe to return row
> >
>
> But you know how many rows were fetched from the outer plan, and this
> number only grows grows. So the number of rows returned by FETCH FIRST
> ... PERCENT also only grows. For example with 10% of rows, you know that
> once you reach 100 rows you should emit ~10 rows, with 200 rows you know
> you should emit ~20 rows, etc. So you may track how many rows we're
> supposed to return / returned so far, and emit them early.
>
>
>

yes that is clear but i don't find it easy to put that in formula. may be
someone with good mathematics will help

regards
Surafel