On Thu, Jan 3, 2019 at 4:51 PM Tomas Vondra <tomas.von...@2ndquadrant.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.von...@2ndquadrant.com <mailto:tomas.von...@2ndquadrant.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
