Re: FETCH FIRST clause PERCENT option - Mailing list pgsql-hackers

On 1/6/19 12:40 PM, Surafel Temesgen wrote:
>
>
> On Fri, Jan 4, 2019 at 5:27 PM Tomas Vondra
> <tomas.vondra@2ndquadrant.com <mailto:tomas.vondra@2ndquadrant.com>> wrote:
>
>
>     What formula? All the math remains exactly the same, you just need to
>     update the number of rows to return and track how many rows are already
>     returned.
>
>     I haven't tried doing that, but AFAICS you'd need to tweak how/when
>     node->count is computed - instead of computing it only once it needs to
>     be updated after fetching each row from the subplan.
>
>     Furthermore, you'll need to stash the subplan rows somewhere (into a
>     tuplestore probably), and whenever the node->count value increments,
>     you'll need to grab a row from the tuplestore and return that (i.e.
>     tweak node->position and set node->subSlot).
>
>     I hope that makes sense. The one thing I'm not quite sure about is
>     whether tuplestore allows adding and getting rows at the same time.
>
>     Does that make sense
>
>
>
> In current implementation in LIMIT_INITIAL state we execute outer
> plan to the end , store the resulting tuples in tuplestore and
> calculate limitCount in number .We are in this state only once and
> did not return any tuple. Once we are at LIMIT_INWINDOW state and
> inner node execution asking for tuple it return from tuple store
> immediately.
>

Right.

> Inorder to do fast startup what I was thinking was dividing the work
> done at LIMIT_INITIAL state in to limitCount. For example we want
> 0.5 percent of the result which means in LIMIT_INWINDOW state we
> execute outer node 200 times ,store the result in tuplestore and
> return the first tuple. if we don’t get that much tuple that means we
> reach end of the limit.
Yes, pretty much.