You could accomplish this task like so:
xlist = []
query = Foo.all().filter("property_x >" 50).order("-timestamp")
for q in query:
xlist.append(q.property_x)
avg = sum(xlist) / len(xlist)
What Nick is saying, I think, is that fetching 1000 entities is going
to be very resource-intensive, so a better way to do it is to
calculate this data at write-time instead of read-time. For example,
every time you add an entity, you could update a separate entity that
has a property like "average = db.FloatProperty()" with the current
average, and then you could simply fetch that entity and get the
current running average.
On Jun 22, 4:25 pm, herbie <4whi...@o2.co.uk> wrote:
> Ok. Say I have many (>1000) Model entities with two properties 'x'
> and 'date'. What is the most efficient query to fetch say the
> latest 200 entities where x > 50. I don't care what their 'date's
> are as long as I get the latest and x > 50
>
> Thanks again for your help.
>
> On Jun 22, 4:11 pm, "Nick Johnson (Google)" <nick.john...@google.com>
> wrote:
>
> > Consider precalculating this data and storing it against another entity.
> > This will save a lot of work on requests.
>
> > -Nick Johnson
>
> > On Mon, Jun 22, 2009 at 3:55 PM, herbie <4whi...@o2.co.uk> wrote:
>
> > > No the users won't need to read 1000 entities, but I want to calculate
> > > the average of a property from the latest 1000 entities.
>
> > > On Jun 22, 3:30 pm, "Nick Johnson (Google)" <nick.john...@google.com>
> > > wrote:
> > > > Correct. Are you sure you need 1000 entities, though? Your users
> > > > probably
> > > > won't read through all 1000.
>
> > > > -Nick Johnson
>
> > > > On Mon, Jun 22, 2009 at 3:23 PM, herbie <4whi...@o2.co.uk> wrote:
>
> > > > > So to be sure to get the latest 1000 entities I should add a datetime
> > > > > property to my entitie model and filter and sort on that?
>
> > > > > On Jun 22, 1:42 pm, herbie <4whi...@o2.co.uk> wrote:
> > > > > > I know that if there are more than 1000 entities that match a query,
> > > > > > then only 1000 will be return by fetch(). But my question is which
> > > > > > 1000? The last 1000 added to the datastore? The first 1000 added to
> > > > > > the datastore? Or is it undedined?
>
> > > > > > Thanks
> > > > > > Ian
>
> > > > --
> > > > Nick Johnson, App Engine Developer Programs Engineer
> > > > Google Ireland Ltd. :: Registered in Dublin, Ireland, Registration
> > > Number:
> > > > 368047
>
> > --
> > Nick Johnson, App Engine Developer Programs Engineer
> > Google Ireland Ltd. :: Registered in Dublin, Ireland, Registration Number:
> > 368047
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