Ahh, thanks Ryan..that makes sense.
So for lenza...you need to try and squeeze your itemBs construction
into one RPC call..so..something like this might do the trick:
def myQuery():
query = A.all().filter('intProperty', val).filter('date >=', date)
itemAs = [itemA for itemA in query]
itemBkeys = [A.typeB.get_value_for_datastore(itemA) for itemA in
itemAs]
itemBs = db.get(itemBkeys)
return itemBs
I haven't tested it, but I think that should boil down itemBs'
construction from n RPC calls to 1, which should bring it more in line
with the cost of itemA's construction.
On Mar 11, 12:02 am, ryan <ryanb+appeng...@google.com> wrote:
> On Mar 10, 4:38 pm, peterk <peter.ke...@gmail.com> wrote:
>
> > Sorry if some redundant points are being raised here.. my lingering
> > question in light of your post is regarding the variation in
> > performance between itemAs' construction and itemBs' if both boil down
> > to the same read behaviour on the datastore. To walk through lenza's
> > code:
>
> sorry, you're right. i misread the original code snippets. let me try
> again.
>
> > query = A.all().filter('intProperty', val).filter('date >=', date) # no
> > reads at this point (?)
>
> correct.
>
> > itemAs = [itemA for itemA in query] # n disk seeks and reads where n =
> > number of itemAs matching query (?)
>
> roughly, yes. it's actually around n + n/20.
>
> > itemBs = [itemA.typeB for itemA in itemAs] # n disk seeks and reads for n
> > automatic dereferencing of itemA.typeB accesses (?)
>
> correct.
>
> > I'm guessing it's not that simple given the variation in performance
> > between line 2 and line 3 reported by lenza. I'm gonna dig into your
> > links tomorrow, so if an answer is in there, don't mind me! :)
>
> the difference in performance is probably due to the RPC overhead
> between the app and the datastore. the query only takes n/20 RPCs, one
> for each result batch. the itemBs list comprehension takes n RPCs,
> since it does a get() for each itemA.typeB. (that's due to the list
> comprehension itself, which is in the app's code, so we can't collapse
> those RPCs because we don't know they're coming ahead of time.)
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