[fdo-internals] Counting rows in managed IFeatureReader

Tue Apr 22 08:46:30 EDT 2008

Hi Maksim,

I think the point around performance issues with Count() on the feature reader is not related to side effect if it isn't used but that if it is used it may be much less efficient than using Count() with Select Aggregates which may use a native Count() implementation. Selecting all the records just to get a count and then selecting them all over again is duplicated processing.

Making an internal copy of a reader may be more of an issue than a simple clone of that reader object. The reader is generated as a result of the execution of a select. If you start reading through the results of the reader, to be able to start reading again, in most cases it means the provider needs to re-execute the select (exception may be providers that support a scrollable reader, but not all have that sort of capability). You can re-execute the select yourself and will get a separate reader.

Could you explain your application scenario where you need this ability and maybe we can think of other alternatives?

Thanks,
Orest.

-----Original Message-----
From: fdo-internals-bounces at lists.osgeo.org [mailto:fdo-internals-bounces at lists.osgeo.org] On Behalf Of Maksim Sestic
Sent: Tuesday, April 22, 2008 8:02 AM
To: fdo-internals at lists.osgeo.org
Subject: RE: [fdo-internals] Counting rows in managed IFeatureReader

Hi Barbara,

I'm aware of it, but Count would not get calculated on IFeatureReader
instantiation (i.e. via ISelect.Execute). The property would act as a
function, when explicitly called (get). If called, it should create an
internal copy of underlying reader, traversing it to return total number of
feature instances. If it's not calculated on IFeatureReader instantiation
then there're no performance side effects. A private member could be used
for storing once-calculated features count for faster consequent calls to
Count() function of the same instance of IFeatureReader.

Now, why IsEmpty when there's a Count? Because IsEmpty traverses only the
first record of possibly many records, returning False if ReadNext doesn't
return Nothing. IsEmpty() works on the same premises as above described
Count() mechanism.

So user can chain calls to get the results with less overhead:

If Not reader.IsEmpty
   Dim cnt As Integer = reader.Count
...
End If

I think that it's better to have it implemented on unmanaged tier, since
doing it using managed bindings may sometimes lead to unexpected results.

Robert's proposal is OK, but the downside is that SelectAggregates can't
lock the records being fetched.

Regards,
Maksim Sestic

Barbara Zoladek wrote:
>
> Maksim,
> Adding Count to IFeatureReader will have performance side effects. There
> is no way to find out the number of rows without either fetching all the
> rows or explicitly executing something like 'select count(*)' .
>
> Robert's suggestion looks better.
>
> Thanks,
> Barbara.
>
> -----Original Message-----
> From: fdo-internals-bounces at lists.osgeo.org
> [mailto:fdo-internals-bounces at lists.osgeo.org] On Behalf Of Maksim Sestic
> Sent: Thursday, April 17, 2008 2:56 AM
> To: fdo-internals at lists.osgeo.org
> Subject: RE: [fdo-internals] Counting rows in managed IFeatureReader
>
>
> Thanks for the tip, Robert. I need to use ISelect because of locking
> (which
> is not supported by ISelectAggregates).
>
> I'd suggest that generic IFeatureReader (neverthless of provider involved)
> expose two additional readonly properties:
>
> 1) Count - creates internal copy of a reader and traverses it, returning
> total number of feature instances found in it.
>
> 2) IsEmpty - boolean returning True if reader has no features instances in
> it, False otherwise.
>
> Regards,
> Maksim Sestic
>
>
>
> Robert Fortin wrote:
>>
>> Maksim,
>>
>> You can use the IselectAggregate command with the COUNT function to
>> retrieve the number of rows for a given class.
>> This will ensure that you use the optimize implementation of the native
>> data source to retrieve the number of row where it is available.
>>
>> Robert Fortin
>>
>> -----Original Message-----
>> From: fdo-internals-bounces at lists.osgeo.org
>> [mailto:fdo-internals-bounces at lists.osgeo.org] On Behalf Of Maksim Sestic
>> Sent: Wednesday, April 16, 2008 6:11 AM
>> To: fdo-internals at lists.osgeo.org
>> Subject: [fdo-internals] Counting rows in managed IFeatureReader
>>
>>
>> Hi all,
>>
>> Since IFeatureReader doesn't provide Count property for total number of
>> records in it, what's the most "proper" way to get it out from managed
>> IFeatureReader? I'm asking this because of necessity to explicitly close
>> a
>> reder after it's returned from, say, ISelect command. How do I get a safe
>> copy of instantiated managed IFeatureReader object to count it's
>> properties,
>> without affecting the original one?
>>
>> It would be a no brainer if IFeatureReader was fully implemented in .NET,
>> but managed bindings always make me wonder if unmanaged resources get
>> disposed in a proper way.
>>
>> Regards,
>> Maksim Sestic
>> --
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