[fdo-internals] Counting rows in managed IFeatureReader

[Maksim Sestic]

Hi Orest,

My reply's hanging on message queue for some time :-) In short, I need it
for processing time predictions and UI response since I'm dealing with
potentially millions (!) of feature instances. After you and Traian
described the implementation details I understood that it can't be done that
easily as I previously thought.

Regards,
Maksim Sestic
 

-----Original Message-----
From: fdo-internals-bounces at lists.osgeo.org
[mailto:fdo-internals-bounces at lists.osgeo.org] On Behalf Of Orest Halustchak
Sent: Wednesday, April 23, 2008 19:02
To: FDO Internals Mail List
Subject: RE: [fdo-internals] Counting rows in managed IFeatureReader

If the server supports a serializable isolation level, then you can rely on
it, but that implies read locks being applied which can impact the
operations of other users. Otherwise, in a multi-user update situation,
there is always a possibility that data changes between selects. I think SQL
Server supports a snapshot type of isolation that doesn't need read locks,
but I'm not sure of the overhead of that option.

A question for Maksim for his application is why he needs the count before
getting the data rather than knowing it after all the objects have been
selected?

Thanks,
Orest.

-----Original Message-----
From: fdo-internals-bounces at lists.osgeo.org
[mailto:fdo-internals-bounces at lists.osgeo.org] On Behalf Of Jason Birch
Sent: Wednesday, April 23, 2008 12:32 PM
To: FDO Internals Mail List
Subject: RE: [fdo-internals] Counting rows in managed IFeatureReader

So, there is no safe way of getting the count of features that are going to
be returned?

Jason

-----Original Message-----
From: Orest Halustchak
Subject: RE: [fdo-internals] Counting rows in managed IFeatureReader

Even with transactional support, you don't have a guarantee that two selects
will return the same results unless the server supports a snapshot or
serializable type of transaction isolation level, which is not usually
default. Typical isolation levels are read-committed. But, this would be the
same case regardless of whether you did a select aggregates and then a
select versus running a select twice.
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