Jason Roberts wrote:
Dear OGR devs,
OGRLayer::TestCapability allows you to determine if an OGRLayer supports
various things, such as creating new features or updating or deleting
existing features. Can you advise me whether there are any drivers for
which the capabilities differ for a given layer depending on whether it
is instantiated as an OGRLayer via OGRDataSource::GetLayer vs.
OGRDataSource::ExecuteSQL? Also, whether the capabilities will change
after OGRLayer::SetAttributeFilter or OGRLayer::SetSpatialFilter is
called on the resulting OGRLayer instance?
Jason,
The results of TestCapability() do differ for regular layers vs
ExecuteSQL layers and also vary depending on whether filters are in
place or not. So, the "answer" to TestCapability() can be quite
dynamic.
In my scenario, I occasionally need to open layers using the equivalent
of a SQL WHERE and/or ORDER BY clauses. WHERE can be achieved by either
including the expression in a call to OGRDataSource::ExecuteSQL, or by
calling OGRDataSource::GetLayer and then OGRLayer::SetAttributeFilter on
the resulting instance. ORDER BY can be only achieved via
OGRDataSource::ExecuteSQL (please correct me if I'm wrong here).
Basically, I would like to know if the following approach is a robust
way of checking capabilities:
1. Call OGRDataSource::GetLayer and then OGRLayer::TestCapability to
determine what capabilities are supported for that layer.
2. Sometime later, call either OGRDataSource::ExecuteSQL or
OGRDataSource::GetLayer again (possibly followed by
OGRLayer::SetAttributeFilter and/or OGRLayer::SetSpatialFilter),
depending on whether ORDER BY or WHERE are ultimately required for the
specific situation.
I would like to know if the capabilities that were tested in step #1
will remain true in step #2, or if OGRLayer::TestCapability must be
called after the final OGRLayer is instantiated and
OGRLayer::SetAttributeFilter and/or OGRLayer::SetSpatialFilter have been
called, just prior to accessing the features.
No, they will not.
Best regards,
--
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I set the clouds in motion - turn up | Frank Warmerdam, warmer...@pobox.com
light and sound - activate the windows | http://pobox.com/~warmerdam
and watch the world go round - Rush | Geospatial Programmer for Rent
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