On 07/19/2010 02:08 PM, Martin Dobias wrote:
Hi,
in order to speed up rendering in QGIS as a part of my GSoC project,
I've took some time to profile reading of shapefiles in OGR. From the
results I'd like to suggest some changes that significantly contribute
to the speed of data retrieval. On a test shapefile of a road network
(about 100 thousand polylines), I have seen 3-4 times faster retrieval
when I've implemented the following changes:
1. allow users of OGR library set which fields they really need. Most
of time is wasted by fetching all the attributes, but typically none
or just one attribute is necessary when rendering. For that, I've
added the following call:
OGRLayer::SetDesiredFields(int numFields, int* fields);
The user passes an array of ints, each item tells whether the field
should be fetched (1) or not (0). The numFields tells the size of the
array. If numFields< 0 then the layer will return all fields (default
behavior). The driver implementation then just before fetching a field
checks whether to fetch the field or not. This optimization could be
easily used in any driver, I've implemented it only for shapefiles.
The speedup will vary depending on the size of the attribute table and
number of desired fields. On my test shapefile containing 16 fields,
the data has been fetched up to 3x faster when no fields were set as
desired.
I have a couple of questions.
Comparing to other Shapefile tools like Arc* and TatukGIS, the OGR-based tool Quantum GIS
has really bad performance because it just does too much. Do you think your solution can
be applied to Qgis? I mean when displaying geometry, it can call SetDesiredFields() with
all 0s. And only when user opens Attribute Table, Qgis can re-get all/selected fields.
Will this be possible with your patch?
--
Yevgen
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