Brock Anderson wrote:
Hi List,
I ran into a curious situation with a quadtree (.qix) index on a
shapefile. Basically the issue is that performance of scanning the
index to fetch features is not as good as I expect. Some details:
I have a shapefile data set with 4 million polygons fairly normally
distributed around British Columbia, Canada. I used 'shptree' to create
a spatial index on that data. I have a very simple layer in my mapfile
pointing at the data. Minimal styling, no labels, etc.
I then make a simple WMS request to fetch exactly 1000 features (limited
by a bbox) from the layer. Mapserver take about 500ms. Seems a bit high.
Geoserver can draw the exact same data, using the *same .qix* index in
about 150ms. Naturally I made every effort to keep the comparison
fair. No reprojection in either case, nearly identical styling, etc.
As a further comparison I noticed that Mapserver and Geoserver are
nearly equal for fetching/drawing 1000 features from a smaller data set
of just 10,000. Response time there is more like 120ms.
So on large shapefile data sets Geoserver's index scanning seems to be
substantially faster. Are there any map file options that might improve
performance? Could it be that Geoserver simply has a faster
implementation for traversing the quadtree?
Brock,
I haven't dug into this, but I think MapServer pulls the features from
the shapefile in the order they are identified traversing the spatial
index. This *could* mean relatively random fetching depending on the
spatial coherence (or lack thereof) of the records in the shapefile.
If (and I haven't actually done any investigation) GeoServer uses the
spatial index to build a list of feature ids, sorts that and then
fetches them in the order they appear in the shapefile then GeoServer
might pull the data in a much more "in order" form from disk which
can give much better read performance.
I stress this is not something I did any investigation into. It is
just a wild theory that might explain it.
BTW, OGR also uses .qix files, and does build one bit list which it
sorts exactly to avoid random fetching.
Best regards,
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