MS RFC 114: Faster retrieval of shape count




Even Rouault





MapServer 7.2


Meteorological Service of Canada

1. Motivation

The WFS protocol has a resultType=hits query to return only the number of features of the specified layer(s), taking into account an optional bounding box and/or filter.

See “resultType parameter” chapter (OGC WFS 2.0 specification, sect.

Currently MapServer implements such requests by retrieving all individual matching features and counting them, as in a full GetFeature response, which is suboptimal in term of speed and bandwidth with the server instance in case of RDBMS data provider.

2. Proposed enhancement

A new virtual method is added to the vtable of MapServer layers

int (*LayerGetShapeCount)(layerObj *layer, rectObj rect, projectionObj *rectProjection);

Its semantics is to return the number of shapes that match the potential filter and extent. rectProjection is the projection in which rect is expressed, or can be NULL if rect should be considered in the layer projection. This should be equivalent to calling msLayerWhichShapes() and counting the number of shapes returned by msLayerNextShape(), honouring layer->maxfeatures limitation if layer->maxfeatures>=0, and honouring layer->startindex if layer->startindex >= 1 and paging is enabled. It should returns -1 in case of failure.

This method will be called by msQueryByRect() and msQueryByFilter() when only feature count is requested (i.e. map->query.only_cache_result_count == 1), and that a layer template is defined (the WFS code has traditionnaly set a fake layer template ‘ttt.html’ if none was defined, so as to disable any class template). Such query is only done by the WFS server code.

A default implementation of LayerGetShapeCount() using msLayerWhichShapes() and msLayerNextShape() is provided, and a specialized implementation is done in the PostGIS driver. This implementation consists in calling SELECT COUNT(*) FROM (the_normal_sql_query).

Why specifying the rectProjection in LayerGetShapeCount(), and not assuming that rect is in the layer projection instead ? The reason is to be completely conformant with the current semantics of msQueryByRect() that checks that the retrieved features intersect the bounding box in the map projection (the map projection being the one specified by SRSNAME in the case of a WFS query). In the case of the PostGIS provider, this can be translated in the following query to do a fast BBOX filtering, followed by a more precision intersection for candidate records : (the_geometry && the_bbox_in_layer_projection) AND ( NOT ST_Disjoint(ST_Transform(the_geometry, rectProjectionSRID), rectProjection) )

2.2 Backwards Compatibility

This change has no backwards compatibility effect, keeping the same semantics as the current implementation.

2.3 Performance Implications

Should result in performance increase for drivers that can implement efficiently LayerGetShapeCount()

Currently, resultType=hits honours the server side limit defined by the wfs_maxfeatures WEB metadata item, i.e. that the maximum number of features returned is limited to wfs_maxfeatures when it is defined. The reason was to limit the number of features retrieved by MapServer. This behaviour can now be controlled with a new WEB metadata item : “wfs_maxfeatures_ignore_for_resulttype_hits” “true” (or “false”). When it is set to true, hits operations are no longer limited by wfs_maxfeatures. The default value of the item depends on the queried layers: if they have all an efficient LayerGetShapeCount() operation (i.e. are PostGIS layers currently), then the default is “true”. Otherwise it is “false”.

Note: of course, any client specified MAXFEATURES / COUNT parameter overrides those server side settings, while the number of features asked by the client is no greater than the server side limit.

2.4 Limitations

A specialized implementation of LayerGetShapeCount() is only done in PostGIS driver for the scope of this work. Could be extended similarly to providers that have SQL semantics and implement LayerTranslateFilter(): Oracle, MSSQL Spatial

3. Implementation Details

3.1 Affected files

  • mapquery.c: to use msLayerGetFeatureCount() in msQueryByRect() and msQueryByFilter()

  • maplayer.c: defines msLayerGetFeatureCount() and LayerDefaultGetFeatureCount()

  • mapwfs.c: to take into account “wfs_maxfeatures_ignore_for_resulttype_hits”

  • mappostgis.c: implement LayerGetFeatureCount()

  • other providers: add comment that LayerGetFeatureCount uses the default implementation

3.2 Tracking Issue

3.3 Documentation

Documentation of the WFS server will be updated to document the “wfs_maxfeatures_ignore_for_resulttype_hits” metadata item.

4. Voting History

+1 from TomK, JeffM, MichaelS, StephenW, ThomasB, DanielM, SteveL, JukkaR, StephanM, PerryN and TamasS.