[QGIS-uk-user] QGIS uses estimated extent of PostGIS layers

[McDonaldR]

Hello folks

I've come across a problem using the geo-processing tools in QGIS and the extents of layers from PostGIS.

It seems like QGIS is using an estimated extent for a PostGIS layer which then leads to issues when using geo-processing tools (creating a TIN or Voronoi polygon set, for example).  Loading the same dataset as a shapefile returns an exact extent.

For a point dataset, for example, I get the following extents in QGIS and PostGIS:

QGIS extents as PostGIS layer
xMin,yMin 318359.56,731243.81 : xMax,yMax 373588.41,779145.12

QGIS extents as Shapefile layer
xMin,yMin 316723.00,731481.00 : xMax,yMax 373315.00,782722.00

PostGIS extents

SELECT ST_EstimatedExtent('schema','tablename','geometry') AS estextent,
 ST_Extent(geometry) AS bextent
 FROM schema.tablename;

Estimated - BOX(318359.5625 731243.8125,373588.40625 779145.125) ;

Exact - BOX(316722.999997809 731480.999996626,373314.999999305 782721.999996855)

The PostGIS estimated extent matches the QGIS estimated extent and is smaller than the exact extent.  The shapefile exact extent matches the PostGIS exact extent. In this case the estimated extent is 91% of the exact extent which is somewhat sub-optimal...

I'm running QGIS 2.6, 2.8.3 and 2.10.1 with PostgreSQL 9.2.13 and PostGIS 2.1.8 on Windows 7 32bit.

I have tried with my PostGIS connection set with and without the "use estimated metadata" and have updated the table statistics in the database with VACUUM ANALYSE but still get the same results.  I imagine the estimated extents make things a bit faster.

There are also a number of reports on this

https://hub.qgis.org/issues/13232
https://hub.qgis.org/issues/10335 (in this case the PostGIS extent was bigger than the shapefile extent)

Anyone got any suggestions as to how to fix this?

Thanks

Ross

Ross McDonald | GIS Data Coordinator | Resources Department, IT Division | Angus Council, Angus House, Orchardbank Business Park, Forfar, DD8 1AT
T: 01307 476419 | F: 01307 476401 | E: mcdonaldr at angus.gov.uk<mailto:mcdonaldr at angus.gov.uk>



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