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 I get the following extents in QGIS and PostGIS:
QGIS extents of PostGIS layer
xMin,yMin 318359.56,731243.81 : xMax,yMax 373588.41,779145.12
QGIS extents of Shapefile layer
xMin,yMin 316723.00,731481.00 : xMax,yMax 373315.00,782722.00
SELECT ST_EstimatedExtent('schema','tablename','geometry') AS estextent,
ST_Extent(geometry) AS bextent
Estimated - BOX(318359.5625 731243.8125,373588.40625 779145.125) ;
Exact - BOX(316722.999997809 731480.999996626,373314.999999305 782721.999996855)
In this case the estimated extent is 91% of the exact extent which is somewhat sub-optimal...
I'm going to submit as a bug. Does anyone have an idea why QGIS would use the estimated extent over the exact extent? Performance?