postgresql statement taking ages to cancel in pgAdmin III

I was running a query in pgAdmin III and it was taking a long time. After 9 minutes I decided to cancel it using the stop button. The UI stated that it was cancelling and stayed in that state for slightly over an hour before it reported as cancelled. For all that time the CPU was around 98-99%. The statement that was running was a call to a function. It was only part way through the function when it got stuck and all the time was taken on the following statement: SELECT ST_UNION(shape) FROM descendant_areas INTO best_fit_areas; where descendant_areas is a temporary table with no indexes containing nine records. Shape is of type geography. Versions: PostgreSQL=PostgreSQL 9.5.2 on x86_64-pc-linux-gnu, compiled by gcc (GCC) 4.8.2 20140120 (Red Hat 4.8.2-16), 64-bit PostGIS=POSTGIS="2.2.2 r14797" GEOS="3.5.0-CAPI-1.9.0 r4084" PROJ="Rel. 4.9.2, 08 September 2015" GDAL="GDAL 2.0.2, released 2016/01/26" LIBXML="2.9.1" LIBJSON="0.12" TOPOLOGY RASTER My question is: Is it normal for cancelling a query to take that long or is it indicative that there are config settings set up badly?