postgis.backend — The backend to service a function where GEOS and SFCGAL overlap. Options: geos or sfcgal. Defaults to geos.
This GUC is only relevant if you compiled PostGIS with sfcgal support. By default
geos backend is used for functions where both GEOS and SFCGAL have the same named function. This variable allows you to override and make sfcgal the backend to service the request.
Sets backend just for life of connection
set postgis.backend = sfcgal;
Sets backend for new connections to database
ALTER DATABASE mygisdb SET postgis.backend = sfcgal;