improved SQL for counting null elements
improved SQL for counting null elements

--HG--
extra : convert_revision : 7fa3c8c981fbcad1a87135abc57353480e837dda

davetcoleman/ompl,sonny-tarbouriech/ompl,sonny-tarbouriech/ompl,florianhauer/ompl,sonny-tarbouriech/ompl,jvgomez/ompl,utiasASRL/batch-informed-trees,davetcoleman/ompl,florianhauer/ompl,jvgomez/ompl,davetcoleman/ompl,florianhauer/ompl,utiasASRL/batch-informed-trees,utiasASRL/batch-informed-trees,jvgomez/ompl,davetcoleman/ompl,jvgomez/ompl,utiasASRL/batch-informed-trees,jvgomez/ompl,jvgomez/ompl,sonny-tarbouriech/ompl,davetcoleman/ompl,sonny-tarbouriech/ompl,davetcoleman/ompl,utiasASRL/batch-informed-trees,florianhauer/ompl,utiasASRL/batch-informed-trees,florianhauer/ompl,sonny-tarbouriech/ompl,florianhauer/ompl