FR #2722 is now implemented. `DT[, c(., lapply(.SD, sum), .), by=grp]` is now optimised for almost all cases, except really complex cases. Check out NEWS for more info.

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/datatable@1242 edb9625f-4e0d-4859-8d74-9fd3b1da38cb