function _sum_ds_f_barrier!(res, ngroups, starts, x, ds, by, i)

for j in 1:ngroups

lo = starts[j]

j == ngroups ? hi = nrow(ds) : hi = starts[j + 1] - 1

res[(i-1)*ngroups+j] = hp_sum(by, view(x, lo:hi))

end

end

# Not ok yet, don't use it

function Base.sum(ds::Dataset, cols::MultiColumnIndex; by = identity, threads = true)

colsidx = index(ds)[cols]

newds = Dataset()

if isgrouped(ds)

ngroups = index(ds).ngroups[]

starts = index(ds).starts

groupcols = index(ds).sortedcols

for j in 1:length(groupcols)

insertcols!(newds, _names(ds)[groupcols[j]]=>repeat(_columns(ds)[groupcols[j]][view(starts,1:ngroups)], length(colsidx)))

end

insertcols!(newds, :var=>repeat(_names(ds)[colsidx], inner = ngroups))

res = Tables.allocatecolumn(Real, nrow(newds))

for i in 1:length(colsidx)

_sum_ds_f_barrier!(res, ngroups, starts, _columns(ds)[colsidx[i]], ds, by, i)

end

insertcols!(newds, ncol(newds)+1, :sum => res, unsupported_copy_cols = false)

else

insertcols!(newds, :var=>_names(ds)[colsidx])

if threads

insertcols!(newds, :sum => [hp_sum(by, _columns(ds)[colsidx[j]]) for j in 1:length(colsidx)])

else

insertcols!(newds, :sum => [sum(by, _columns(ds)[colsidx[j]]) for j in 1:length(colsidx)])

end

end

newds

end

Base.sum(ds::Dataset, col::ColumnIndex; by = identity, threads = true) = sum(ds, [col], by = by, threads = threads)