in src/table.jl [441:455]
function Base.iterate(tp::TablePartitions, i=1)
i > tp.npartitions && return nothing
tp.npartitions == 1 && return tp.table, i + 1
cols = columns(tp.table)
newcols = AbstractVector[cols[j].arrays[i] for j = 1:length(cols)]
nms = names(tp.table)
tbl = Table(
nms,
types(tp.table),
newcols,
Dict{Symbol,AbstractVector}(nms[i] => newcols[i] for i = 1:length(nms)),
schema(tp.table),
)
return tbl, i + 1
end