diff --git a/src/operations/operations.jl b/src/operations/operations.jl
index 76e6c92..dbbe76b 100644
--- a/src/operations/operations.jl
+++ b/src/operations/operations.jl
@@ -24,15 +24,15 @@ julia> fetch(m)
 ```
 """
 function map(f, d::DTable)
-    chunk_wrap = (_chunk, _f) -> begin
+    chunk_wrap = (_chunk, _f, tabletype) -> begin
         return if isnonempty(_chunk)
-            sink = materializer(_chunk)
+            sink = materializer(tabletype === nothing ? _chunk : tabletype())
             sink(TableOperations.map(_f, _chunk))
         else
             _chunk
         end
     end
-    chunks = map(c -> Dagger.spawn(chunk_wrap, c, f), d.chunks)
+    chunks = map(c -> Dagger.spawn(chunk_wrap, c, f, d.tabletype), d.chunks)
     return DTable(chunks, d.tabletype)
 end
 
diff --git a/src/table/dtable.jl b/src/table/dtable.jl
index ce96fe7..1edfcf4 100644
--- a/src/table/dtable.jl
+++ b/src/table/dtable.jl
@@ -1,4 +1,5 @@
-const VTYPE = Vector{Union{Dagger.Chunk,Dagger.EagerThunk}}
+const ELTYPE = Union{Dagger.Chunk,Dagger.EagerThunk}
+const VTYPE = Vector{ELTYPE}
 
 """
     DTable
@@ -153,6 +154,11 @@ function _file_load(filename::AbstractString, loader_function::Function, tablety
     return tpart
 end
 
+function DTable(files::Vector{Dagger.File}; tabletype=nothing)
+    chunks = ELTYPE[file.chunk for file in files]
+    return DTable(chunks, tabletype)
+end
+
 """
     fetch(d::DTable)
 
@@ -229,7 +235,11 @@ Removes empty chunks from `d`.
 """
 function trim!(d::DTable)
     check_result = [Dagger.@spawn isnonempty(c) for c in d.chunks]
-    d.chunks = getindex.(filter(x -> fetch(check_result[x[1]]), collect(enumerate(d.chunks))), 2)
+    for idx in length(d.chunks):-1:1
+        if !fetch(check_result[idx])
+            deleteat!(d.chunks, idx)
+        end
+    end
     return d
 end
 
diff --git a/src/table/tables.jl b/src/table/tables.jl
index cb2907c..8536b67 100644
--- a/src/table/tables.jl
+++ b/src/table/tables.jl
@@ -43,7 +43,14 @@ end
 
 function determine_columnnames(table::DTable)
     s = determine_schema(table)
-    return s === nothing ? nothing : s.names
+    if s !== nothing
+        return s.names
+    end
+    if length(table.chunks) == 0
+        return ()
+    end
+    c = first(table.chunks)
+    return fetch(Dagger.@spawn columnnames(c))
 end
 
 function _getcolumn(table::DTable, col::Union{Symbol,Int})