store ncols in Table and TablePartitions

src/simple_reader.jl

@@ -19,6 +19,7 @@ df = DataFrame(read_parquet(path; copycols=false))
 """
 struct Table <: Tables.AbstractColumns
     path::String
+    ncols::Int
     rows::Union{Nothing,UnitRange}
     batchsize::Union{Nothing,Signed}
     use_threads::Bool
@@ -33,8 +34,9 @@ struct Table <: Tables.AbstractColumns
             use_threads::Bool=(nthreads() > 1))
         parfile = Parquet.File(path)
         sch = tables_schema(parfile)
+        ncols = length(sch.names)
         lookup = Dict{Symbol, Int}(nm => i for (i, nm) in enumerate(sch.names))
-        new(path, rows, batchsize, use_threads, parfile, sch, lookup, AbstractVector[])
+        new(path, ncols, rows, batchsize, use_threads, parfile, sch, lookup, AbstractVector[])
     end
 end
 
@@ -47,19 +49,18 @@ end
 
 struct TablePartitions
     table::Table
+    ncols::Int
     cursor::BatchedColumnsCursor
 
     function TablePartitions(table::Table)
-        new(table, cursor(table))
+        new(table, getfield(table, :ncols), cursor(table))
     end
 end
 length(tp::TablePartitions) = length(tp.cursor)
 function iterated_partition(partitions::TablePartitions, iterresult)
     (iterresult === nothing) && (return nothing)
     chunk, batchid = iterresult
-    sch = Tables.schema(getfield(partitions, :table))
-    ncols = length(sch.names)
-    TablePartition(partitions.table, AbstractVector[chunk[colidx] for colidx in 1:ncols]), batchid
+    TablePartition(partitions.table, AbstractVector[chunk[colidx] for colidx in 1:partitions.ncols]), batchid
 end
 Base.iterate(partitions::TablePartitions, batchid) = iterated_partition(partitions, iterate(partitions.cursor, batchid))
 Base.iterate(partitions::TablePartitions) = iterated_partition(partitions, iterate(partitions.cursor))
@@ -75,8 +76,7 @@ loaded(table::Table) = !isempty(getfield(table, :columns))
 load(table::Table) = load(table, cursor(table))
 function load(table::Table, chunks::BatchedColumnsCursor)
     chunks = [chunk for chunk in chunks]
-    sch = Tables.schema(table)
-    ncols = length(sch.names)
+    ncols = getfield(table, :ncols)
     columns = getfield(table, :columns)
 
     empty!(columns)