Don't use ChainedVector as DictEncoding data array unless necessary (#…

…110)

Fixes #109. The issue here was when reading arrow record batches with
dict encoded columns, we eagerly used `ChainedVector` for the underlying
array backing the `DictEncoding` in case there were subsequent
record batches that added additional elements to the dict encoding. This
is too eager though, since it's probably common, like for "feather"
files, where the dict encoding values are always known and provided in
the first record batch. In fact, several language implementations don't
even support these kind of "delta" dict updates in subsequent record
batches. This PR, therefore, uses a regular array for the dict encoding
backing for the first record batch, and only promotes to a ChainedVector
if we happen to get a delta update.

src/table.jl

@@ -234,13 +234,18 @@ function Table(bytes::Vector{UInt8}, off::Integer=1, tlen::Union{Integer, Nothin
                 field = dictencoded[id]
                 values, _, _ = build(field, field.type, batch, recordbatch, dictencodings, Int64(1), Int64(1), convert)
                 dictencoding = dictencodings[id]
-                append!(dictencoding.data, values)
+                if typeof(dictencoding.data) <: ChainedVector
+                    append!(dictencoding.data, values)
+                else
+                    A = ChainedVector([dictencoding.data, values])
+                    dictencodings[id] = DictEncoding{eltype(A), typeof(A)}(id, A, field.dictionary.isOrdered, values.metadata)        
+                end
                 continue
             end
             # new dictencoding or replace
             field = dictencoded[id]
             values, _, _ = build(field, field.type, batch, recordbatch, dictencodings, Int64(1), Int64(1), convert)
-            A = ChainedVector([values])
+            A = values
             dictencodings[id] = DictEncoding{eltype(A), typeof(A)}(id, A, field.dictionary.isOrdered, values.metadata)
             @debug 1 "parsed dictionary batch message: id=$id, data=$values\n"
         elseif header isa Meta.RecordBatch