Fix type instability in getproperty(::Schema, ::Symbol) (#340)

src/Tables.jl

@@ -499,8 +499,17 @@ function Base.getproperty(sch::Schema{names, types}, field::Symbol) where {names
     if field === :names
         return names === nothing ? getfield(sch, :storednames) : names
     elseif field === :types
-        T = getfield(sch, :storedtypes)
-        return types === nothing ? (T !== nothing ? T : nothing) : Tuple(fieldtype(types, i) for i = 1:fieldcount(types))
+        if types === nothing
+            return getfield(sch, :storedtypes)
+        else
+            ncol = fieldcount(types)
+            if ncol <= 512
+                # Type stable, but slower to compile
+                return ntuple(i -> fieldtype(types, i), Val(ncol))
+            else
+                return Tuple(fieldtype(types, i) for i=1:ncol)
+            end
+        end
     else
         throw(ArgumentError("unsupported property for Tables.Schema"))
     end

test/runtests.jl

@@ -211,6 +211,17 @@ end
 
     # 228
     @test Tables.columntable(NamedTuple[]) === NamedTuple()
+
+    @testset "Type stability of schema(x).types" begin
+        _get_types(X) = Tables.schema(X).types
+
+        x = (a=[1.0], b=[1.0])
+        @inferred Tuple{Type{Float64},Type{Float64}} _get_types(x)
+
+        # Trigger other branch which lacks type stability:
+        x_wide = NamedTuple([Symbol("x$(i)") => [1.0] for i=1:513])
+        @inferred NTuple{513,DataType} _get_types(x_wide)
+    end
 end
 
 @testset "Materializer" begin