materializer behavior differs from its docs

[aplavin]

Docs:

help?> Tables.materializer
  Tables.materializer(x) => Callable

  <...>
  The default materializer is Tables.columntable, which converts any table input into a NamedTuple of Vectors.

  It is recommended that for users implementing MyType, they define only materializer(::Type{<:MyType}). materializer(::MyType) will then
  automatically delegate to this method.

Actual behavior:

julia> using StructArrays, Tables

julia> sa = StructArray(a=[1,2])


# default: returns columntable for the type, ...
julia> Tables.materializer(StructArray)
columntable (generic function with 5 methods)

# ...  but rowtable for an instance
julia> Tables.materializer(sa)
rowtable (generic function with 1 method)


# if I define materializer() for the type, ...
julia> Tables.materializer(::Type{StructArray}) = StructArray ∘ columntable

# ... it works for the type ...
julia> Tables.materializer(StructArray)
StructArray ∘ Tables.columntable

# ... but still returns rowtable for an instance
julia> Tables.materializer(sa)
rowtable (generic function with 1 method)

These are not due to StructArrays doing something weird – it doesn't have materializer definitions yet. I noticed these inconsistencies when adding materializer support there, JuliaArrays/StructArrays.jl#276.

Would be nice to either fix the behavior to match the docs, or fix the docs. I'm not familiar with Tables internals, so it's hard to tell which is correct.