PooledVectors for isbitstypes of small size?

[sairus7]

data = """a	b	c	d
A	BB	CCC	DDDD
A	BB	CCC	DDDD
""" |> IOBuffer
df = CSV.File(data; header = 1) |> DataFrame

df[!, :a] # 2-element PooledVector{String1, UInt32, Vector{UInt32}}
df[!, :b] # 2-element PooledVector{String3, UInt32, Vector{UInt32}}
df[!, :c] # 2-element PooledVector{String3, UInt32, Vector{UInt32}}
df[!, :d] # 2-element PooledVector{String7, UInt32, Vector{UInt32}}