diff --git a/src/DataFrames.jl b/src/DataFrames.jl
index 874c4d85c1..b7a071a3f4 100644
--- a/src/DataFrames.jl
+++ b/src/DataFrames.jl
@@ -31,7 +31,6 @@ export AbstractDataFrame,
        Tables,
        allowmissing!,
        antijoin,
-       by,
        columnindex,
        combine,
        completecases,
@@ -72,9 +71,6 @@ export AbstractDataFrame,
        unstack,
        valuecols
 
-# TODO: remove these exports in year 2021
-export by, aggregate
-
 if VERSION >= v"1.1.0-DEV.792"
     import Base.eachcol, Base.eachrow
 else
diff --git a/src/deprecated.jl b/src/deprecated.jl
index 2de96d8be3..47d5585515 100644
--- a/src/deprecated.jl
+++ b/src/deprecated.jl
@@ -1,3 +1,5 @@
+export by, aggregate
+
 by(args...; kwargs...) = throw(ArgumentError("by function was removed from DataFrames.jl. " *
                                              "Use the `combine(groupby(...), ...)` or `combine(f, groupby(...))` instead."))
 
diff --git a/src/other/precompile.jl b/src/other/precompile.jl
index 7122f7b75e..da211ac8b9 100644
--- a/src/other/precompile.jl
+++ b/src/other/precompile.jl
@@ -1579,9 +1579,15 @@ function precompile(all=false)
         Base.precompile(Tuple{Reduce{typeof(max), Nothing, Nothing},Vector{Int},GroupedDataFrame{DataFrame}})
         Base.precompile(Tuple{Type{OnCol},Vector{String},Vararg{AbstractVector{T} where T, N} where N})
 
-        combine(groupby(DataFrame(), []), identity)
-        innerjoin(DataFrame(v1=[]), DataFrame(v1=[]), on=:v1)
-        outerjoin(DataFrame(v1=[]), DataFrame(v1=[]), on=:v1)
+        for v in ([1, 2], [2, 1], [2, 2, 1]),
+            op in (identity, x -> string.(x), x -> PooledArrays.PooledArray(string.(x))),
+            on in (:v1, [:v1, :v2])
+            df = DataFrame(v1=op(v), v2=v)
+            combine(groupby(df, on), identity, :v1 => identity,
+                    :v2 => ByRow(identity), :v2 => sum)
+            innerjoin(df, select(df, on), on=on)
+            outerjoin(df, select(df, on), on=on)
+        end
     end
     return nothing
 end