allow passing empty sets of columns to ByRow and filter

[bkamins]

No description provided.

[bkamins]

Fixes #2457

[bkamins]

This is a change before a final GroupedDataFrame rules rewrite for returning multiple columns that is relatively orthogonal so I split it to a separate PR to make reviews simpler (this PR should be relatively easy to review).

[bkamins]

@nalimilan - last two commits are attempts to improve code and compilation latency (hopefully not sacrificing performance for large tables)

@pdeffebach - some benchmarking would be welcome (following the discussion on Slack)

[bkamins]

This is what I have now. I think it is good enough for now. I will merge this PR once the tests pass (I have extracted one common case to a separate function and added @nospecialize where tests indicated that it is beneficial; further optimizations are left for the future).

After this PR:

julia> using DataFrames

julia> df = DataFrame(a=1);

julia> @time begin
           @time select(df, :a => identity)
           @time select(df, :a => identity)
           @time select(df, :a => x -> 1)
           @time select(df, :a => x -> 1)
           @time select(df, :a => identity => :c)
           @time select(df, :a => identity => :c)
           @time select(df, :a => (x -> 1) => :c)
           @time select(df, :a => (x -> 1) => :c)
           @time select(df, [:a] => identity)
           @time select(df, [:a] => identity)
           @time select(df, [:a] => x -> 1)
           @time select(df, [:a] => x -> 1)
           @time select(df, [:a] => (x -> [[1]]) => [:c])
           @time select(df, [:a] => (x -> [[1]]) => [:c])
       end;
  1.242834 seconds (4.81 M allocations: 251.046 MiB, 7.05% gc time)
  0.000143 seconds (84 allocations: 5.000 KiB)
  0.099327 seconds (215.07 k allocations: 11.366 MiB)
  0.096288 seconds (215.05 k allocations: 11.376 MiB)
  0.063307 seconds (160.21 k allocations: 8.505 MiB)
  0.000091 seconds (79 allocations: 4.875 KiB)
  0.111864 seconds (201.95 k allocations: 10.764 MiB, 12.66% gc time)
  0.098550 seconds (201.95 k allocations: 10.762 MiB)
  0.310056 seconds (899.44 k allocations: 45.872 MiB, 3.69% gc time)
  0.000123 seconds (95 allocations: 5.641 KiB)
  0.105463 seconds (291.17 k allocations: 15.455 MiB)
  0.120932 seconds (291.16 k allocations: 15.453 MiB, 8.62% gc time)
  0.364569 seconds (1.50 M allocations: 76.191 MiB, 2.57% gc time)
  0.100328 seconds (224.71 k allocations: 12.368 MiB, 11.53% gc time)
  2.762119 seconds (9.06 M allocations: 472.373 MiB, 5.24% gc time)

Before this PR:

julia> using DataFrames

julia> df = DataFrame(a=1);

julia> @time begin
           @time select(df, :a => identity)
           @time select(df, :a => identity)
           @time select(df, :a => x -> 1)
           @time select(df, :a => x -> 1)
           @time select(df, :a => identity => :c)
           @time select(df, :a => identity => :c)
           @time select(df, :a => (x -> 1) => :c)
           @time select(df, :a => (x -> 1) => :c)
           @time select(df, [:a] => identity)
           @time select(df, [:a] => identity)
           @time select(df, [:a] => x -> 1)
           @time select(df, [:a] => x -> 1)
           @time select(df, [:a] => (x -> [[1]]) => [:c])
           @time select(df, [:a] => (x -> [[1]]) => [:c])
       end;
  1.365161 seconds (5.46 M allocations: 284.203 MiB, 6.30% gc time)
  0.000120 seconds (82 allocations: 5.000 KiB)
  0.117532 seconds (236.30 k allocations: 12.589 MiB)
  0.137019 seconds (236.28 k allocations: 12.601 MiB, 14.71% gc time)
  0.091981 seconds (184.44 k allocations: 9.884 MiB)
  0.000107 seconds (73 allocations: 4.703 KiB)
  0.120085 seconds (224.36 k allocations: 12.062 MiB)
  0.119065 seconds (224.36 k allocations: 12.060 MiB)
  0.347304 seconds (803.06 k allocations: 40.986 MiB, 8.86% gc time)
  0.000157 seconds (87 allocations: 5.484 KiB)
  0.218455 seconds (314.21 k allocations: 16.766 MiB, 35.85% gc time)
  0.127758 seconds (314.20 k allocations: 16.769 MiB)
  0.470066 seconds (1.51 M allocations: 76.974 MiB, 14.84% gc time)
  0.110272 seconds (247.74 k allocations: 13.680 MiB)
  3.274517 seconds (9.81 M allocations: 511.783 MiB, 8.70% gc time)

release 0.21 (does not support last two operations):

julia> using DataFrames

julia> df = DataFrame(a=1);

julia> @time begin
           @time select(df, :a => identity)
           @time select(df, :a => identity)
           @time select(df, :a => x -> 1)
           @time select(df, :a => x -> 1)
           @time select(df, :a => identity => :c)
           @time select(df, :a => identity => :c)
           @time select(df, :a => (x -> 1) => :c)
           @time select(df, :a => (x -> 1) => :c)
           @time select(df, [:a] => identity)
           @time select(df, [:a] => identity)
           @time select(df, [:a] => x -> 1)
           @time select(df, [:a] => x -> 1)
           #@time select(df, [:a] => (x -> [[1]]) => [:c])
           #@time select(df, [:a] => (x -> [[1]]) => [:c])
       end;
  0.802099 seconds (3.12 M allocations: 160.918 MiB, 6.74% gc time)
  0.000086 seconds (48 allocations: 3.453 KiB)
  0.186024 seconds (342.27 k allocations: 18.242 MiB, 7.50% gc time)
  0.161856 seconds (340.47 k allocations: 18.134 MiB)
  0.099809 seconds (214.92 k allocations: 11.570 MiB)
  0.000075 seconds (46 allocations: 3.344 KiB)
  0.187048 seconds (323.27 k allocations: 17.314 MiB, 6.39% gc time)
  0.164101 seconds (323.26 k allocations: 17.309 MiB)
  0.381791 seconds (954.31 k allocations: 49.240 MiB, 2.82% gc time)
  0.000108 seconds (64 allocations: 4.297 KiB)
  0.190896 seconds (417.56 k allocations: 22.295 MiB, 5.30% gc time)
  0.173000 seconds (417.53 k allocations: 22.289 MiB)
  2.399654 seconds (6.52 M allocations: 340.514 MiB, 4.20% gc time)

[bkamins]

Thank you!