use 5.018001;
use warnings;

use Benchmark qw(:hireswallclock cmpthese );
use Test::More;

my $AoH;

my @cmpf = qw( map_map map_direct map_keys grep_map list_join );
my %test = map { $_ => \&$_ } "slice_join", @cmpf;

my $n_rec = 1;
for (1 .. 5) {
    say "";
    say "=== num of records is: ", $n_rec;
    $AoH = create_data (1, $n_rec);
    my $sj = [ sort &slice_join ];
    is_deeply ([ sort $test{$_}->() ], $sj, "slice_join === $_") for @
+cmpf;

    cmpthese (-3, \%test);

    $n_rec *= 10;
    }

done_testing;

sub list_join {
    my %H;
    %H = (%H, %$_) for @$AoH;
    return keys %H;
    } # list_join

sub slice_join {
    my %H;
    @H{keys %$_} = () for @$AoH;
    return keys %H;
    } # slice_join

sub grep_map {
    my %seen;
    return grep { !$seen{$_}++ } map { keys %$_ } @$AoH;
    } # grep_map

sub map_map {
    my %H = map { $_ => 1 } map { keys %$_ } @$AoH;
    return keys %H;
    } # map_map

sub map_direct {
    my %H = (map %$_, @$AoH);
    return keys %H
    } # map_direct

sub map_keys {
    my %H;
    @H{map keys %$_, @$AoH} = ();
    return keys %H
    } # map_keys

sub create_data {
    my ($density, $records) = @_;

    my @AoH;
    push @AoH, {map { rand 100 <= $density ? ("$_" => $_) : () } "A" .
+. "ZZ"}
        for 1 .. $records;
    return \@AoH;
    }
[download]

=== num of records is: 1
ok 1 - slice_join === map_map
ok 2 - slice_join === map_direct
ok 3 - slice_join === map_keys
ok 4 - slice_join === grep_map
ok 5 - slice_join === list_join
                Rate   map_map map_direct list_join grep_map map_keys 
+slice_join
map_map     754695/s        --       -14%      -17%     -25%     -42% 
+      -43%
map_direct  882464/s       17%         --       -3%     -12%     -32% 
+      -33%
list_join   913980/s       21%         4%        --      -9%     -30% 
+      -31%
grep_map   1002034/s       33%        14%       10%       --     -23% 
+      -24%
map_keys   1302904/s       73%        48%       43%      30%       -- 
+       -2%
slice_join 1323455/s       75%        50%       45%      32%       2% 
+        --

=== num of records is: 10
ok 6 - slice_join === map_map
ok 7 - slice_join === map_direct
ok 8 - slice_join === map_keys
ok 9 - slice_join === grep_map
ok 10 - slice_join === list_join
               Rate list_join   map_map map_direct  grep_map slice_joi
+n map_keys
list_join   11635/s        --      -80%       -80%      -84%       -88
+%     -89%
map_map     58659/s      404%        --         0%      -18%       -42
+%     -44%
map_direct  58659/s      404%        0%         --      -18%       -42
+%     -44%
grep_map    71323/s      513%       22%        22%        --       -29
+%     -32%
slice_join 100407/s      763%       71%        71%       41%         -
+-      -4%
map_keys   104260/s      796%       78%        78%       46%         4
+%       --

=== num of records is: 100
ok 11 - slice_join === map_map
ok 12 - slice_join === map_direct
ok 13 - slice_join === map_keys
ok 14 - slice_join === grep_map
ok 15 - slice_join === list_join
              Rate list_join map_direct   map_map  grep_map  map_keys 
+slice_join
list_join    161/s        --       -97%      -97%      -98%      -98% 
+      -98%
map_direct  5584/s     3363%         --       -3%      -22%      -45% 
+      -46%
map_map     5748/s     3465%         3%        --      -20%      -44% 
+      -44%
grep_map    7175/s     4350%        29%       25%        --      -29% 
+      -30%
map_keys   10175/s     6210%        82%       77%       42%        -- 
+       -1%
slice_join 10288/s     6280%        84%       79%       43%        1% 
+        --

=== num of records is: 1000
ok 16 - slice_join === map_map
ok 17 - slice_join === map_direct
ok 18 - slice_join === map_keys
ok 19 - slice_join === grep_map
ok 20 - slice_join === list_join
             Rate list_join    map_map map_direct  grep_map  map_keys 
+slice_join
list_join  6.95/s        --       -99%       -99%      -99%      -99% 
+      -99%
map_map     512/s     7267%         --       -13%      -48%      -56% 
+      -60%
map_direct  587/s     8341%        15%         --      -41%      -50% 
+      -54%
grep_map    991/s    14157%        94%        69%        --      -15% 
+      -22%
map_keys   1172/s    16754%       129%       100%       18%        -- 
+       -8%
slice_join 1273/s    18202%       148%       117%       28%        9% 
+        --

=== num of records is: 10000
ok 21 - slice_join === map_map
ok 22 - slice_join === map_direct
ok 23 - slice_join === map_keys
ok 24 - slice_join === grep_map
ok 25 - slice_join === list_join
            (warning: too few iterations for a reliable count)
              Rate list_join   map_map map_direct  grep_map  map_keys 
+slice_join
list_join  0.597/s        --      -98%       -98%      -99%      -99% 
+      -99%
map_map     31.0/s     5098%        --        -9%      -63%      -65% 
+      -73%
map_direct  34.2/s     5625%       10%         --      -59%      -62% 
+      -70%
grep_map    84.0/s    13964%      171%       146%        --       -6% 
+      -27%
map_keys    89.4/s    14873%      188%       162%        6%        -- 
+      -23%
slice_join   115/s    19235%      272%       238%       37%       29% 
+        --
1..25
[download]