PERF: Recover `ActiveRecord::pluck` performance.

```ruby
require 'active_record'
require 'benchmark/ips'

ActiveRecord::Base.establish_connection(ENV.fetch('DATABASE_URL'))
ActiveRecord::Migration.verbose = false

ActiveRecord::Schema.define do
  create_table :users, force: true do |t|
    t.string :name, :email
    t.timestamps null: false
  end
end

attributes = {
  name: 'Lorem ipsum dolor sit amet, consectetur adipiscing elit.',
  email: 'foobar@email.com'
}

class User < ActiveRecord::Base; end

1000.times do
  User.create!(attributes)
end

Benchmark.ips do |x|
  x.config(time: 10, warmup: 2)

  x.report('pluck 1 column') do
    User.pluck(:id)
  end

  x.report('pluck 2 columns') do
    User.pluck(:id, :email)
  end

  x.report('pluck 1 column with scope') do
    User.where(id: 1000).pluck(:id)
  end

  x.report('pluck 2 columns with scope') do
    User.where(id: 1000).pluck(:id, :email)
  end
end
```

```
Calculating -------------------------------------
      pluck 1 column   122.000  i/100ms
     pluck 2 columns    74.000  i/100ms
pluck 1 column with scope
                       615.000  i/100ms
pluck 2 columns with scope
                       515.000  i/100ms
-------------------------------------------------
      pluck 1 column      1.272k (± 3.9%) i/s -     12.810k
     pluck 2 columns    750.096  (± 3.3%) i/s -      7.548k
pluck 1 column with scope
                          6.074k (± 4.1%) i/s -     60.885k
pluck 2 columns with scope
                          5.158k (± 2.7%) i/s -     52.015k
```

```
Calculating -------------------------------------
      pluck 1 column   126.000  i/100ms
     pluck 2 columns    78.000  i/100ms
pluck 1 column with scope
                       457.000  i/100ms
pluck 2 columns with scope
                       434.000  i/100ms
-------------------------------------------------
      pluck 1 column      1.266k (± 2.1%) i/s -     12.726k
     pluck 2 columns    795.061  (± 3.0%) i/s -      7.956k
pluck 1 column with scope
                          4.660k (± 2.1%) i/s -     46.614k
pluck 2 columns with scope
                          4.355k (± 2.3%) i/s -     43.834k
```

```
Calculating -------------------------------------
      pluck 1 column   126.000  i/100ms
     pluck 2 columns    78.000  i/100ms
pluck 1 column with scope
                       539.000  i/100ms
pluck 2 columns with scope
                       481.000  i/100ms
-------------------------------------------------
      pluck 1 column      1.308k (± 3.4%) i/s -     13.104k
     pluck 2 columns    798.604  (± 2.8%) i/s -      8.034k
pluck 1 column with scope
                          5.530k (± 3.4%) i/s -     55.517k
pluck 2 columns with scope
                          4.914k (± 2.7%) i/s -     49.543k
```

```
Calculating -------------------------------------
      pluck 1 column   139.000  i/100ms
     pluck 2 columns    79.000  i/100ms
pluck 1 column with scope
                       580.000  i/100ms
pluck 2 columns with scope
                       526.000  i/100ms
-------------------------------------------------
      pluck 1 column      1.337k (± 3.0%) i/s -     13.483k
     pluck 2 columns    806.776  (± 2.7%) i/s -      8.137k
pluck 1 column with scope
                          5.924k (± 4.1%) i/s -     59.160k
pluck 2 columns with scope
                          5.276k (± 3.1%) i/s -     53.126k
```

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