Pluck 与 ActiveRecord 关系的 Rails 性能

Question

pluck当只需要一两个字段时，我一直默认使用，但我最近对性能差异进行了基准测试，并且勇气丢失了：

ActiveRecord::Base.logger.level = 1
n = 5000
Benchmark.bm do |x|
  x.report('Country.all:              ') { n.times { Country.all } }
  x.report('Country.unscoped:         ') { n.times { Country.unscoped } }
  x.report('base priority_order:      ') { n.times { Country.unscoped.with_translations(I18n.locale).order(list_at_top:   :desc).order(:name) } }
  x.report('.includes(:translations): ') { n.times { Country.unscoped.with_translations(I18n.locale).order(list_at_top: :desc).order(:name).includes(:translations) } }
  x.report('.pluck(:name):            ') { n.times { Country.unscoped.with_translations(I18n.locale).order(list_at_top: :desc).order(:name).includes(:translations).pluck(:name) } }
  x.report('.pluck(:name) only:       ') { n.times { Country.unscoped.with_translations(I18n.locale).order(list_at_top: :desc).order(:name).pluck(:name) } }
end

# Results
=begin
                                  user     system      total        real
  Country.all:                0.990000   0.020000   1.010000 (  1.023518)
  Country.unscoped:           0.050000   0.000000   0.050000 (  0.050440)
  base priority_order:        1.350000   0.010000   1.360000 (  1.356728)
  .includes(:translations):   1.460000   0.000000   1.460000 (  1.462635)
  .pluck(:name):              8.230000   0.150000   8.380000 ( 11.168291)
  .pluck(:name) only:         6.980000   0.150000   7.130000 (  9.903130) 
=end

正如您所看到的，Pluck 的实际时间要慢得多。我选择第四个.includes(:translations)是为了避免 N+1 查询（前两个实际上只是基线，没有提供所需的数据）。

这个基准测试是否准确，而勇气真的更慢，还是这并没有说明全部情况？由于没有创建 ActiveRecord 对象，因此应该可以节省大量内存，并节省一些时间。

Answer 1

你的测试不准确。Country.all实际上并没有获取行，它只是返回一个活动记录关系。与您的其他范围相同。只有 pluck 才会真正查询数据。

我有一个包含 243 行的国家/地区表。列不多（id、name、iso2、iso3）。我将您的查询调整为map(&:name)前两个，因此结果与采摘相同。普拉克赢了很多。

  ActiveRecord::Base.logger.level = 1
  n = 1000
  Benchmark.bm do |x|
    x.report('Country.all.map(&:name):      ') { n.times { Country.all.map(&:name) } }
    x.report('Country.unscoped.map(&:name): ') { n.times { Country.unscoped.map(&:name) } }
    x.report('Coutry.pluck(:name):          ') { n.times { Country.unscoped.pluck(:name) } }
  end

输出：

                                    user     system      total        real
Country.all.map(&:name):        3.830000   0.140000   3.970000 (  4.328655)
Country.unscoped.map(&:name):   3.780000   0.030000   3.810000 (  4.159162)
Coutry.pluck(:name):            1.550000   0.040000   1.590000 (  1.879490)