01_csv_benchmark.rb

require "benchmark/ips"
require "csv"

# Data: Fielding.csv from Lahman Baseball Database, sorted
# http://seanlahman.com/baseball-archive/statistics/
FILENAME = File.expand_path("Fielding-sorted.csv", __dir__)
MATCH_FIELD = "kruegot01" # Record in the middle of the file
NUM_ROWS = 5 # Grab this many rows (for lack of anything better to do)

def method_a
  rows = []

  CSV.foreach(FILENAME, headers: true) do |row|
    next unless row["playerID"] == MATCH_FIELD
    rows << row
    return rows if rows.size == NUM_ROWS
  end
end

def method_b
  file = File.open(FILENAME)
  headers = CSV.parse_line(file.gets)
  match = "#{MATCH_FIELD},"

  while line = file.gets
    break if line.start_with?(match)
  end

  # back up to the beginning of the line
  file.seek(-line.size, IO::SEEK_CUR)

  CSV.new(file, headers: headers).take(NUM_ROWS)
ensure
  file.close
end

Benchmark.ips do |x|
  x.report("method_a") { method_a }
  x.report("method_b") { method_b }
  x.compare!
end

02_benchmark_result.txt

Warming up --------------------------------------
            method_a     1.000  i/100ms
            method_b     1.000  i/100ms
Calculating -------------------------------------
            method_a      0.100  (± 0.0%) i/s -      1.000  in   9.987337s
            method_b      6.575  (±15.2%) i/s -     33.000

Comparison:
            method_b:        6.6 i/s
            method_a:        0.1 i/s - 65.67x slower