foo.rb

=begin
$ export PGUSER=<your_pg_user>
$ createdb foo
$ bundle
$ ruby foo.rb
=end

require 'bundler/setup'

require 'logger'
require 'sequel'
require 'timeout'

logdev = '/dev/null' # or $stdout

conn1 = Sequel.connect('postgres:///foo', loggers: [Logger.new(logdev).tap {|l| l.progname = 'conn1' }])
conn2 = Sequel.connect('postgres:///foo', loggers: [Logger.new(logdev).tap {|l| l.progname = 'conn2' }])

conn1.create_table? :t

at_exit do
  conn1.drop_table :t
end

t1 = conn1[:t]
t2 = conn2[:t]

puts '# transaction (read comitted)'

conn1.transaction do
  p [t1.count, t2.count] #=> [0, 0]

  t1.insert

  p [t1.count, t2.count] #=> [1, 0]
end

t1.truncate

puts '-' * 30
puts
puts '# transaction (serializable)'

conn1.transaction isolation: :serializable do
  p [t1.count, t2.count] #=> [0, 0]

  t1.insert

  p [t1.count, t2.count] #=> [1, 0]
end

t1.truncate

puts '-' * 30
puts
puts '# table-level locks (access exclusive)'

t1.lock 'ACCESS EXCLUSIVE' do
  puts 'begin lock'

  # 同一スレッド、同一コネクションからは参照できる
  puts "conn1: #{t1.count}"

  # 別コネクションからの参照はロック開放までブロックされる
  begin
    Timeout.timeout 1 do
      puts "conn2: #{t2.count}"
    end
  rescue Timeout::Error
    puts 'conn2: blocked'
  end

  # 別スレッドからの参照はロック開放までブロックされる
  Thread.new { puts "conn1 (another thread): #{t1.count}" }
  Thread.new { puts "conn2 (another thread): #{t2.count}" }

  t1.insert

  puts "conn1: #{t1.count}"
end

puts 'end lock'
sleep 1

Gemfile

source 'https://rubygems.org'

gem 'pg'
gem 'sequel'

out.txt

$ ruby foo.rb
# transaction (read comitted)
[0, 0]
[1, 0]
------------------------------

# transaction (serializable)
[0, 0]
[1, 0]
------------------------------

# table-level locks (access exclusive)
begin lock
conn1: 0
conn2: blocked
conn1: 1
end lock
conn2 (another thread): 1
conn1 (another thread): 1