Stream rows from one PostgreSQL database to another, eliminating the majority of ActiveRecord and SQL overhead

clone_data.rb

def clone_data(src_conn, dest_conn, table_name, options = {})
  puts "Cloning #{table_name} data"

  copyable = if options[:conditions]
    "(SELECT * FROM #{table_name} WHERE #{options[:conditions]})"
  else
    table_name
  end

  copy_command = "COPY #{copyable} TO STDOUT"

  src_conn.raw_connection.exec(copy_command)
  dest_conn.raw_connection.exec("COPY #{table_name} FROM STDIN")

  row_count = 0
  while row = src_conn.raw_connection.get_copy_data do
    dest_conn.raw_connection.put_copy_data(row)
    row_count += 1
  end

  dest_conn.raw_connection.put_copy_end

  puts "=> #{row_count} rows"
end

src_conn = SomeActiveRecordModel.connection
dest_conn = SomeOtherActiveRecordModel.connection
clone_data(src_conn, dest_conn, :schema_migrations)
clone_data(src_conn, dest_conn, :users, :conditions => "status = 'Active'")