luajava.bindClass('java.lang.Class'):forName('org.postgresql.Driver')
conn = luajava.bindClass('java.sql.DriverManager'):getConnection('jdbc:postgresql://localhost/fran?user=fran&password=fran')
conn:prepareStatement([[
  create temporary table a(k serial primary key, t text);
  insert into a values(42, 'foo');]]):execute();
function list()
  local st = conn:prepareStatement('select * from a')
  local rs = st:executeQuery()
  while rs:next() do
    print(rs:getString(1), rs:getString(2))
  end
  rs:close()
  st:close()
end
upsert = (function()
  local stmtIns = conn:prepareStatement([[insert into a values(?, ?);]])
  local stmtUpd = conn:prepareStatement([[update a set t=? where k=?;]])
  return function(k, t)
    stmtIns:setInt(1, k)
    stmtIns:setString(2, t)
    if not pcall(function() stmtIns:execute() end) then
      stmtUpd:setInt(2, k)
      stmtUpd:setString(1, t)
      stmtUpd:execute()
    end
  end
end)()

list()
print('running upsert')
upsert(30, 'bar')
upsert(42, 'pelota')
list()

-- ejecución:
--  42      foo
--  running upsert
--  30      bar
--  42      pelota