database <- data.frame(start = seq(1, 150000000, 1000),
                       end = seq(1000, 150000000, 1000),
                       code = paste0(sample(LETTERS, 15000, replace = TRUE),
                                      sample(LETTERS, 15000, replace = TRUE)))

identityCodes <- sample(1:15000000, 1000)

fun <- function (x, y) {
  z <- rep(NA, length(x)
  for (i in 1:length(x)){
    z[i] <- as.character(y[y["start"] <= x[i] & y["end"] >= x[i], "code"])
  }
  return(z)
}

a <- fun(identityCodes, database)

system.time(fun(identityCodes, database))
user   system elapsed
15.36    0.00   15.50