Warns if a join will produce a very large result due to many-to-many relationships (Cartesian product explosion).
Examples
# Dangerous: both tables have duplicates
x <- data.frame(id = c(1, 1, 2, 2), val_x = 1:4)
y <- data.frame(id = c(1, 1, 2, 2), val_y = 1:4)
check_cartesian(x, y, by = "id")