Subset data table without using <-

Question

I want to subset some rows of a data table. Like this:

# load data
  data("mtcars")

# convert to data table
  setDT(mtcars,keep.rownames = T)

# Subset data
  mtcars <- mtcars[like(rn,"Mer"),] # or
  mtcars <- mtcars[mpg > 20,]

However, I'm working with a huge data set and I wanted to avoid using <-, which is not memory efficient because it makes a copy of the data.

Is this correct? Is it possible to update the filtered data without <- ?