undant copies, <em>data.table</em> utilised the <a href="https://stackoverflow.com/q/7033106/559784">available but unused <code>:=</code> operator in R</a>.</p>
<p>Great performance improvements were made in <code>R v3.1</code> as a result of which only a <em>shallow</em> copy is made for (1) and not <em>deep</em> copy. However, for (2) still, the entire column is <em>deep</em> copied even in <code>R v3.1+</code>. This means the more columns one subassigns to in the <em>same query</em>, the more <em>deep</e