title: fast merge(..., all = TRUE) with data.table in R
body: <p>Is is possible to make the equivalent of a merge(..., all = TRUE) with the data.table syntax (like X[Y]) ?</p>
  
  <p>Specifically, I would need a very fast way of getting the result of:</p>
  
  <pre><code>item_length = data.table(index = 1:10, length =  c(2,5,4,6,3),key ="index")
  item_weigth = data.table(index = c(2,4,6,7,8,11), weight= c(.3,.5,.2), key = "index")
  merge(x2,y2, all=TRUE)
  </code></pre>
  
  <p>Which is :</p>
  
  <pre><code>&gt; merge(item_length ,item_weigth , all=TRUE)
        index length weight
  [1,]     1      2     NA
  [2,]     2      5    0.3
  [3,]     3      4     NA
  [4,]     4      6    0.5
  [5,]     5      3     NA
  [6,]     6      2    0.2
  [7,]     7      5    0.3
  [8,]     8      4    0.5
  [9,]     9      6     NA
  [10,]    10      3     NA
  [11,]    11     NA    0.2
  </code></pre>