title: Select NA in a data.table in R
body: <p>How do I select all the rows that have a missing value in the primary key in a data table.</p>
  
  <pre><code>DT = data.table(x=rep(c("a","b",NA),each=3), y=c(1,3,6), v=1:9)
  setkey(DT,x)   
  </code></pre>
  
  <p>Selecting for a particular value is easy  </p>
  
  <pre><code>DT["a",]  
  </code></pre>
  
  <p>Selecting for the missing values seems to require a vector search. One cannot use binary search. Am I correct?</p>
  
  <pre><code>DT[NA,]# does not work
  DT[is.na(x),] #does work
  </code></pre>