How do I select all the rows that have a missing value in the primary key in a data table.

DT = data.table(x=rep(c("a","b",NA),each=3), y=c(1,3,6), v=1:9)
setkey(DT,x)

Selecting for a particular value is easy 

DT["a",]

Selecting for the missing values seems to require a vector search. One cannot use binary search. Am I correct?

DT[NA,]# does not work
DT[is.na(x),] #does work
r select data.table missing-data na
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