When using the data.table package, I am a bit unsure of when i need to setkey(). For example, when using the := operator with the by option, things seem to still be very fast even though I have not set a key. Could someone please elucidate when setkey() is necessary and when it is not? And if it is not necessary prior to calling := with by then how is the data.table package so fast since presumably it has to do the same thing as apply in standard data.frame R by doing a sequential search rather than a binary one since it doesn't know whether my data.table is actually sorted by the argument to by.
Thanks