setkey.rd

Author: venom1204

man/setkey.Rd

@@ -74,18 +74,12 @@ The sort is \emph{stable}; i.e., the order of ties (if any) is preserved.
 For character vectors, \code{data.table} takes advantage of R's internal global string cache, also exported as \code{\link{chorder}}.
 }
 
-\section{Key vs. Index Subsetting}{
-When using \code{setkey}:
-\itemize{
-  \item Subsetting can omit \code{on} (e.g., \code{DT[.(value)]})
-  \item Data is physically reordered in RAM
-}
-When using \code{setindex}:
-\itemize{
-  \item Must specify \code{on} (e.g., \code{DT[.(value), on = "col"]})
-  \item Multiple indices can coexist via \code{setindexv(x, list(cols))}
-  \item Original row order is preserved
-}
+\section{Keys vs. Indices}{
+Setting a key (with \code{setkey}) and an index (with \code{setindex}) are similar, but have very important distinctions.
+
+Setting a key physically reorders the data in RAM.
+
+Setting an index computes the sort order, but instead of applying the reordering, simply \emph{stores} this computed ordering. That means that multiple indices can coexist, and that the original row order is preserved.
 }
 
 \section{Good practice}{