setorder can keep keys in more cases (#3458)

NEWS.md

@@ -19,6 +19,8 @@
     space for new columns) before adding new columns by reference to it.
     ```
 
+3. `setorder` on a superset of a keyed `data.table`'s key now retains its key, [#3456](https://github.com/Rdatatable/data.table/issues/3456). For example, if `a` is the key of `DT`, `setorder(DT, a, -v)` will leave `DT` keyed by `a`.
+
 
 ### Changes in v1.12.2  (submitted to CRAN on 28 Mar 2019)
 

R/setkey.R

@@ -333,7 +333,9 @@ setorderv <- function(x, cols = colnames(x), order=1L, na.last=FALSE)
     if (is.data.frame(x) & !is.data.table(x)) {
       setattr(x, 'row.names', rownames(x)[o])
     }
-    setattr(x, 'sorted', NULL) # if 'forderv' is not 0-length, it means order has changed. So, set key to NULL, else retain key.
+    k = key(x)
+    if (!identical(head(cols, length(k)), k) || any(head(order, length(k)) < 0))
+      setattr(x, 'sorted', NULL) # if 'forderv' is not 0-length & key is not a same-ordered subset of cols, it means order has changed. So, set key to NULL, else retain key.
     setattr(x, 'index', NULL)  # remove secondary keys too. These could be reordered and retained, but simpler and faster to remove
   }
   invisible(x)

inst/tests/tests.Rraw

@@ -13971,6 +13971,21 @@ test(2020.09, DT[, prod(v), by=id], error="'complex' not supported by GForce pro
 DT = data.table(id = c(1L,1L,2L,2L), v = c(1L, 2L, NA, NA))
 test(2020.10, DT[, median(v), id], data.table(id=1:2, V1=c(1.5, NA))) # median whole group has NAs
 
+# setorder can keep key if the order does not change, #3456
+DT = data.table(
+  a = rep(4:1, 1:4),
+  b = 1:10,
+  v = c(2L, 4L, 10L, 5L, 9L, 6L, 7L, 8L, 3L, 1L)
+)
+setkey(DT, a)
+setorder(DT, a, -v)
+test(2021.1, key(DT), 'a')
+setorder(DT, -a, v)
+test(2021.2, key(DT), NULL)
+setkey(DT, a, b)
+setorder(DT, b)
+test(2021.3, key(DT), NULL)
+
 
 ###################################
 #  Add new tests above this line  #