[Rd] Small performance bug in [.Date

[Martin Maechler]

>>>>> Hadley Wickham <h.wickham at gmail.com>
>>>>>     on Mon, 20 Nov 2017 12:50:24 -0600 writes:

    > Hi all,
    > I think there's an unnecessary line in [.Date which has a considerable
    > impact on performance when subsetting large dates:

    > x <- Sys.Date() + 1:1e6

    > microbenchmark::microbenchmark(x[1])
    > #> Unit: microseconds
    > #>  expr     min       lq     mean   median       uq      max neval
    > #>  x[1] 920.651 1039.346 3624.833 2294.404 3786.881 41176.38   100

    > `[.Date` <- function(x, ..., drop = TRUE) {
    > cl <- oldClass(x)
    > # class(x) <- NULL
    > val <- NextMethod("[")
    > class(val) <- cl
    > val
    > }
    > microbenchmark::microbenchmark(x[1])
    > #> Unit: microseconds
    > #>  expr   min     lq     mean median    uq      max neval
    > #>  x[1] 2.738 3.0225 28.40893  3.269 3.513 2470.068   100

    > Setting the class of x to NULL is problematic because it forces a
    > copy, and I'm pretty sure it's unnecessary as NextMethod() does not
    > consult the class of x, but instead uses .Class.

Yes, at least so it looks in  src/main/objects.c

Also, we had a very similar change a while ago :
------------------------------------------------------------------------
r65926 | luke | 2014-06-12 15:54:38 +0200 (Thu, 12. Jun 2014) | 2 Zeilen
Geänderte Pfade:
   M src/library/base/R/datetime.R

Commented out class(x) <- NULL in [.POSIXct and [[.POSICct.
------------------------------------------------------------------------

and we never seemed to have followed up in a systematic manner
finding other places where this happens and could be
eliminated.  I see about half a dozen examples in
base/R/dates.R  alone and am trying to find more in other places.

[maybe this used to be necessary for very early different
 versions of NextMethod() which were not yet optimized using  .Class etc]

Thank you very much,
Hadley!

Martin