Only print progress if more than one group (#6693)

* only print progress if ngrp>1

* Add NEWS entry

---------

Co-authored-by: Michael Chirico <[email protected]>

Author: joshhwuu

NEWS.md

@@ -139,6 +139,8 @@ rowwiseDT(
 
 7. The `dcast()` and `melt()` generics no longer attempt to redirect to {reshape2} methods when passed non-`data.table`s. If you're still using {reshape2}, you must use namespace-qualification: `reshape2::dcast()`, `reshape2::melt()`. We have been warning about the deprecation since v1.12.4 (2019). Please note that {reshape2} is retired.
 
+8. `showProgress` in `[` is disabled for "trivial" grouping (`.NGRP==1L`), [#6668](https://github.com/Rdatatable/data.table/issues/6668). Thanks @MichaelChirico for the request and @joshhwuu for the PR.
+
 # data.table [v1.16.2](https://github.com/Rdatatable/data.table/milestone/35) (9 October 2024)
 
 ## BUG FIXES

src/dogroups.c

@@ -70,10 +70,7 @@ SEXP dogroups(SEXP dt, SEXP dtcols, SEXP groups, SEXP grpcols, SEXP jiscols, SEX
   SEXP ans=NULL, jval, thiscol, BY, N, I, GRP, iSD, xSD, rownames, s, RHS, target, source;
   Rboolean wasvector, firstalloc=FALSE, NullWarnDone=FALSE;
   const bool verbose = LOGICAL(verboseArg)[0]==1;
-  const bool showProgress = LOGICAL(showProgressArg)[0]==1;
   double tstart=0, tblock[10]={0}; int nblock[10]={0}; // For verbose printing, tstart is updated each block
-  double startTime = (showProgress) ? wallclock() : 0; // For progress printing, startTime is set at the beginning
-  double nextTime = (showProgress) ? startTime+3 : 0; // wait 3 seconds before printing progress
   bool hasPrinted = false;
 
   if (!isInteger(order)) internal_error(__func__, "order not integer vector"); // # nocov
@@ -89,6 +86,10 @@ SEXP dogroups(SEXP dt, SEXP dtcols, SEXP groups, SEXP grpcols, SEXP jiscols, SEX
   // fix for longstanding FR/bug, #495. E.g., DT[, c(sum(v1), lapply(.SD, mean)), by=grp, .SDcols=v2:v3] resulted in error.. the idea is, 1) we create .SDall, which is normally == .SD. But if extra vars are detected in jexp other than .SD, then .SD becomes a shallow copy of .SDall with only .SDcols in .SD. Since internally, we don't make a copy, changing .SDall will reflect in .SD. Hopefully this'll workout :-).
   SEXP SDall = PROTECT(findVar(install(".SDall"), env)); nprotect++;  // PROTECT for rchk
   SEXP SD = PROTECT(findVar(install(".SD"), env)); nprotect++;
+  
+  const bool showProgress = LOGICAL(showProgressArg)[0]==1 && ngrp > 1; // showProgress only if more than 1 group
+  double startTime = (showProgress) ? wallclock() : 0; // For progress printing, startTime is set at the beginning
+  double nextTime = (showProgress) ? startTime+3 : 0; // wait 3 seconds before printing progress
 
   defineVar(sym_BY, BY = PROTECT(allocVector(VECSXP, ngrpcols)), env); nprotect++;  // PROTECT for rchk
   SEXP bynames = PROTECT(allocVector(STRSXP, ngrpcols));  nprotect++;   // TO DO: do we really need bynames, can we assign names afterwards in one step?