add a fast path for gatherby

Author: sl-solution

src/other/utils.jl

@@ -429,7 +429,22 @@ function _gather_groups(ds, cols, ::Val{T}; mapformats = false, stable = true, t
     colidx = index(ds)[cols]
     _max_level = nrow(ds)
 
-
+    # fast path for a common scenario
+    # PooledVectors are already gathered. However, note stable must be false, since the result is not stable
+    if (ds isa Dataset) && length(colidx) == 1 && (_columns(ds)[colidx[1]] isa PooledVector) && !stable
+        _f = _date_value
+        if mapformats
+            _f = _date_value∘getformat(ds, colidx[1])
+        end
+        if _f == _date_value∘identity || !mapformats
+            v = DataAPI.refarray(_columns(ds)[colidx[1]])
+        else
+            v = DataAPI.refarray(map(_f, _columns(ds)[colidx[1]]))
+        end
+        prev_groups = Vector{T}(undef, nrow(ds))
+        copy!(prev_groups, v)
+        return prev_groups, T[], threads ? hp_maximum(prev_groups) : stat_maximum(prev_groups)
+    end
 	if nrow(ds) > 2^23 && !stable && 5<length(colidx)<16 # the result is stable anyway
 		if !mapformats || all(==(identity), getformat.(Ref(ds), colidx))
 			return _gather_groups_hugeds_multicols(ds, cols, Val(T); threads = threads)