fix: faster picking when no slices are made

Author: henryiii

docs/CHANGELOG.md

@@ -1,8 +1,8 @@
 # What's new in boost-histogram
 
-## Version 1.1
+## Version 1.2
 
-### Version 1.1.1
+### Version 1.2.0
 
 #### User changes
 * Python 3.10 officially supported, with wheels.
@@ -11,6 +11,7 @@
 
 #### Bug fixes
 * Support custom setters on AxesTuple subclasses. [#627][]
+* Faster picking if slices are not also used [#645][]
 * Throw an error when an AxesTuple setter is the wrong length (inspired by zip strict in Python 3.10) [#627][]
 * Fix error thrown on comparison with axis and non-axis object [#631][]
 * Static typing no longer thinks `storage=` is required [#604][]
@@ -26,8 +27,11 @@
 [#627]: https://github.com/scikit-hep/boost-histogram/pull/627
 [#631]: https://github.com/scikit-hep/boost-histogram/pull/631
 [#636]: https://github.com/scikit-hep/boost-histogram/pull/636
+[#645]: https://github.com/scikit-hep/boost-histogram/pull/645
 [#647]: https://github.com/scikit-hep/boost-histogram/pull/647
 
+## Version 1.1
+
 ### Version 1.1.0
 
 #### User changes

src/boost_histogram/_internal/hist.py

@@ -841,8 +841,16 @@ def __getitem__(  # noqa: C901
                 assert isinstance(stop, int)
                 slices.append(_core.algorithm.slice_and_rebin(i, start, stop, merge))
 
-        logger.debug("Reduce with %s", slices)
-        reduced = self._hist.reduce(*slices)
+        if slices:
+            logger.debug("Reduce with %s", slices)
+            reduced = self._hist.reduce(*slices)
+        elif pick_set or pick_each or integrations:
+            # Can avoid a copy in these cases, will be copied anyway
+            logger.debug("Reduce is empty, but picking or slicing, so no copy needed")
+            reduced = self._hist
+        else:
+            logger.debug("Reduce is empty, just making a copy")
+            reduced = copy.copy(self._hist)
 
         if pick_set:
             warnings.warn(