align: Avoid reindexing when join="exact" (#7736)

* align: Avoid reindexing when join="exact"

xref #7730

* Aligner.copy always copies ecen with join="exact"

* Move logic to Aligner.align

* Add whats-new

Author: dcherian

doc/whats-new.rst

@@ -41,6 +41,12 @@ Breaking changes
 Deprecations
 ~~~~~~~~~~~~
 
+Performance
+~~~~~~~~~~~
+- Optimize alignment with ``join="exact", copy=False`` by avoiding copies. (:pull:`7736`)
+  By `Deepak Cherian <https://github.com/dcherian>`_.
+- Avoid unnecessary copies of ``CFTimeIndex``. (:pull:`7735`)
+  By `Deepak Cherian <https://github.com/dcherian>`_.
 
 Bug fixes
 ~~~~~~~~~

xarray/core/alignment.py

@@ -574,6 +574,8 @@ def align(self) -> None:
 
         if self.join == "override":
             self.override_indexes()
+        elif self.join == "exact" and not self.copy:
+            self.results = self.objects
         else:
             self.reindex_all()