Needs performance check / improvements in value assignment of DataArray

[fujiisoup]

xarray/xarray/core/dataarray.py

Lines 482 to 489 in 5e80189

	def __setitem__(self, key, value):
	if isinstance(key, basestring):
	self.coords[key] = value
	else:
	# Coordinates in key, value and self[key] should be consistent.
	# TODO Coordinate consistency in key is checked here, but it
	# causes unnecessary indexing. It should be optimized.
	obj = self[key]

In #1746, we added a validation in xr.DataArray.__setitem__ whether the coordinates consistency of array, key, and values are checked.
In the current implementation, we call xr.DataArray.__getitem__ to use the existing coordinate validation logic, but it does unnecessary indexing and it may decrease the __setitem__ performance if the arrray is multidimensional.

We may need to optimize the logic here.

Is it reasonable to constantly monitor the performance of basic operations, such as Dataset construction, alignment, indexing, and assignment?
(or are these operations too light to make a performance monitor?)

cc @jhamman @shoyer

[jhamman]

@fujiisoup in #1457, we added a framework (Airspeed-velocity) for benchmarking xarray operations. It is certainly within the scope of that framework to include indexing performance benchmarks. I just implemented a few IO related benchmarks with the expectation that more issues, like this one, would be added later on.