Better "merge" defaults for Dataset.__setitem__ and Dataset.update

[dcherian]

What is your issue?

In dataset_update_method we call merge_core without specific values for join and compat.

xarray/xarray/structure/merge.py

Lines 1063 to 1068 in e71c341

	return merge_core(
	[dataset, other],
	priority_arg=1,
	indexes=dataset.xindexes,
	combine_attrs="override",
	)

These values default to join="outer" and compat="broadcast_equals"

xarray/xarray/structure/merge.py

Lines 639 to 649 in e71c341

	def merge_core(
	objects: Iterable[CoercibleMapping],
	compat: CompatOptions = "broadcast_equals",
	join: JoinOptions = "outer",
	combine_attrs: CombineAttrsOptions = "override",
	priority_arg: int | None = None,
	explicit_coords: Iterable[Hashable] | None = None,
	indexes: Mapping[Any, Any] | None = None,
	fill_value: object = dtypes.NA,
	skip_align_args: list[int] | None = None,
	) -> _MergeResult:

.

We should probably migrate to join="left" and compat="override". Using override will avoid any potentially expensive equality comparisons for non-indexed coordiante variables. I am not sure how compat="override" interacts with priority_arg.

cc @shoyer