Xslt 3.0 defines a feature called streamability: a technique to write xslt code that is able to handle arbitrary sized inputs.
This contrasts with conventional xslt code (and xslt engines) where inputs are completely loaded in memory.
To make code streamable a developer should declare her code as such, and the code should pass Streamability analysis.
The goal is to define subset of xslt/xpath operations that allow to process input in one pass.
In simple case it's indeed a simple task to verify that code is streamable, but the more complex your code is the less trivial it's to witness it is streamable.
On the forums we have seen a lot of discussions, where experts were trying to figure out whether particular xslt is streamable. At times it's remarkably untrivial task!
This, in our opinion, clearly manifests that the feature is largerly failed attempt to inscribe some optimization technique into xslt spec.
The place of such optimization is in the implementation space, and not in spec. Engine had to attempt such optimization and fallback to the traditional implementation.
The last such example is: Getting SXST0060 "No streamable path found in expression" when trying to push a map with grounded nodes to a template of a streamable mode, where both xslt code and engine developers are not sure that the code is streamable in the first place.
By the way, besides streamability there is other optimization technique that works probably in all SQL engines. When data does not fit into memory engine may spill it on disk. Thus trading memory pressure for disk access. So, why didn't such techninque find the way into the Xslt or SQL specs?