Improvements to Bindings
------------------------
Key: JENA-121
URL: https://issues.apache.org/jira/browse/JENA-121
Project: Jena
Issue Type: Improvement
Components: ARQ
Reporter: Stephen Allen
Priority: Minor
The Binding interface is a key object for query execution. It has some issues
such that it may be a good idea to think about tweaking the design a bit. Some
thoughts:
1) Bindings should be immutable (in the strong Java sense:
http://www.ibm.com/developerworks/java/library/j-jtp02183/index.html)
2) Add a BindingPair class that represents a Var/Node value (could be called
something else, BindingValue?)
2a) Binding constructor/factory method takes an Iterable<BindingPair> to
initialize it
2b) Binding can now implement Iterable<BindingPair> which would be more
efficient than iterating over the variables then looking up each node
3) An implementation that has better memory usage than BindingMap (a HashMap
may be overkill here, if we can use the iterator from 2b in more places)
4) An implementation that copies parent BindingPairs instead of maintaining a
reference. If the parent bindings are not held onto by themselves after being
incorporated into a parent, we can save memory by copying and letting the
parent be GCed (indeed in the common join case, this appears to be what
happens). We would also get speed benefits from storing the BindingPairs in a
single data structure, making iterating and looking up values faster.
Additionally, more Binding objects die young instead of being held as part of a
higher level algebra collection (like sort or distinct), which can help with GC
overhead.
5) Expose an iterator of BindingPairs ordered by variable. This is needed for
BindingComparator, and may be an option for Algebra.merge()/compatible() if we
eliminate fast get(Var) lookups of nodes (as a consequence of 3). The ordering
could be determined at construction or be initialized lazily.
6) Method for estimating memory size for the binding. Would be very useful for
setting threshold policies for DataBags. Although this may be tough to do,
especially if Nodes are shared between bindings.
Some of these points need some more investigation, and some good profiling to
ensure that they are beneficial, especially 3, 4, and 5.
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