Hi all,
this is to keep you informed what we are currently working on in Munich.
Pathfinder's purely relational approach so far cannot handle *recursion* in XQuery expression in a sensible way (recursion in the W3C XQuery language is possible by means of user-defined recursive function). The milprint_summer branch includes some hacky implementation (with help of MIL PROCs) that provides recursion nevertheless.
Together with people from UVA (Loredana Afanasiev in particular), we are currently working on support for recursion in an algebraic manner.
A generic handling of recursion seems quite difficult. The W3C XQuery language syntax does not pose any restrictions on the style of recursion that can be described. The common approaches to deal with recursion in relational systems, in contrast, cannot cope with all these variants of recursion. A second problem is that the kind of recursion used in a given query (e.g., tail recursion) used in a given query seems quite tricky to infer (and a consistent treatment of this case in the algebraic compiler adds another challenge).
Hence, we decided to go for a limited recursion feature in Pathfinder first. We may later on try to add a more generic handling. We are currently implementing a *syntax extension* to Pathfinder that allows to describe a limited notion of recursion. The new construct
with $var seeded by expr1 recurse expr2
will have the following semantics:
1. expr1 is evaluated once and then used as a seed to initiate the recursion. $var is bound to the result of expr1 in the first recursion step.
2. Then, expr2 is evaluated over and over. Variable $var is visible in expr2 and for each recursion step will be bound to the union of all items (nodes) obtained in the previous recursion steps. (The results of all recursion steps are collected into the overall result using set semantics.)
3. Recursion terminates if a recursion step does not contribute any new items (nodes) to the overall result (i.e., we reached a fixpoint).
(Remark: There still is some discussion if (for performance reasons) each recursion step should only observe the *new* items obtained in the previous recursion step, or whether the entire result set collected so far should be observable in expr2.)
The `union' semantics of this new recursion construct implies that we can do recursion only on *nodes*, not on atomic values. This quite directly matches the requirements of the UVA group (and others) that are looking for a "transitive closure" operator.
Note that the above syntax extension that we are about to introduce does not interfere with any existing constructs. So our changes will not affect any (existing) valid XQuery expression.
Motivation: There is a high demand for transitive closure functionality in the group at UVA (i.e., Loredana and others), but also in other research groups (as I was told). Transitive closure is, e.g., a vital component of an XPath dialect known as "Regular XPath".
Regards from Garching
Jens