Hi Maurice, Thanks for your clarifications. They still leave me wondering about my question what the goal of this proposal is. From: (a) the compilation limitations of recursion are well known and there is no hope at all to eliminate recursion in the general case (b) XQuery contains full recrsion It follows that the only way to implement the recursion feature in XQuery is to use an execution environment that supports recursion. Call it hacky or not. In case of MIL and MAL, recursion happens to be supported. Some variants of PL/SQL (i.e. the scripted procedural language offerered by your RDBMS) also support recursion. To be precise, such a PL/SQL must support recursive table functions for it to be useful. The specific cases of XQuery recursion that can be eliminated or translated to fixpoint notation are rare. With the set-union semantic only very very special cases of easily recognizable functions could conceivably be translated to fixpoint. Thus my question to Jens what is the goal: (1) translation of existing XQuery recursion to fixpoint notation? (2) provide a separate different recursive language construct in XQuery? Contrary to what you say, this proposal is not just an internal algebraic operator, but foremost (2) a syntax extension. So all evidence points to (2) and the absence of a real ambition to do (1). After all, we know beforehand that if automatic translation were attempted, it would only work for rare cases. On milprint-summer: I did not say it *eliminates* recursion; rather it *reduces* the amount of calls. In recursive functions that do all recursive calls inside the same for-loop it reduces the number of calls to one per calling-depth. Peter -----Original Message----- From: Maurice van Keulen [mailto:m.vankeulen@utwente.nl] Sent: Tuesday, September 26, 2006 9:21 AM To: p.a.boncz Cc: monetdb-developers@lists.sourceforge.net Subject: Re: [Monetdb-developers] Monetdb-developers Digest, Vol 4, Issue 13 Hi Peter, I can of course wait for Jens to reply, but I guess it doesn't hurt to add my 2ct ... p.a.boncz wrote: Hi Jens, I read your recursion proposal, but I do not really understand the need or goal for such a syntax extension: - is't the default XQuery function recursion more powerful? - why not confine oneself to have an *internal* algebraic closure operator, if that is such a useful abstraction? The idea, as I understand it, is to have an "internal algebraic closure operator" (or fixpoint operator). The problem is that XQuery function recursion is too powerful. It is an unsolved problem how to translate arbitrary function recursion to algebra expressions with a fixpoint operator. The quickest way to (1) have some support for recursion, that (2) we know how to translate into an algebra and execute, is to extend the syntax with special recursion construct and disallow any function recursion. This is, I guess, exactly how I would approach it. First restrict to manageable proportions, then try to extend. Nor do I understand the need for the focus on nodes and set fixpoint semantics. If the result expression is a sequence of nodes you may define this to be duplicate-free and in document order, but it is less obvious why atomic sequences should be treated like that. Order is a first-class citizen in XQuery, and atomic types are also first-class. And what about element construction? Isn't it obvious given the recursive nature of XML that people may want to build XML documents with recursion -- so the choice to union all results always is limiting. The restrictions on set fixpoint semantics and nodes is, I believe, to simply confine first to simple things that the  research community researched before. I believe that we first need to check what known techniques apply here, and only then see if we can extend these techniques so that they are better suitable to XQuery. I can imagine that *some* XQuery recursion patterns could be translated to this fixpoint operator. Tail recursion is the big "success story" here (given your node-union fixpoint semantics it will actually only be possible in rare cases). However, given the fact that XQuery has for-loops, it is quite unlikely in the first place that in the real world people will use tail-recursion for iteration (this may distinguish XQuery from "purer" functional languages). I conclude that this extension per-se does not help in translating recursion in XQuery for the algebra backend at all (nor is it its goal), rather places a second recursive vehicle beside it. I don't agree. The syntax extension does help in translating recursion in XQuery to the algebra backend, because it is a construct for which it is clear how to translate it to a fixpoint operator. "Real-world people" will of course want to write queries with all kinds of weird recursions in them. But "real-world users" will also want their queries to be executed. I think that with a syntax extension it is much easier for "real-world people" to understand what is supported and what is not. If they can squeeze their recursion into this syntax, it is supported. If they cannot, it is not supported. Otherwise, you would have to document which patterns are supported, supply detailed and concise error messages, etc. etc. I am completely puzzled to hear such proposals from the usually XQuery-standard-respecting community in Garching. Finally, what exactly is "hacky" about the milprint_summer recursion approach? Maybe you mean with "hacky" that it does not try to eliminate recursion, and thus only works if the target language offers recursion. The reality is that only trivial recursion patterns can be eliminated, so this limitation will apply always. And you may forget, that a lot of recursion in milprint_summer *is* eliminated, as all calls inside a for-loop are reduced to one thanks to "loop-lifting" (maybe we could even "sell" this idea in the FP community). So actually it surely is not as primitive/hacky as the implementation of recursion in say Galax. How is recursion eliminated by loop-lifting or function inlining? Any recursion would still remain after this, won't it? Defining myself as an engineer, I am aware of the limitations of my knowledge and understanding of such formal issues. Let's not forget that (any variant of) SQL only has a very restricted form of recursion as well. Recursion in a database systems is simply an unsolved problem. In the context of XQuery nothing less. ready to be educated.. Peter Just my 2cts, Maurice. -- ---------------------------------------------------------------------- Dr.Ir. M. van Keulen - Assistant Professor, Data Management Technology Univ. of Twente, Dept of EEMCS, POBox 217, 7500 AE Enschede, Netherlands Email: m.vankeulen@utwente.nl, Phone: +31 534893688, Fax: +31 534892927 Room: INF3039, WWW: http://www.cs.utwente.nl/~keulen

Thanks Torsten! All clear here. I may have created the impression to be opposed to this, but my emotions are instead better summarized as surprise. -----Original Message----- From: Torsten Grust [mailto:torsten.grust@gmail.com] Sent: Tuesday, September 26, 2006 10:05 AM To: p.a.boncz Cc: 'Maurice van Keulen'; monetdb-developers@lists.sourceforge.net Subject: Re: [Monetdb-developers] Monetdb-developers Digest, Vol 4, Issue 13 Hi all, before this diverges and Jens can chime in, a quick clarification: On Sep 26, 2006, 9:54 AM, p.a.boncz wrote with possible deletions:

[...] Thus my question to Jens what is the goal: (1) translation of existing XQuery recursion to fixpoint notation? (2) provide a separate different recursive language construct in XQuery?

Contrary to what you say, this proposal is not just an internal algebraic operator, but foremost (2) a syntax extension. So all evidence points to (2) and the absence of a real ambition to do (1). After all, we know beforehand that if automatic translation were attempted, it would only work for rare cases.

The motiviation behind 'with $v seeded by e1 recurse e2' purely stems from our desire to collaborate with the UvA people (Loredana, in particular). The construct will bring Regular XPath (search Google) to MonetDB/XQuery -- and all chances are that MonetDB/XQuery gains some visibility because Regular XPath currently is a very prominent dialect of XPath in the data integration and DB theory communities. No less, no more. We clearly have the real ambition to do (1). And it indeed seems that a ''recursive back-end'' is required to achieve this. (But not for Regular XPath.) There is more involved, though, than ''simply'' map to the recursion feature in the back-end. We'd want to look at controlled unrolling (schema knowledge may help here) and identityfing some cases of tail recursion (presence of 'for' does not alleviate the need for tail recursion -- it's the other way around). More on this later today?!, --Teggy -- | Prof. Dr. Torsten Grust grust@in.tum.de | | http://www-db.in.tum.de/~grust/ | | Database Systems - Technische Universität München (Germany) |