[XML-DEV Mailing List Archive Home] [By Thread] [By Date] [Recent Entries] [Reply To This Message] Re: The use of XML syntax in XML Query
I wrote: > >Dare Obasanjo wrote: >> I'm still waiting for someone with a theoretical background to debunk the >> statements in the paper "On Database Theory and XML"[0] that it is is an >> unsolvable problem to guarantee that one can create a query that for any >> given XML input will generate output that conforms to a specified XML schema. >> > [0] http://www.cs.washington.edu/homes/suciu/files/_F2066943700.ps > [...] > As for Suciu's claim that "When one adds joins, typechecking > becomes undecidable" [p. 5]: I'm not quite up to debunking it, > but it sounds bogus to me; regular languages are closed under > cartesian product and homomorphisms, which is basically what > a join gives you. > Having taken a look at the paper where Dan actually *proves* this to be the case [1], I take that back :-) But this paper appears to be using a stronger than usual definition of the typechecking problem: "Given an input DTD tau_1, an output DTD tau_2 [...], and a query q, we say that q _typechecks_ (wrt. tau_1 and tau_2) iff q(inst(tau_1)) [is a proper subset of] inst(tau_2)" That is, it demands that the algorithm be both sound *and* complete. The paper then presents some (impressive!) results showing that this problem is decidable for certain restricted classes of query languages and DTDs [2], but for "specialized DTDs" (roughly speaking: something at the level of RELAX-NG) or full XQuery, the typechecking problem as stated is undecidable. On the other hand, in practice a weaker definition of typechecking is often satisfactory: "if a query q typechecks wrt. L1 and L2, then for all s in L1, q(s) is in L2". A guarantee from a type checker that a program never produces bad output from good input is a useful thing to have, even if the typechecker rejects some correct programs. [1] "XML with Data Values: Typechecking Revisited", <URL: http://www.cs.washington.edu/homes/suciu/files/_F1359680672.ps > [2] the term "DTD" is used in the paper as a generic term for grammar-based schema languages, not XML DTDs per se. --Joe English jenglish@f...
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