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...ased primarily on familiar relational operators. The relational "tuples" we operate on are Object Assignments, or OAs. We use a recursive iterator approach in query processing, as described =-=in, e.g., [Gra93]-=-. We now explain how OAs are constructed and operated upon by the nodes in our logical plan. An OA is a simple data structure containing slots corresponding to range variables in the query, along with...

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...dcards. 1.2 Lorel and OQL The data model underlying Lorel is called OEM (for Object Exchange Model). OEM is a simple and flexible object model, introduced initially in the TSIMMIS project at Stanford =-=[PGMW95]-=-. Roughly speaking, a database conforming to OEM can be thought of as a graph with complex values at internal nodes, atomic values at leaf nodes, and labeled edges. 2 Although the Lorel language could...

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...now define Lorel in terms of OQL, coercion takes on an importance in Lorel96 that it did not have in Lorel1. A detailed comparison of Lorel1 with more conventional languages such as OQL [Cat94], XSQL =-=[KKS92]-=-, and SQL [MS93] appears in [QRS + 95a]; most comparisons carry over directly to Lorel96. Another OEM-based language called MSL has been designed for mediator specification in the Stanford TSIMMIS pro...

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..., and WebLog [LSS96], which is based on a Datalog-like syntax. Additional relevant work includes query languages for hypertext structures, e.g., [MW95, BK94, CM89, MW93], and work on integrating SGML =-=[GR90]-=- documents with relational databases [BCK + 94] or object-oriented databases such as OpenODB [YA94] or O 2 [CACS94], since SGML documents can be viewed as semistructured. In the area of heterogeneous ...

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...el in a "home grown" DBMS designed specifically for semistructured data, the data model underlying Lorel can be defined as an extension to the ODMG model and the language as an extension to =-=OQL. (See [Cat94]-=- for a specification of ODMG and OQL.) Thus, Lorel can be implemented on top of a conventional object-oriented DBMS, yielding a flexible system suitable for managing both structured and semistructured...

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... not expressible in Lorel, which was designed primarily as a simple to use query language. In [CACS94, CCM96], extensions to OQL are proposed that are somewhat similar in spirit or goals to Lorel. In =-=[CACS94]-=-, a more rigidly typed approach is followed, but because heterogeneous collections are introduced, the model still has a strong similarity to OEM. However, the language proposed in [CACS94], called OQ...

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...in terms of OQL, coercion takes on an importance in Lorel96 that it did not have in Lorel1. A detailed comparison of Lorel1 with more conventional languages such as OQL [Cat94], XSQL [KKS92], and SQL =-=[MS93]-=- appears in [QRS + 95a]; most comparisons carry over directly to Lorel96. Another OEM-based language called MSL has been designed for mediator specification in the Stanford TSIMMIS project [PGMU96, PA...

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...echanism. An alternative method would be to compute a finite representation of the infinite set of data paths matching a given path expression, which is possible because of the regularity of this set =-=[Cou83]-=-. However, this approach would seriously complicate the implementation. 5.2 Wildcards The regular expressions specified above already allow some flexibility in querying. However, when querying semistr...

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...for the World-Wide Web that have emerged recently, e.g., W3QL [KS95], which focuses on extensibility, WebSQL [MMM96], which provides a formal semantics and introduces a notion of locality, and WebLog =-=[LSS96]-=-, which is based on a Datalog-like syntax. Additional relevant work includes query languages for hypertext structures, e.g., [MW95, BK94, CM89, MW93], and work on integrating SGML [GR90] documents wit...

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...onsider adapting the optimization techniques of [CCM96] to OEM using the data guide. Also related to our work are several query languages for the World-Wide Web that have emerged recently, e.g., W3QL =-=[KS95]-=-, which focuses on extensibility, WebSQL [MMM96], which provides a formal semantics and introduces a notion of locality, and WebLog [LSS96], which is based on a Datalog-like syntax. Additional relevan...

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...e data guide and external objects: ffl The data guide for a given OEM database is an OEM object that encapsulates the structure of the graph in terms of edge labels, without repeating identical paths =-=[NUWC96]-=-. Essentially, the data guide provides a structural summary of the current database, which in a semistructured environment can be extremely useful in understanding how the data is structured and formu...

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...L. This process is depicted by the dashed arrows in Figure 1. Section 8 discusses how this approach can be used to implement Lorel on top of an ODMG-conforming database management system, such as O 2 =-=[BDK92]-=-. 1.3 Related work A first version of Lorel (now dubbed Lorel1) was introduced in [QRS + 95a] and implemented in the initial version of the Lore system. Lorel1 was designed and defined from scratch, i...

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...cterize the relative expressiveness of MSL and Lorel. A work closely related to ours is a language called UnQL, also designed for querying semistructured data. UnQL is based on a model similar to OEM =-=[BDS95]-=-. A primary feature of UnQL is a powerful construct called traverse that allows restructuring of trees to arbitrary depth. Such restructuring operations not expressible in Lorel, which was designed pr...

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...called OQL-doc, does not use coercion the way it is used in Lorel, and the treatment of path expressions is quite different. Optimizing the evaluation of generalized path expressions is considered in =-=[CCM96]-=-. Their optimization is based on two object algebra operators, one dealing with paths at the schema level and one with paths at the data level. Since we are in a schema-less context, we cannot directl...

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...structured data. Semistructured data arises in a number of common situations. Some data sources are designed with non-rigid structures for convenience. A concrete example is the ACeDB genome database =-=[TMD92]-=-, while a somewhat less concrete but certainly well-known example is the World-Wide Web. The Web imposes no constraints on the internal structure of HTML pages, although structural primitives such as ...

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...ery languages for hypertext structures, e.g., [MW95, BK94, CM89, MW93], and work on integrating SGML [GR90] documents with relational databases [BCK + 94] or object-oriented databases such as OpenODB =-=[YA94]-=- or O 2 [CACS94], since SGML documents can be viewed as semistructured. In the area of heterogeneous database integration, which as we have suggested is a common scenario for semistructured data, most...

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...[RAK + 92] and UniSQL/M [Kim94] are designed to integrate data in object-oriented and relational databases. At the other end of the spectrum, systems such as GAIA [RJR94], Willow [Fre94], and ACL/KIF =-=[GF94]-=- provide uniform access to data with minimal structure. Note that environments such as CORBA [OMG92] and OLE2 [Mic94] operate at a different level from Lorel. These approaches provide a common protoco...

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... [CCM96] to OEM using the data guide. Also related to our work are several query languages for the World-Wide Web that have emerged recently, e.g., W3QL [KS95], which focuses on extensibility, WebSQL =-=[MMM96]-=-, which provides a formal semantics and introduces a notion of locality, and WebLog [LSS96], which is based on a Datalog-like syntax. Additional relevant work includes query languages for hypertext st...

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... particular, systems such as Pegasus [RAK + 92] and UniSQL/M [Kim94] are designed to integrate data in object-oriented and relational databases. At the other end of the spectrum, systems such as GAIA =-=[RJR94]-=-, Willow [Fre94], and ACL/KIF [GF94] provide uniform access to data with minimal structure. Note that environments such as CORBA [OMG92] and OLE2 [Mic94] operate at a different level from Lorel. These...

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...tems such as Pegasus [RAK + 92] and UniSQL/M [Kim94] are designed to integrate data in object-oriented and relational databases. At the other end of the spectrum, systems such as GAIA [RJR94], Willow =-=[Fre94]-=-, and ACL/KIF [GF94] provide uniform access to data with minimal structure. Note that environments such as CORBA [OMG92] and OLE2 [Mic94] operate at a different level from Lorel. These approaches prov...

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...ther end of the spectrum, systems such as GAIA [RJR94], Willow [Fre94], and ACL/KIF [GF94] provide uniform access to data with minimal structure. Note that environments such as CORBA [OMG92] and OLE2 =-=[Mic94]-=- operate at a different level from Lorel. These approaches provide a common protocol for passing messages between objects in a distributed object environment. In distributed settings, Lorel could cert...

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...tabases. At the other end of the spectrum, systems such as GAIA [RJR94], Willow [Fre94], and ACL/KIF [GF94] provide uniform access to data with minimal structure. Note that environments such as CORBA =-=[OMG92]-=- and OLE2 [Mic94] operate at a different level from Lorel. These approaches provide a common protocol for passing messages between objects in a distributed object environment. In distributed settings,...

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... we have suggested is a common scenario for semistructured data, most work has focused on integrating data in well structured databases. In particular, systems such as Pegasus [RAK + 92] and UniSQL/M =-=[Kim94]-=- are designed to integrate data in object-oriented and relational databases. At the other end of the spectrum, systems such as GAIA [RJR94], Willow [Fre94], and ACL/KIF [GF94] provide uniform access t...

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...the comparison 4 This particular table was the outcome of a lively Lore meeting at Stanford. An interesting issue (not addressed in this paper) is the development of access techniques, e.g., indexing =-=[Raj96] or hashin-=-g, to support such comparisons. arg2 arg1 string real int string \Gamma string ! real both ! real real \Gamma int ! real int \Gamma Table 1: Coercion for basic comparison operators "4.3" ! 5...

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... name may be assigned to any new object as part of the load. Lorel's load statement can also add additional subobjects to existing named objects. The load file syntax and further details are given in =-=[HW96]-=-. 8 Implementation on Top of an OODB In this section, we briefly consider how Lorel can be implemented on top of a standard ODMG database. We first reconsider the type OEM defined in Section 2. Since ...