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Understanding OQL: A Declarative Query Language for Object Databases

OQL (Object Query Language) is a declarative query language designed for object databases, similar to SQL. It allows users to perform ad-hoc queries, invoking methods easily and accessing any object or value within the database. OQL differentiates itself with its unique approach to object identity and equality, offering three comparison methods: identical, shallow equal, and deep equal. With features such as freely composable queries, built-in forall and exists constructs, and support for advanced optimization techniques, OQL facilitates clean syntax and efficient data retrieval from complex object-oriented structures.

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Understanding OQL: A Declarative Query Language for Object Databases

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  1. Object Database Queries: OQL

  2. Object Query Language (OQL) • Declarative (like SQL) • No updates • Computationally incomplete (like SQL) • Query result: Any object or value • Ad-hoc or embedded (like SQL) • Can invoke methods easily • All queries “start with” named, top-level database objects (vs. relations, in SQL) • OQL is a freely composable, functional language

  3. Object identity and equalities • Can compare referenced objects in 3 ways • Two referenced objects are identical if both references are the same OID • Two referenced objects are shallow equal if the referenced objects contain the same values in all value (e.g. scalar) fields and identical references in object fields • Two referenced objects are deep equal if they contain the same values in all value fields and the objects they reference are also deep equal • ID  shallow equal  deep equal

  4. OQL: Summary • SQL-like; declarative and functional • Freely composable: clean semantics • No need for having or order by • forall and exists both available • Query processing/optimization techniques: • Algebras can be defined: • Extended relational algebras • New algebras • Indexes used (O2, for example) • Query Processing/Optimization research results slowly making their way into products

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