218 Epilogue Imagine a situation where you have
Beyond JDO 1.0 219 Or perhaps it would be useful to define a query and execute it in separate steps. This might even facilitate the chaining of query executions, illustrated below with a pipe | operator. DEFINE-QUERY q1 CANDIDATE com.ogilviepartners.jdobook.op.BusinessPartner IMPORTS com.ogilviepartners.jdobook.op.* VARIABLES Order o PARAMETERS Double searchValue FILTER customer.orders.contains(o) && o.totalValue > searchValue ORDERING name ascending; EXECUTE-QUERY q1 PARAMETER-VALUES 1000 | q2 PARAMETER-VALUES … ; Please note that the above is not supported, or even suggested syntax, but should be sufficient to give the reader an idea of the possibilities for a standard string-based JDOQL representation. 13.1.3.5 Query object model I have already pointed out that SQL suffers from loose typing and deferred compilation, which makes it easy to compile and deploy applications with syntactically incorrect queries. The current form of JDOQL reduces this risk significantly, since all mapping between JDO instances and the underlying data store is handled internally. However, it is still possible to write a query in JDOQL that will not execute correctly, but which will be compiled by the Java compiler. This is due to the presence of string elements within a programmatic query definition. JDOQL will not have its current string elements (variable declaration, filter, ordering declaration, parameter declaration, etc.) removed, as their presence enables a wide variety of dynamic applications. However, it is likely that a future specification of JDOQL will provide the classes and methods necessary to describe a query in a purely programmatic manner, with no reliance on string data. This is known as a query object model (QOM). In the interim, vendors will be quick to provide tools for the generation and verification of queries and query strings in JDOQL. 13.1.4 Pre-fetch patterns JDO currently defines the default fetch group. This is a grouping of fields which, in addition to primary key fields, will be retrieved from the data store when the instance is first read. However, it has been argued successfully that this simplistic treatment is inadequate in certain scenarios. It is possible that work in this area will provide application-level functionality for influencing the data that is initially retrieved for each instance. This functionality could then
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