Translating query objects

Translation is the process that transforms a query object into a form feasible for execution in a data store and/or object model. The result of translating a query is a query execution plan.

Query translators are the components responsible for translation. xotl.ql does not provide production quality translators, but other packages are planned to have implementation of translators. Nevertheless, the module xotl.ql.translation.py provides an implementation of a naive translator that matches the Python object model and fetches objects from the current process memory.

General requirements about translators

Take query expressions and query objects

Translators are required to take either a query expression or a query object. For this, translators could use xotl.ql.core.normalize_query().

Re-usability of execution plans

Translators must allow the reuse of execution plans; i.e. once a query is translated you may execute the plan several times to fetch the objects that matches the query at the time the plan is executed.

This way you may use the translator only once per query and then reuse plan for several calls to retrieve objects.

This does not mean that each time you execute the plan it will return the same objects since the underlying data storage system might have been changed between calls.

This does not imply that plans should be cached either. If the translation process is executed many times for the same query, translators may return different (although equivalent) plans each time.

Documentation requirements

Translators authors are encouraged to provide as much documentation as necessary, so that application writers have the guidance they need for writing queries.

The following information is required in order for a translator documentation be complete:

• A list of the supported expression operations. For instance, a translator might reject operations involving max() or min() or allow them only in some cases.
• A list of additionally supported operations, and their related documentation. Package that implements translators may provide non-standards functions (e.g, a coalesce function available for SQL translators).
• Documentation of functions, classes applications writers may use to access the translator functionality directly if they have to.

See also

The documentation for the module xotl.ql.translation.py as an example.

Writing translators

Writing a translator is much like writing a compiler. You get a query syntax tree and you need to produce a program that operates according a well established semantics. In this regard, writing a translator involves giving denotational and/or operational semantics for the queries.

In this section we provide some pointers to this task, but fail to be comprehensive.