The 5800 System Query Model

One of the primary methods for retrieving data is to specify the characteristics of the desired data and then let the system find it for you. In the 5800 system, a query expression specifies a set of conditions on metadata field values. The system then returns a list of all the objects whose metadata values match the query conditions. Each object is considered individually without reference to any other objects. There are no queries that compare fields in one object with fields in a different object.

Query expressions can use much of the power of Structured Query Language (SQL). Each query expression combines SQL functions and operators, field names from the metadata schema, and literal values. There are no query expressions that select objects based on the data stored in the object itself; all queries apply only to the metadata fields associated with the object. Only queryable fields can be used in query expressions. For an object to show up in a query result set, the object must have a value for each of the fields mentioned in the query; in other words, there is an implicit INNER JOIN between the fields in the query.

A query may optionally specify that the result set should include not just the OID of each matching object, but also the values from a set of selected fields of each matched object . The value retrieved by Query With Select for some field may be a canonical equivalent of the value originally stored in that field. For example, values in numeric fields may have been converted to standard numeric format. Trailing spaces at the end of string fields will have been truncated (The value that is returned will be some value that would match the original data as stored, in the SQL sense.) To be included in the result set, an object must include values for all queried fields and all selected fields. In other words, there is an implicit INNER JOIN between all the fields in the query and in the select list.

There are significant limitations on which queries may be executed efficiently, or at all. See Chapter 4, Sun StorageTek 5800 System Query Language and Chapter 5, Programming Considerations and Best Practicess for details of these limitations.

There are no ordering guarantees between queries and store operations that are proceeding at the same time. If an object is added to the 5800 system while a query is being performed, and the object matches the query, then the object may or may not show up in the query result set.

For a detailed description of query syntax and query semantics, including a description of exactly what it means for an object to match a query, see Chapter 4, Sun StorageTek 5800 System Query Language.