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Object views allow you to access and manipulate relational data as if the data were stored in tables containing object type or collection columns. Object views give you the flexibility to store data in relational tables. Thus you can selectively choose to use object features when it makes sense (e.g., to retrieve data directly as objects for your Java applications). For the most part, you can use relational SQL directly on the underlying relational schema, thus avoiding the code complexity and performance problems associated with storing data in tables containing objects. The process of creating object views on relational tables involves the following steps: 1. Define an object type. In this step, we define an object type where each attribute corresponds to a relational table column. This object type will be used in creating the object view.
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CHAPTER 8 ORACLE OBJECTS: AN OBJECTIVE ANALYSIS
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2. Define an object view. Next, we define an object view with a query that represents the object-oriented view we have in mind. 3. Define the instead of triggers. In this step, we write instead of triggers on the view to support required DML statements such as insert, update, and delete that work directly on the object view. This step is required only if we want to insert, delete, and update directly on the object view. The alternative is to do these operations directly on the underlying tables. As you ll see shortly, the instead of triggers can be quite complex, and the resulting DMLs on the object view can result in very poor performance. Hence I recommend that you avoid creating these triggers and instead use object views only to perform selects on them. We ll look at each of the preceding steps in the following sections. We will demonstrate the use of object views by creating an object view on top of the relational tables, components_rel and parts_rel, that we created in the earlier section Relational Tables Based Approach. We assume that our relational schema consisting of the tables components_rel and parts_rel has the following data to begin with: benchmark@ORA10G> select * from components_rel; COMPONENT_ID -----------1 2 COMPONENT_NAME -------------component1 component2
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benchmark@ORA10G> select * from parts_rel; COMPONENT_ID PART_ID PART_NAME PART_DESC ------------ ------- ---------- -------------------1 1 part11 part 11 desc 1 2 part12 part 12 desc 2 3 part21 part 21 desc 2 4 part22 part 22 desc Now, let s look at each of the previous three steps involved in creating an object view on top of the tables components_rel and parts_rel.
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Defining an Object Type
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The first step is to define an object type based on which we will create an object view. Since we want our view to be equivalent in functionality to table components_nt created in the section Using Nested Tables to Store Data, we create an object type as follows: benchmark@ORA10G> create or replace type components_nt_tab as object 2 ( 3 component_id number, 4 component_name varchar2(50), 5 parts part_type_tab 6 ); 7 / Type created.
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CHAPTER 8 ORACLE OBJECTS: AN OBJECTIVE ANALYSIS
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Defining an Object View
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The next step is to define an object view called components_or_view on top of our relational tables components_rel and parts_rel, based on the object type components_nt_tab created in the previous section, as illustrated in the following code. Notice how we give the object identifier to be the primary key column, component_id, to indicate that the component_id column uniquely identifies one row in the object view. (The syntax using cast and multiset was explained in earlier sections. For a more detailed explanation of this syntax, please see the Oracle Database Application Developer s Guide Object Relational Features [10g Release 1].) benchmark@ORA10G> create or replace view components_or_view of 2 components_nt_tab with object identifier( component_id ) 3 as 4 select component_id, component_name, 5 cast 6 ( 7 multiset 8 ( 9 select component_id, part_id, part_name, part_desc 10 from parts_rel p 11 where p.component_id = c.component_id 12 ) 13 as part_type_tab 14 ) 15 from components_rel c; View created. In the next section, we ll examine examples of how to query data from the object view.
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