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Now that we ve looked at how we can use either varrays or nested tables to store data, let s summarize the various reasons for not doing so in the first place. In the case of nested tables, the following are the major disadvantages of using them to store data in tables: Oracle ultimately stores nested tables data in the relational tables. The overhead of doing so comes in the form of hidden columns, which may be mostly extraneous had we decided to use simple relational schema. For example, Oracle internally created two hidden columns of 16 bytes each in the parent table and the hidden nested tables as part of its attempt to simulate the foreign key relationship. These two columns are thrust upon the user. In the equivalent relational schema, you would simply have created a foreign key on the child table. You also have the flexibility of creating a sequence-based column as a primary key if need be. The bottom line is that the two hidden columns take additional (unnecessary, in most cases) space in your schema and constrain your design choices.

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ToString(Bdd idx) = let rec fmt depth idx = if depth > 3 then ".." else let (Node(p,l,r)) = idxToNode(idx) if p = "" then if l = trueIdx then "T" else "F" else sprintf "(%s => %s | %s)" p (fmt (depth+1) l) (fmt (depth+1) r) fmt 1 idx.

creating the nested tables as index-organized tables with the compress option in the first place. However, all other disadvantages mentioned in this section still hold true. For more details on using index-organized tables for creating nested tables, please see section Nested Table in an IndexOrganized Table (IOT) in Oracle Database Application Developer s Guide Object Relational Features (10g Release 1).

Concurrency, or more specifically, the problem of what to do with a dirty read is always an issue in distributed application development. The problem is simple. User A reads a row of data, and begins to examine it within his browser window, contemplating changing it. Meanwhile, user B comes along and makes a change to the same row. User A finally applies a change, and posts it to the server for submission to the database. The row in the table has changed since user A first retrieved the data, so user A is said to have a dirty read of the data, and if user A posts those changes, it will result in a dirty write (see Figure 10-6).

The code becomes overly complex if you use nested tables You have to be aware of the nested table type, the nested table column in the parent table, and the internally created relational table where the nested table column data is actually stored Moreover, the syntax of retrieving, inserting, and modifying data becomes more complicated, as you saw in the examples presented in earlier sections of this chapter The hint nested_table_get_refs (or the equivalent exotic syntax using a combination of the table and cast keywords) needs to be used if you want to select, insert, delete, or update the individual columns The fact that in the majority of cases you do need to manipulate the child table contents independent of the parent tables and end up using this hint really begs the question as to why you need to use nested tables in the first place.