19.1 Types of Collections

Oracle now supports three types of collections:

• PL/SQL tables are singly dimensioned, unbounded, sparse collections of homogeneous elements and are available only in PL/SQL (see Chapter 10). These are now called index-by tables.

• Nested tables are also singly dimensioned, unbounded collections of homogeneous elements. They are initially dense but can become sparse through deletions. Nested tables are available in both PL/SQL and the database (for example, as a column in a table).

• VARRAYs, like the other two collection types, are also singly dimensioned collections of homogeneous elements. However, they are always bounded and never sparse. Like nested tables, they can be used in PL/SQL and in the database. Unlike nested tables, when you store and retrieve a VARRAY, its element order is preserved.

Using a nested table or VARRAY, you can store and retrieve nonatomic data in a single column. For example, the employee table used by the HR department could store the date of birth for each employee's dependents in a single column, as shown in Table 19.1.

It's not terribly difficult to create such a table. First we define the collection type:

CREATE TYPE Dependent_birthdate_t AS VARRAY(10) OF DATE;

Now we can use it in the table definition:

CREATE TABLE employees (
   id NUMBER,
   name VARCHAR2(50),
   ...other columns...,
   Dependents_ages Dependent_birthdate_t
);

We can populate this table using the following INSERT syntax, which relies on the type's default constructor to transform a list of dates into values of the proper datatype:

INSERT INTO employees VALUES (42, 'Zaphod Beeblebrox', ...,
   Dependent_birthdate_t( '12-JAN-1765', '4-JUL-1977', '22-MAR-2021'));

One slight problem: most of us have been trained to view nonatomic data as a design flaw. So why would we actually want to do this? In some situations (for those in which you don't need to scan the contents of all the values in all the rows), theoreticians and practitioners alike consider nonatomic data to be perfectly acceptable. Even the conscience of the relational model, Chris Date, suggests that relational domains could contain complex values, including lists.[1] Some database designers have believed for years that the large percentage of nonatomic data inherent in their applications demands a nonrelational solution.

[1] See Hugh Darwen and C. J. Date, "The Third Manifesto," SIGMOD Record, Volume 24 Number 1, March 1995.

Setting aside theoretical arguments about "natural" data representations, Oracle collections provide a dramatic advantage from an application programmer's perspective: you can pass an entire collection between the database and PL/SQL using a single fetch. This feature alone could have significant positive impact on application performance.

As we've mentioned, within PL/SQL both nested tables and VARRAYs are ordered collections of homogeneous elements. Both bear some resemblance to the PL/SQL Version 2 table datatype, the elder member of the "collection" family. The new types are also singly dimensioned arrays, but they differ in areas such as sparseness (not exactly), how they're initialized (via a constructor), and whether they can be null (yes).

One chief difference between nested tables and VARRAYs surfaces when we use them as column datatypes. Although using a VARRAY as a column's datatype can achieve much the same result as a nested table, VARRAY data must be predeclared to be of a maximum size, and is actually stored "inline" with the rest of the table's data.

Nested tables, by contrast, are stored in special auxiliary tables called store tables, and there is no pre-set limit on how large they can grow. For this reason, Oracle Corporation says that VARRAY columns are intended for "small" arrays, and that nested tables are appropriate for "large" arrays.

As we've mentioned, the old Version 2 table datatype is now called an index-by table , in honor of the INDEX BY BINARY_INTEGER syntax required when declaring such a type. Despite the many benefits of the new collection types, index-by tables have one important unique feature: initial sparseness. Table 19.2 illustrates many of the additional differences among index-by tables and the new collection types.

The inevitable question is: Which construct should I use? This chapter reviews some examples of the new collections and offers some suggestions in this area. The short answer:

• Nested tables are more flexible than VARRAYs for table columns.

• VARRAYs are best when you need bounded arrays that preserve element order.

• Index-by tables are the only option that allows initial sparseness.

• If your code must run in both Oracle7 and Oracle8, you can use only index-by tables.

We'll revisit these suggestions in more detail at the end of the chapter. Before diving in, though, let's review a few of the new terms:

Collection

A term which can have several different meanings:

• A nested table, index-by table, or VARRAY datatype

• A PL/SQL variable of type nested table, index-by table, or VARRAY

• A table column of type nested table or VARRAY

Outer table

A term referring to the "enclosing" table in which you have used a nested table or VARRAY as a column's datatype

Inner table

The "enclosed" collection that is implemented as a column in a table; also known as a "nested table column"

Store table

The physical table which Oracle creates to hold values of the inner table

Unfortunately, the term "nested table" can be a bit misleading. A nested table, when declared and used in PL/SQL, is not nested at all! It is instead fairly similar to an array. Even when you use a nested table as a table column, in Oracle 8.0 you can only nest these structures to a single level. That is, your column cannot consist of a nested table of nested tables.

"Variable-size array" is also a deceptive name; one might assume, based on the fact that it is supposed to be "variable size," that it can be arbitrarily extended; quite the opposite is true. Although a VARRAY can have a variable number of elements, this number can never exceed the limit that you define when you create the type.