OTN's Oracle9i PL/SQL User Guide introduces collections as:

"A collection is an ordered group of elements, all of the same type. It is a general concept that encompasses lists, arrays, and other familiar datatypes. Each element has a unique subscript that determines its position in the collection."

So what are these used for? Well, when you first start using PL/SQL, you generally end up declaring variables, then retrieving a value from a table to go into the variable. If you're going to retrieve a range of values from a table, you declare a cursor, retrieve the values one at a time from the database, and process them sequentially. Sometimes though, you want to retrieve a load of values all in one go, and load them into an array, so that you can carry out some sort of operation on the group of values as a whole.

With Oracle7, you could create what was called an index-by table, or 'PL/SQL table,' that consisted of a series of value pairs; an index value, and a scalar datatype (such as varchar2 or number). You referred to an individual PL/SQL table entry by using the index (i.e. CUSTOMER_NAME(10)). What made it interesting was that, as well as using scalar datatypes, you could also create PL/SQL tables using PL/SQL records, which could consist of a number of individual columns. By creating a PL/SQL record type based on an existing table (for example, by using the SCOTT.EMP%ROWTYPE), you could load a table row, or an entire table, into a variable and process it within your PL/SQL package.

For example:

DECLARE
TYPE EmpTabTyp IS TABLE OF emp%ROWTYPE
    INDEX BY BINARY_INTEGER;
emp_tab EmpTabTyp;


BEGIN
/* Retrieve employee record. */
SELECT * INTO emp_tab(7468) FROM emp WHERE empno = 7468;
...
END;

This would set up a PL/SQL table type that consists of PL/SQL records with the same definition as the columns in SCOTT.EMP, and then create a variable based on this type. It would then retrieve a single row from the EMP table, then place it into the EMP_TAB variable, using the index value of 7468. Note that you don't need to initialize the variable before it's used, and you can arbitrarily put values into the variable using any valid BINARY_INTEGER value.

As of Oracle8, PL/SQL tables were renamed collections and supplemented by two new composite types: nested tables, and VARRAYs.

Nested tables extend the functionality of index-by tables by adding extra collection methods (known as table attributes for index-by tables), and in a new development, nested tables can also be stored in database tables and can be directly manipulated using SQL. Collectively, both types are known as PL/SQL tables.

To declare a nested table, you use the syntax (note the lack of 'INDEX BY BINARY INTEGER')

TYPE type_name IS TABLE OF element_type [NOT NULL];

There are some key differences between traditional index-by tables, and nested tables, and some good reasons why you'd want to start using nested tables, which are detailed in the online documentation. In practical terms, one major difference between nested tables and index-by tables, is that you have to initialize nested tables using a constructor (like you get in Java), definining how many elements can initially be stored in it (although you can later EXTEND the nested table); however, as mentioned above, you can store nested tables within the database (embedded in database columns), which is a distinct advantage over index-by tables. So, if you want to put together a database that is object orientated, and you need the data to be persistent, nested tables are the way to go.

The other new type of collection, as of Oracle8, is VARRAYs. A VARRAY(variable length array) is a datatype that would be familiar to Java or C programmers. It has a maximum size, which you must specify in its type definition. Its index has a fixed lower bound of 1 and an extensible upper bound. Thus, a VARRAY can contain a varying number of elements, from zero (when empty) to the maximum specified in its type definition.

So, how do you choose which of these three types (index-by tables, nested tables and varrays) to use? According to the docs (which also gives detailed reasons to choose nested tables over associative arrays, and between nested tables and varrays):

"If you already have code or business logic that uses some other language, you can usually translate that language's array and set types directly to PL/SQL collection types.
• Arrays in other languages become VARRAYs in PL/SQL.
• Sets and bags in other languages become nested tables in PL/SQL.
• Hash tables and other kinds of unordered lookup tables in other languages become associative arrays in PL/SQL."

So, why do the answers suggest using collections -- in this case, nested tables?

The original method for loading the table as put forward in the question was:

CREATE TABLE t1 AS
SELECT *
FROM all_objects
WHERE 1=0;

CREATE OR REPLACE PROCEDURE test_proc IS

BEGIN
    FOR x IN (SELECT * FROM all_objects)
    LOOP
        INSERT INTO t1
        (owner, object_name, subobject_name, object_id,
        data_object_id, object_type, created, last_ddl_time,
        timestamp, status, temporary, generated, secondary)
        VALUES
        (x.owner, x.object_name, x.subobject_name, x.object_id,
        x.data_object_id, x.object_type, x.created,
        x.last_ddl_time, x.timestamp, x.status, x.temporary,
        x.generated, x.secondary);
    END LOOP;
COMMIT;
END test_proc;

Ignoring the initial table creation script, the test_proc producedure does three things:

1. Declares a cursor that points to the resultset from SELECT * FROM ALL_OBJECTS
2. Starts at record one, and inserts into the t1 table the columns from the first row in the cursor
3. Then, it loops back and gets the next row of data, until all rows from the cursor have been retrieved.

The data is then committed, and the procedure ends.

The first solution put forward uses a nested table to hold the data from the ALL_OBJECTS table, and does something called BULK COLLECT to load all of the source tables' data into the nested table.

CREATE OR REPLACE PROCEDURE fast_proc (p_array_size IN PLS_INTEGER DEFAULT 100)
IS

TYPE ARRAY IS TABLE OF all_objects%ROWTYPE;
l_data ARRAY;

CURSOR c IS
SELECT *
FROM all_objects;

BEGIN
    OPEN c;
    LOOP
    FETCH c BULK COLLECT INTO l_data LIMIT p_array_size;

    FORALL i IN 1..l_data.COUNT
    INSERT INTO t2 VALUES l_data(i);

    EXIT WHEN c%NOTFOUND;
    END LOOP;
    CLOSE c;
END fast_proc;
/

Ignore the fact that the table's called ARRAY -- it's a nested table, not a VARRAY.

The second example answer is a variation on this, that does much the same thing with slightly more compact code;

SQL> create or replace procedure fast_proc is
2         type TObjectTable is table of ALL_OBJECTS%ROWTYPE;
3         ObjectTable$ TObjectTable;
4         begin
5         select
6                     * BULK COLLECT INTO ObjectTable$
7         from ALL_OBJECTS;
8
9         forall x in ObjectTable$.First..ObjectTable$.Last
10       insert into t1 values ObjectTable$(x) ;
11       end;
12 /

Again, a nested table is declared, but this time the cursor is dispensed with, and the data is just bulk loaded directly into the nested table. Again, the FORALL statement is used afterwards to run through the nested table, and I'll go into how BULK COLLECT and FORALL work tomorrow.

So what is the message here? Well, first of all, if you need to process lots of rows in one go, loading the data into memory first, use a collection and BULK COLLECT the data into them.