Collection_Array

PL/SQL has three collection types, Tom often demos with Nested Table and Associative Array.
Associative Array is most flexible on element indexing. Nested Table is good enough for Bulk Fetching.

But Bryn Llewellyn, Oracle PL/SQL Product Manager likes to use VArray in his demo.
"the collection is best declared as a varray with a maximum size equal to Batchsize."
See http://www.oracle.com/technology/tech/pl_sql/pdf/doing_sql_from_plsql.pdf
Probably he knows that VARRAY implemented with the most efficient internal storage, and delivery the best performance, when correctly used; it may meet most of the requirements for loop batch bulk fetching.
To find out the details, you may do a simple RunStats benchmark.

• Associative Array type(or index-by table)

TYPE population IS TABLE OF NUMBER INDEX BY VARCHAR2(64);

• Nested Tables

TYPE nested_type IS TABLE OF VARCHAR2(30);

• invoke EXTEND method to add elements later

• Collection of ADT = UDT, Abstract datatype, User defined datatype:

CREATE OR REPLACE TYPE INVDB.NUMBER_TAB_TYPE is table of number;
/

select ... from TABLE(ADT_Table_Instance);

Comments, Bulk fetch into ADT is not efficient, you may see the workaround in paper doing_sql_from_plsql.pdf

• Variable-size array (varray)

-- Code_30 Many_Row_Select.sql
Batchsize constant pls_integer := 1000;

type Result_t is record(PK t.PK%type, v1 t.v1%type);
type Results_t is varray(1000) of Result_t;
Results Results_t;

Concept

• Associative Array: sparse array.
• Nested Table or ADT/UDT : dense array

REM Associative Array
declare
   type date_aat is table of date index by binary_integer;
   l_data date_aat;
begin
   l_data(-200) := sysdate;
   l_data(+200) := sysdate+1;
end;
/

collection(elements), e.g. collection_instance(element_unique_subscript_index_number)
record_name.field_name