Best way to do a Object which holds a collection of another object type.

Question

I'm writing a caching object to store another object. The cache is only valid for a session, so I want a store the data in a nested table.
I have try to simplify my example down to its core.
How do I make this work and what is the best to index the index the items stored for fastest retrieval.
CREATE OR REPLACE TYPE ty_item AS OBJECT (
id_object VARCHAR2 (18),
ORDER MEMBER FUNCTION compare (other ty_item)
RETURN INTEGER
CREATE OR REPLACE TYPE BODY ty_item
AS
ORDER MEMBER FUNCTION compare (other ty_item)
RETURN INTEGER
IS
BEGIN
IF SELF.id_object < other.id_object
THEN
RETURN -1;
ELSIF SELF.id_object > other.id_object
THEN
RETURN 1;
ELSE
RETURN 0;
END IF;
END;
END;
CREATE OR REPLACE TYPE ty_item_store AS TABLE OF ty_item;
CREATE OR REPLACE TYPE ty_item_holder AS OBJECT (
CACHE ty_item_store,
MEMBER FUNCTION get (p_id_object IN VARCHAR2)
RETURN REF ty_item,
MEMBER FUNCTION find (p_id_object IN VARCHAR2)
RETURN REF ty_item,
MEMBER FUNCTION ADD (p_id_object IN VARCHAR2)
RETURN REF ty_item
CREATE OR REPLACE TYPE BODY ty_item_holder
AS
MEMBER FUNCTION get (p_id_object IN VARCHAR2)
RETURN REF ty_item
IS
rtn REF ty_item;
BEGIN
rtn := find (p_id_object);
IF rtn IS NULL
THEN
rtn := ADD (p_id_object);
END IF;
RETURN rtn;
END;
MEMBER FUNCTION find (p_id_object IN VARCHAR2)
RETURN REF ty_item
IS
rtn ty_item;
BEGIN
SELECT VALUE (ch)
INTO rtn
FROM CACHE ch
WHERE ch.id_object = p_id_object;
RETURN rtn;
END;
MEMBER FUNCTION ADD (p_id_object IN VARCHAR2)
RETURN REF ty_item
IS
item ty_item;
BEGIN
item := ty_item (p_id_object);
INSERT INTO CACHE
VALUES (item);
END;
END;
/

Answer 1

Best way to do a Object which holds a collection of another object type. The best place for data in a database is.. no real surprise.. in tables. If that data is temporary of nature, global temporary tables cater for that.
Storing/caching data using PL/SQL requires very expensive private process memory (PGA) from the server. This does not scale.
I'm writing a caching object to store another object. Irrespective of how l33t your haxor skillz are, you will not be able to code as a sophisticated, performant and scalable PL/SQL data cache, as what already exists (as the database buffer cache) in Oracle.
The cache is only valid for a session, so I want a store the data in a nested table.Not sure how you take one (session local data) to mean the other (oh, let's use a nested table).
Session local data can be done using PL/SQL static variables. Can be done using name-value pairs residing in a context (Oracle namespace). Can be done using a global temporary table.
The choice is dependent on the requirements that need to be addressed. However, the term +"caching+" has very specific connotations that say that a global temporary table is likely the best suited candidate.