Oracle CAST MULTISET ORA-00902: invalid datatype - oracle

Question

I'm trying to sort an Oracle PL/SQL collection (an associative array) using the approach presented here. I modified the example from the linked page in order to use an associative array, but I guess I'm having some casting issues.
Here it is my code:
DECLARE
TYPE TABLE_TYPE IS TABLE OF NUMBER INDEX BY VARCHAR2(10);
table_in TABLE_TYPE;
table_out TABLE_TYPE;
BEGIN
-- 1. Populating the collection with random numbers between 1 and 50
FOR i IN 1..9 LOOP
SELECT ROUND(DBMS_RANDOM.VALUE(1,50))
INTO table_in('key-'||i)
FROM DUAL;
END LOOP;
-- 2. Trying to order the collection -> throws ORA-00902: invalid datatype
SELECT CAST (MULTISET(
SELECT * FROM TABLE(table_in)
ORDER BY 2
) AS TABLE_TYPE
)
INTO table_out
FROM DUAL;
END;
What am I doing wrong?

Answer 1

You can't use associative array as a table in select, because that time is working SQL Engine and SQL Engine doesn't know what is associative array, SQL Engine can only works with Table Type, and you don't need first select (select from dual), you can do it via pl/sql engine, change it like this
....................
TYPE NumbersTableType IS TABLE OF NUMBER; -- it must be in global types, not in your local package
....................
DECLARE
table_in NumbersTableType := NumbersTableType();
table_out NumbersTableType;
BEGIN
-- 1. Populating the collection with random numbers between 1 and 50
FOR i IN 1..9 LOOP
table_in.Extend;
table_in(i):=ROUND(DBMS_RANDOM.VALUE(1,50));
END LOOP;
SELECT CAST (MULTISET(
SELECT * FROM TABLE(table_in)
ORDER BY 1
) AS NumbersTableType
)
INTO table_out
FROM DUAL;
END;

Answer 2

For sake of completeness I'll report here the trivial workaround I used to order a collection (index by varchar) in Oracle version before 12c. The solution just replaces the associative array collection with a custom Table type (in order to make it visible to the SQL Engine).
CREATE OR REPLACE TYPE MY_TABLE_TYPE_OBJ AS OBJECT (
THE_KEY VARCHAR2(10),
THE_VALUE NUMBER
);
/
CREATE OR REPLACE TYPE MY_TABLE_TYPE IS TABLE OF MY_TABLE_TYPE_OBJ;
/
DECLARE
my_key VARCHAR2(10);
table_in MY_TABLE_TYPE := MY_TABLE_TYPE();
table_out MY_TABLE_TYPE := MY_TABLE_TYPE();
BEGIN
-- Creating an unsorted collection
FOR i IN 1..9 LOOP
table_in.EXTEND;
table_in(i) := MY_TABLE_TYPE_OBJ(
'key-'||i,
ROUND(DBMS_RANDOM.VALUE(1,50))
);
END LOOP;
-- Sorting the collection
SELECT CAST (MULTISET(
SELECT * FROM TABLE(table_in)
ORDER BY 2
) AS MY_TABLE_TYPE
)
INTO table_out
FROM DUAL;
END;
Hope this helps.