Huge memory leaks in using PL/SQL tables and collections

Question

I have faced a very interesting problem recently.
I use PL/SQL tables ( Type TTab is table of ... index by binary_integer; ) and collections ( Type TTab is table of ...; ) in my packages very widely. And have noticed avery strange thing Oracle does. It seems to me that there are memory leaks in PGA when I use PL/SQL tables or collections. Let me a little example.
CREATE OR REPLACE PACKAGE rds_mdt_test IS
TYPE TNumberList IS TABLE OF NUMBER INDEX BY BINARY_INTEGER;
PROCEDURE test_plsql_table(cnt INTEGER);
END rds_mdt_test;
CREATE OR REPLACE PACKAGE BODY rds_mdt_test IS
PROCEDURE test_plsql_table(cnt INTEGER) IS
x TNumberList;
BEGIN
FOR indx IN 1 .. cnt LOOP
x(indx) := indx;
END LOOP;
END;
END rds_mdt_test;
I run the following test code:
BEGIN
rds_mdt_test.test_plsql_table (1000000);
END;
and see that my session uses about 40M in PGA.
If I repeat this example in the same session creating the PL/SQL table of smaller size, for instance:
BEGIN
rds_mdt_test.test_plsql_table (1);
END;
I see again that the size of used memory in PGA by my session was not decreased and still be the same.
The same result I get if I use not PL/SQL tables, but collections or varrays.
I have tried some techniques to make Oracle to free the memory, for instance to rewrite my procedure in the following ways:
PROCEDURE test_plsql_table(cnt INTEGER) IS
x TNumberList;
BEGIN
FOR indx IN 1 .. cnt LOOP
x(indx) := indx;
END LOOP;
x.DELETE;
END;
or
PROCEDURE test_plsql_table(cnt INTEGER) IS
x TNumberList;
BEGIN
FOR indx IN 1 .. cnt LOOP
x(indx) := indx;
END LOOP;
FOR indx in 1 .. cnt LOOP
x.DELETE(indx);
END LOOP;
END;
or
PROCEDURE test_plsql_table(cnt INTEGER) IS
x TNumberList;
empty TNumberList;
BEGIN
FOR indx IN 1 .. cnt LOOP
x(indx) := indx;
END LOOP;
x := empty;
END;
and so on, but result was the same.
This is a huge problem for me as I have to manipulate collections and PL/SQL tables of very big size (from dozens of thousand of rows to millions or rows) and just a few sessions running my procedure may cause server's fall due to memory lack.
I can not understand what Oracle reseveres such much memory for (I use local variables) -- is it a bug or a feature?
I will be appreciated for any help.
I use Oracle9.2.0.1.0 server under Windows2000.
Thank you in advance.
Dmitriy.

Answer 1

Thank you, William!
Your advice about using DBMS_SESSION.FREE_UNUSED_USER_MEMORY was very useful. Indeed it is the tool I was looking for.
Now I write my code like this
declare
type TTab is table of ... index binary_integer;
res TTab;
empty_tab TTab;
begin
res(1) := ...;
res := empty_tab;
DBMS_SESSION.FREE_UNUSED_USER_MEMORY;
end;
I use construction "res := empty_tab;" to mark all memory allocated to PL/SQL table as unused according to Tom Kyte's advices. And I could live a hapy life if everything were so easy. Unfortunately, some tests I have done showed that there are some troubles in cleaning complex nested PL/SQL tables indexed by VARCHAR2 which I use in my current project.
Let me another example.
CREATE OR REPLACE PACKAGE rds_mdt_test IS
TYPE TTab0 IS TABLE OF NUMBER INDEX BY BINARY_INTEGER;
TYPE TRec1 IS RECORD(
NAME VARCHAR2(4000),
rows TTab0);
TYPE TTab1 IS TABLE OF TRec1 INDEX BY BINARY_INTEGER;
TYPE TRec2 IS RECORD(
NAME VARCHAR2(4000),
rows TTab1);
TYPE TTab2 IS TABLE OF TRec2 INDEX BY BINARY_INTEGER;
TYPE TStrTab IS TABLE OF NUMBER INDEX BY VARCHAR2(256);
PROCEDURE test_plsql_table(cnt INTEGER);
PROCEDURE test_str_tab(cnt INTEGER);
x TTab2;
empty_tab2 TTab2;
empty_tab1 TTab1;
empty_tab0 TTab0;
str_tab TStrTab;
empty_str_tab TStrTab;
END rds_mdt_test;
CREATE OR REPLACE PACKAGE BODY rds_mdt_test IS
PROCEDURE test_plsql_table(cnt INTEGER) IS
BEGIN
FOR indx1 IN 1 .. cnt LOOP
FOR indx2 IN 1 .. cnt LOOP
FOR indx3 IN 1 .. cnt LOOP
x(indx1) .rows(indx2) .rows(indx3) := indx1;
END LOOP;
END LOOP;
END LOOP;
x := empty_tab2;
dbms_session.free_unused_user_memory;
END;
PROCEDURE test_str_tab(cnt INTEGER) IS
BEGIN
FOR indx IN 1 .. cnt LOOP
str_tab(indx) := indx;
END LOOP;
str_tab := empty_str_tab;
dbms_session.free_unused_user_memory;
END;
END rds_mdt_test;
1. Running the script
BEGIN
rds_mdt_test.test_plsql_table ( 100 );
END;
I see that usage of PGA memory in my session is close to zero. So, I can judge that nested PL/SQL table indexed by BINARY_INTEGER and the memory allocated to it were cleaned successfully.
2. Running the script
BEGIN
rds_mdt_test.test_str_tab ( 1000000 );
END;
I can see that plain PL/SQL table indexed by VARCHAR2 and memory allocated to it were cleaned also.
3. Changing the package's type
TYPE TTab2 IS TABLE OF TRec2 INDEX BY VARCHAR2(256);
and running the script
BEGIN
rds_mdt_test.test_plsql_table ( 100 );
END;
I see that my session uses about 62M in PGA. If I run this script twice, the memory usage is doubled and so on.
The same result I get if I rewrite not highest, but middle PL/SQL type:
TYPE TTab1 IS TABLE OF TRec1 INDEX BY VARCHAR2(256);
And only if I change the third, most nested type:
TYPE TTab0 IS TABLE OF NUMBER INDEX BY VARCHAR2(256);
I get the desired result -- all memory was returned to OS.
So, as far as I can judge, in some cases Oracle does not clean complex PL/SQL tables indexed by VARCHAR2.
Is it true or not? Perhaps there are some features in using such way indexed tables?