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Tuning the Shared Pool and resolving ORA-4031 in Oracle7

Creation Date: 12-SEP-1997
Last Revision Date: 19-JAN-2001

This document discusses some of the common issues associated with the shared
pool in Oracle7 and describes how to diagnose and respond to these issues.


With Release 7.2 and 7.3, changes have been made to reduce usage of shared
memory as well as per-user (UGA) memory. Also, memory is not being allocated
in large contiguous chunks, resulting in better shared-pool utilization and
reduction in fragmentation.


The primary problem that occurs is that free memory in the shared pool becomes
fragmented into small pieces over time. Any attempt to allocate a large piece
of memory in the shared pool will cause large amount of objects in the library
cache to be flushed out and may result in an ORA-04031 out of shared memory


i) ORA-04031 ERROR

One way to diagnose that this is happening is to look for ORA-04031 errors
being returned from applications. When an attempt is made to allocate a large
contiguous piece of shared memory, and not enough contiguous memory can be
created in the shared pool, the database will signal this error.

Before this error is signalled, all objects in the shared pool that are not
currently in use will be flushed from the shared pool, and their memory will be
freed and merged. This error only occurs when there is still not a large
enough contiguous piece of free memory after this happens. There may be very
large amounts of total free memory in the shared pool, but just not enough
contiguous memory.


An init.ora parameter can be set so that whenever an ORA-04031 error is
signalled a dump will occur into a trace file. By looking for these trace
files, the DBA can determine that these errors are occurring. This is useful
when applications do not always report errors signalled by oracle, or if users
do not report the errors to the DBAs. The parameter is the following:

event = "4031 trace name errorstack"

If you are using 7.0.16 or higher you can use the following:

event = "4031 trace name errorstack level 4"

This will cause a dump of the Oracle state objects to occur when this error is
signalled. By looking in the dump for 'load=X' and then looking up a few lines
for 'name=' you can often tell whether an object was being loaded into the
shared pool when this error occurred. If an object was being loaded then it is
likely that this load is the cause of the problem and the object should be
'kept' in the shared pool. The object being loaded is the object printed after
the 'name='. Do not use the 'level 4' option in versions before 7.0.16 because
a bug existed that often caused the system to crash with this option enabled
due to a latch level violation.

Prior to version 7.3, there were a handful of cases where the RDBMS or PL/SQL
would attempt to allocate large pieces of contiguous memory. Most of this has
been fixed for 7.3. This problem was especially acute when running MTS, when
the UGA would be located in the SGA. This should also be fixed in 7.3 and using
MTS for a high OLTP scenario is recommended. As a result of all these changes,
the ORA-04031 error should be virtually eliminated. If an ORA-04031 error is
signalled, quite likely the shared pool is over 90% utilized and the
alternative is to increase the shared pool. The only known situation is PL/SQL
packages (like STANDARD) where the package contains a very large number (over
400) procedure/function definitions. This still needs to be in contiguous
memory and may request memory chunks as large as 15K. Packages like this should
be the only ones that should be kept.


There is a fixed table called x$ksmlru that tracks allocations in the
shared pool that cause other objects in the shared pool to be aged out. This
fixed table can be used to identify what is causing the large allocation.
The columns of this fixed table are the following:

KSMLRCOM - allocation comment that describes the type of allocation.

If this comment is something like 'MPCODE' or 'PLSQL%' then there is a large
PL/SQL object being loaded into the shared pool. This PL/SQL object will need
to be 'kept' in the shared pool.

If this comment is 'kgltbtab' then the allocation is for a dependency table in
the library cache. This is only a problem when several hundred users are logged
on using distinct user ids. The solution in this case is to use fully qualified
names for all table references.

If you are running MTS and the comment is something like 'Fixed UGA' then the
problem is that the init.ora parameter 'open_cursors' is set too high.

KSMLRSIZ - amount of contiguous memory being allocated. Values over around 5K
start to be a problem, values over 10K are a serious problem, and values over
20K are very serious problems. Anything less then 5K should not be a problem.

KSMLRNUM - number of objects that were flushed from the shared pool in order
allocate the memory.

In release 7.1.3 or later, the following columns also exist:

KSMLRHON - the name of the object being loaded into the shared pool if the
object is a PL/SQL object or a cursor.

KSMLROHV - hash value of object being loaded

KSMLRSES - SADDR of the session that loaded the object.

The advantage of X$KSMLRU is that it allows you to identify problems with
fragmentation that are effecting performance, but that are not bad enough to be
causing ORA-04031 errors to be signalled. If a lot of objects are being
periodically flushed from the shared pool then this will cause response time
problems and will likely cause library cache latch contention problems when the
objects are reloaded into the shared pool. With version 7.2, the library cache
latch contention should be significantly reduced with the breaking up of the
library cache pin latch into a configurable set of symmetric library cache

One unusual thing about the x$ksmlru fixed table is that the contents of the
fixed table are erased whenever someone selects from the fixed table. This is
done since the fixed table stores only the largest allocations that have
occurred. The values are reset after being selected so that subsequent large
allocations can be noted even if they were not quite as large as others that
occurred previously. Because of this resetting, the output of selecting from
this table should be carefully noted since it cannot be reselected if it is
forgotten. Also you should take care that there are not multiple people on one
database that select from this table because only one of them will select the
real data.

To monitor this fixed table just run the following:

select * from x$ksmlru where ksmlrsiz > 5000;

iv) MTS

Oracle users using SQL*Net V2 can connect to the database using dedicated
servers, or multiple clients can use a pool of shared (or MTS) servers. The
biggest memory implication of this mode is that the session memory (also known
as the UGA) for every session needs to be accessible to every MTS server. This
implies that the logical UGA comes out of the physical SGA (or the shared
pool) instead of the PGA (process memory).

In versions prior to 7.3, there were a few components in the UGA that would
request large contiguous chunks of memory, contributing to fragmentation of
the shared pool if using MTS. If the system had been up for a while, users
would have failures when attempting to connect or executing sql. Starting with
7.3, all these allocations have been segmented such that the average size of
memory chunks allocated to the UGA should be about 5K.



The primary source of problems is large PL/SQL objects. The means of correcting
these errors is to 'keep' large PL/SQL object in the shared pool at startup
time. This will load the objects into the shared pool and will make sure that
the objects are never aged out of the shared pool. If the objects are never
aged out then there will not be a problem with trying to load them and not
having enough memory.

Objects are 'kept' in the shared pool using the dbms_shared_pool package that
is defined in the dbmspool.sql file. For example:

execute dbms_shared_pool.keep('SYS.STANDARD');

All large packages that are shipped should be 'kept' if the customer uses
PL/SQL. This includes 'STANDARD', 'DBMS_STANDARD', and 'DIUTIL'. With 7.3, the
only package left in this list is 'STANDARD'.

All large customer packages should also be marked 'kept'.

To mark all packages in the system 'kept' execute the following:

own varchar2(100);
nam varchar2(100);
cursor pkgs is
select owner, object_name
from dba_objects
where object_type = 'PACKAGE';
open pkgs;
fetch pkgs into own, nam;
exit when pkgs%notfound;
dbms_shared_pool.keep(own || '.' || nam, 'P');
end loop;

The dbms_shared_pool package was introduced in 7.0 and has evolved over the
versions. Until 7.1.5, 'keep' could only be used for packages. Starting with
7.1.6, this was extended to standalone procedures, cursors as well as triggers.
For detailed usage instructions, see the dbmspool.sql file. So, prior to this
version, if you have large procedures or large anonymous blocks, then these
will need to be put into packages and marked kept. With 7.3, most packages do
not need to be kept any longer since PL/SQL no longer requires large amounts of
contiguous memory to load packages/procedures in memory.

You can determine what large stored objects are in the shared pool by selecting
from the v$db_object_cache fixed view. This will also tell you which objects
have been marked kept. This can be done with the following query:

select * from v$db_object_cache where sharable_mem > 10000;

Note that this query will not catch PL/SQ: objects that are only rarely used
and therefore the PL/SQL object is not currently loaded in the shared pool.

To determine what large PL/SQL objects are currently loaded in the shared pool
and are not marked 'kept' and therefore may cause a problem, execute the

select name, sharable_mem
from v$db_object_cache
where sharable_mem > 10000
and (type = 'PACKAGE' or type = 'PACKAGE BODY' or type = 'FUNCTION'
or type = 'PROCEDURE')
and kept = 'NO';

Another approach to the above is to use the dbms_shared_pool.sizes procedure.
To use this in SQLDBA:

set serveroutput on;
execute dbms_shared_pool.sizes(10);

This should show you the names of all the objects in the shared pool that take
more that 10K of memory as well as if they are marked kept or not. For SQL
statements, if there are multiple versions of a query (usually a bug if the
count is more than 3), they will also be indicated in parenthesis. Use the
following query to check for problems:

select sql_text, loaded_versions, version_count, sharable_mem
from v$sqlarea where loaded_versions > 3
order by sharable_mem;

In Oracle7.3 onwards the best candidates for keeping can be seen by querying
the table X$KSMSP to see if there are any chunks in the shared-pool that have
the KSMCHSIZ larger than 5K and KSMCHCOM like '%PL/SQL%'. If so then one can
identify the object name and owner of this chunk using the following SQL:

select distinct
||' - '||kglnaown||'.'||kglnaobj "Eligible PL/SQL objects"
from x$kglob
where kglobhd4 in
(select ksmchpar from x$ksmsp
where ksmchcom='PL/SQL MPCODE' and ksmchsiz>5120)

If you are 'keeping' PL/SQL objects today and migrate to 7.3 or higher there is
no need to re-assess the list of objects that you are keeping.


One of the best things that can be done to reduce the amount of fragmentation
is to reduce or eliminate the number of sql statements in the shared pool that
are duplicates of each other except for a constant that is embedded in the
statement. The statements should be replaced with one statement that uses
a bind variable instead of a constant.

For example:

select * from emp where empno=1;
select * from emp where empno=2;
select * from emp where empno=3;

Should all be replaced with:

select * from emp where empno=:1;

You can identify statements that potentially fall into this class with a query
like the following:

select substr(sql_text, 1, 30) sql, count(*) copies
from v$sqlarea
group by substr(sql_text, 1, 30)
having count(*) > 3;


It is possible for a SQL statement to not be shared because the max bind
variable lengths of the bind variables in the statement do not match. This is
automatically taken care of for precompiler programs and forms programs, but
could be a problem for programs that directly use OCI. The bind call in OCI
takes two arguments, one is the max length of the value, and the other is a
pointer to the actual length. If the current length is always passed in as the
max length instead of the max possible length for the variable, then this could
cause the SQL statement not to be shared.

To identify statements that might potentially have this problem execute the
following statement:

select sql_text, version_count from v$sqlarea where version_count > 5;

Starting with 7.1.6 this should no longer be an issue as the server can
graduate bind buffers even when the user's max bind lengths are jumping up or
down and continue to share cursors that are built for larger buffer lengths
and flush the smaller sql compilation from the shared pool.


Large anonymous PL/SQL blocks should be turned into small anonymous PL/SQL
blocks that call packaged functions. The packages should be 'kept' in memory.
For version earlier that 7.3, this includes anonymous PL/SQL blocks that are
used for trigger definitions. With 7.3, triggers are compiled and stored to
disk like standalone procedures and should be treated as such. Large anonymous
blocks can be identified with the following query:

select sql_text from v$sqlarea
where command_type=47 -- command type for anonymous block
and length(sql_text) > 500;

Note that this query will not catch PL/SQL blocks that are only rarely used and
therefore the PL/SQL block is not currently loaded in the shared pool.

Another option that can be used when an anonymous block cannot be turned into
a package is to mark the anonymous block with some string so that it can be
identified in v$sqlarea and marked 'kept'.

For example, instead of using:
declare x number; begin x := 5; end;;

you can use:
declare /* KEEP_ME */ x number; begin x := 5; end;

You can then use the following procedure to select these statements out of the
shared pool and mark them 'kept' using the dbms_shared_pool.keep package.

addr varchar2(10);
hash number;
cursor anon is
select address, hash_value
from v$sqlarea
where command_type = 47 -- command type for anonymous block
and sql_text like '% KEEP_ME %'
and sql_text not like '%DONT_KEEP_ME%';
open anon;
fetch anon into addr, hash;
exit when anon%notfound;
dbms_shared_pool.keep(addr || ',' || to_char(hash), 'C');
end loop;


Another way to reducing fragmentation is to reduce consumption. This is of
special importance when using MTS, when every user's session memory is in the
shared pool and the impact is multiplied by the total concurrent users.

Insert, update, delete and anonymous blocks complete the execution in one round
trip. All the memory that is allocated on the server for the execute comes from
the PGA and is freed before the call returns to the user. But in the case of
selects, memory required to execute the statement - which could be large if a
sort was involved - is not freed until the end-of-fetch is reached or the query
is cancelled. In these situations using the OCI features to do an exact fetch
and cancel helps free memory back to the pool.

If the application logic has been embedded into server side PL/SQL, a large
number of cursors may be getting cached on the server for every user. Though
this results in reduced latch contention and faster response, it does use more
memory in the UGA. Setting the close_cached_open_cursors init.ora to TRUE
closes the PL/SQL cached cursors on the server, freeing the memory.



There are a number of common fallacies about the shared pool that are often
stated as fact.


One fallacy is that the amount of 'free memory' reported in v$sgastat needs to
be kept high. This is incorrect. The free memory reported in this table is not
like the free memory reported by operating system statistics. Since the shared
pool acts as a cache, nothing will ever be aged out of the shared pool until all
the free memory has been used up. This is entirely normal.

Free memory is more properly thought of as 'wasted memory'. You would rather
see this value be low than very high. In fact, a high value of free memory is
sometimes a symptom that a lot of objects have been aged out of the shared pool
and therefore the system is experiencing fragmentation problems.


Some people think that frequently executing 'alter system flush shared_pool'
improves the performance of the system and decreases the amount of
fragmentation. This is incorrect. Executing this statement causes a big spike
in performance and does nothing to improve fragmentation.

The only time when it might be useful to run this statement is between shifts of
users so that the objects that are relevant to the last shift of users can be
flushed out before the next shift of users starts to use the system. This is
almost never needed though.



One very difficult judgement that needs to be make in Oracle7 is to determine
the proper size of the shared pool. The following provides some guidelines for
this. It should be emphasized that these are just guidelines, there are no
hard and fast rules here and experimentation will be needed to determine a good

The shared pool size is highly application dependent. To determine the shared
pool size that will be needed for a production system it is generally necessary
to first develop the application and run it on a test system and take some
measurements. The test system should be run with a very large value for the
shared pool size to make the measurements meaningful.


The amount of shared pool that needs to be allocated for objects that are
stored in the database like packages and views is easy to measure. You can
just measure their size directly with the following statement:

select sum(sharable_mem) from v$db_object_cache;

This is especially effective because all large pl/sql object should be 'kept'
in the shared pool at all times.


The amount of memory needed to store SQL statements in the shared pool is more
difficult to measure because of the needs of dynamic SQL. If an application
has no dynamic SQL then the amount of memory can simply be measured after the
application has run for a while by just selecting it out of the shared pool as

select sum(sharable_mem) from v$sqlarea;

If the application has a moderate or large amount of dynamic SQL like most
applications do, then a certain amount of memory will be needed for the shared
SQL, plus more for the dynamic SQL, and more so that the dynamic SQL does not
age the shared SQL out of the shared pool.

The amount of memory for the shared SQL can be approximated by the following:

select sum(sharable_mem) from v$sqlarea where executions > 5;

The remaining memory in v$sqlarea is for dynamic SQL Some shared pool will
need to be budgeted for this also, but there are few rules here.


You will need to allow around 250 bytes of memory in the shared pool per
concurrent user for each open cursor that the user has whether the cursor is
shared or not. During the peak usage time of the production system, you can
measure this as follows:

select sum(250 * users_opening) from v$sqlarea;

In a test system you can measure it by selecting the number of open cursors
for a test user and multiplying by the total number of users:

select 250 * value bytes_per_user
from v$sesstat s, v$statname n
where s.statistic# = n.statistic#
and n.name = 'opened cursors current'
and s.sid = 23; -- replace 23 with session id of user being measured

The per-user per-cursor memory is one of the classes of memory that shows up as
'library cache' in v$sgastat.


If you are using multi-threaded server, then you will need to allow enough
memory for all the shared server users to put their session memory in the
shared pool. This can be measured for one user with the following query:

select value sess_mem
from v$sesstat s, v$statname n
where s.statistic# = n.statistic#
and n.name = 'session uga memory'
and s.sid = 23; -- replace 23 with session id of user being measured

A more conservative value to use is the maximum session memory that was
ever allocated by the user:

select value sess_max_mem
from v$sesstat s, v$statname n
where s.statistic# = n.statistic#
and n.name = 'session uga memory max'
and s.sid = 23; -- replace 23 with session id of user being measured

To select this value for all the currently logged on users the following query
can be used:

select sum(value) all_sess_mem
from v$sesstat s, v$statname n
where s.statistic# = n.statistic#
and n.name = 'session uga memory max';


You will need to add a minimum of 30% overhead to the values calculated above
to allow for unexpected and unmeasured usage of the shared pool.



The most important point that needs to be understood by everyone using Oracle7
and PL/SQL (prior to release 7.3) is that all large PL/SQL objects must be made
into packages and those packages must be kept in the shared pool. This point
cannot be over emphasized. Many customers, especially those running a lot of
users, have had terrible performance problems that were completely cleared up
by doing this.

APPENDIX I: Reserved Shared Pool

On busy systems, the RDBMS may have difficulty finding a contiguous piece of
memory to satisfy a large request for memory. Because the RDBMS will search
for and free currently unused memory, the search for this large piece of memory
may disrupt the behavior of the share pool, leading to more fragmentation and
poor performance.

RDBMS 7.1.5 allows DBAs to reserve memory within the shared pool to satisfy
these large allocations during RDBMS operations such as PL/SQL compilation and
trigger compilation. Smaller objects will not fragment the reserved list,
helping to ensure the reserved list will have large contiguous chunks of memory.
Once the memory allocated from the reserved list is freed, it returns to the
reserved list.

The size of the reserved list, as well as the minimum size of the objects that
can be allocated from the reserved list are controlled via init.ora parameters:
shared_pool_reserved_size and shared_pool_reserved_min_alloc.

1.1 shared_pool_reserved_size
The init.ora parameter shared_pool_reserved_size controls the amount of
shared_pool_size reserved for large allocations. In order to create a reserved
list, shared_pool_reserved_size must be greater than

units : bytes
default: 0 (no reserved list)
minimum: > shared_pool_reserved_min_alloc
maximum: 1/2 shared_pool_size

1.2 shared_pool_reserved_min_alloc
The init.ora parameter shared_pool_reserved_min_alloc controls allocation for
the reserved memory. Only allocations larger than
shared_pool_reserved_min_alloc are allowed to allocate space from the reserved
list if a chunk of memory of sufficient size is not found on the shared pool's
free lists.

units : bytes
default: 5000
minimum: 5000
maximum: < shared_pool_reserved_size

The default value for shared_pool_reserved_min_alloc should be adequate for
almost all systems.

RDBMS 7.1.5 also provides a new procedure, aborted_request_threshold, in package
dbms_shared_pool, which allows users to set the limit on the size of allocations
allowed to flush the shared pool if the free lists cannot satisfy the request

Before the RDBMS signals the ORA-04031 error, it incrementally flushes unused
objects from the shared pool until there is sufficient memory to satisfy the
allocation request. In most cases, incrementally flushing objects from the
shared pool frees enough memory for the allocation to complete succesfully. If
the RDBMS signals an ORA-04031 error, it has flushed all objects currently not
in use on the system without finding a large enough piece of contiguous memory.

On a busy system, the larger the space allocation, the more likely the RDBMS
will signal the ORA-04031 error. Flushing all objects, however, will impact
other users on the system, possibly causing a degradation in performance.

The aborted_request_threshold procedure allows the DBA to localize the impact
the ORA-04031 error to the process that couldn't allocate memory. The procedure
takes a numeric value between 5000 and 2147483647, representing the size, in
bytes, of the threshold.

RDBMS 7.1.5 has a new fixed view to help tune the reserved pool and space within
the shared pool. The name of the new fixed view is V$SHARED_POOL_RESERVED and
has the following columns:

Name Null? Type
------------------------------- -------- --------------

These columns of V$SHARED_POOL_RESERVED are only valid if the parameter
shared_pool_reserved_size is set to a valid value.

FREE_SPACE is the total amount of free space on the reserved list.

AVG_FREE_SIZE is the average size of the free memory on the reserved list.

FREE_COUNT is the number of free pieces of memory on the reserved list.

MAX_FREE_SIZE is the size of the largest free piece of memory on the reserved

USED_SPACE is the total amount of used memory on the reserved list.

AVG_USED_SIZE is the average size of the of the used memory on the reserved

USED_COUNT is the number of used pieces of memory on the reserved list.

MAX_USED_SIZE is the size of the largest used piece of memory on the reserved

REQUESTS is the number of times that the reserved list was searched for a
free piece of memory.

REQUEST_MISSES is the number of times the reserved list didn't have a free
piece of memory to satisfy the request, and proceeded to start
flushing objects from the LRU list.

LAST_MISS_SIZE is the request size of the last REQUEST_MISS.

MAX_MISS_SIZE is the request size of the largest REQUEST_MISS.

The next set of columns contain values which are valid even if
shared_pool_reserved_size is not set.

REQUEST_FAILURES is the number of times that no memory was found to satisfy a
request (example: number of times ORA-04031 occurred)

LAST_FAILURE_SIZE is the request size of the last failed request
(example: the request size of last ORA-04031).

ABORTED_REQUEST_THRESHOLD is the minimum size of a request which will signal
an ORA-04031 error without flushing objects. See
the procedure aborted_request_threshold described

LAST_ABORTED_SIZE is the last size of the request which returned an ORA-04031
error without flushing objects from the LRU list.

Information in V$SHARED_POOL_RESERVED can help to set values for
shared_pool_reserved_size and even shared_pool_size. This section assumes the
DBA has performed all other shared pool tuning on his system.

4.1 Initial Value for shared_pool_reserved_size
The DBA should make shared_pool_reserved_size 10% of the shared_pool_size. For
most systems, this value should be sufficient, if the DBA has already spent time
tuning the shared pool.

4.2 Initial Value for shared_pool_reserved_min_alloc
In most cases, the default value for this parameter is adequate. If the DBA
increases this value, then the RDBMS will allow fewer allocations from the
reserved list and will request more memory from the shared pool list.

4.4 Tuning shared_pool_reserved_size
Ideally, shared_pool_reserved_size should be made large enough to satisfy any
request scanning for memory on the reserved list without flushing objects from
the shared pool. The amount of operating system memory, however, may constrain
the size of the SGA, and therefore the size of the shared pool such that this
is not a feasible goal.

If the DBA has a system with ample free memory to increase his SGA, the goal is
to have:


If the DBA is constrained for OS memory, his goal is:

REQUEST_FAILURES = 0 or not increasing
LAST_FAILURE_SIZE > shared_pool_reserved_min_alloc
AVG_FREE_SIZE > shared_pool_reserved_min_alloc

If neither of these goals are met, increase shared_pool_reserved_size; the DBA
also needs to increase shared_pool_size by the same amount, since the reserved
list is taken from the shared pool.

4.5 shared_pool_reserved_size too low
The reserved pool is too small when:

REQUEST_FAILURES > 0 (and increasing)

and at least one of the following is true:

LAST_FAILURE_SIZE > shared_pool_reserved_min_alloc
MAX_FREE_SIZE < shared_pool_reserved_min_alloc
FREE_MEMORY < shared_pool_reserved_min_alloc

The DBA has two options, depending on his SGA size constraints:

o Increase shared_pool_reserved_size and shared_pool_size, accordingly
o Increase shared_pool_reserved_min_alloc (but may need to increase

The first option will increase the amount of memory available on the reserved
list without impacting users not allocating memory from the reserved list. The
second options reduces the number of allocations allowed to use memory from the
reserved list; doing so, however, will increase normal shared pool perhaps
impacting other users on the system.

4.6 shared_pool_reserved_size too high
It is possible that too much memory has been allocated to the reserved list.

REQUEST_MISS = 0 or not increasing
FREE_MEMORY = > 50% of shared_pool_reserved_size minimum

The DBA has two options:

o Decrease shared_pool_reserved_size
o Decrease shared_pool_reserved_min_alloc (if not the default

4.7 shared_pool_size too small
The new fixed table can also indicate when shared_pool_size is too small.

REQUEST_FAILURES > 0 and increasing
LAST_FAILURE_SIZE < shared_pool_reserved_min_alloc

Then the DBA has two options if he has enabled the reserved list:

o Decrease shared_pool_reserved_size
o Decrease shared_pool_reserved_min_alloc (if set larger than the default)

Otherwise, the DBA the could:

o Increase shared_pool_size

APPENDIX 2: Procedure free_unused_memory

This text is also in the specification for this procedure in dbmsutil.sql.
It is part of package dbms_session.

Procedure free_unused_memory --

Procedure for users to reclaim unused memory after performing operations
requiring large amounts of memory (where large is >100K). Note that this
procedure should only be used in cases where memory is at a premium.

Examples operations using lots of memory are:

o large sorts where entire sort_area_size is used and
sort_area_size is hundreds of KB
o compiling large PL/SQL packages, procedures, or functions
o storing hundreds of KB of data within PL/SQL indexed tables

One can monitor user memory by tracking the statistics "session uga memory" and
"session pga memory" in the v$sesstat/v$statname fixed views. Monitoring these
statistics will also show how much memory this procedure has freed. The behavior
of this procedure depends upon the configuration of the server operating on
behalf of the client:

o dedicated server - returns unused PGA memory to the OS
o MTS server - returns unused session memory to the shared_pool

In order to free memory using this procedure, the memory must not be in use.

Once an operation allocates memory, only the same type of operation can reuse
the allocated memory. For example, once memory is allocated for sort, even if
the sort is complete and the memory is no longer in use, only another sort can
reuse the sort-allocated memory. For both sort and compilation, after the
operation is complete, the memory is no longer in use and the user can invoke
this procedure to free the unused memory.

An indexed table implicitly allocates memory to store values assigned to the
indexed table's elements. Thus, the more elements in an indexed table, the more
memory the RDBMS allocates to the indexed table. As long as there are elements
within the indexed table, the memory associated with an indexed table is in use.

The scope of indexed tables determines how long their memory is in use. Indexed
tables declared globally are indexed tables declared in packages or package
bodies. They allocate memory from session memory. For an indexed table
declared globally, the memory will remain in use for the lifetime of a user's
login (lifetime of a user's session), and is freed after the user disconnects
from ORACLE.

Indexed tables declared locally are indexed tables declared within functions,
procedures, or anonymous blocks. These indexed tables allocate memory from PGA
memory. For an indexed table declared locally, the memory will remain in use
for as long as the user is still executing the procedure, function, or anonymous
block in which the indexed table is declared. After the procedure, function, or
anonymous block is finished executing, the memory is then available for other
locally declared indexed tables to use (i.e., the memory is no longer in use).

Assigning an uninitialized, "empty," indexed table to an existing index table is
a method to explicitly re-initialize the indexed table and the memory associated
with the indexed table. After this operation, the memory associated with the
indexed table will no longer be in use, making it available to be freed by
calling this procedure. This method is particularly useful on indexed tables
declared globally which can grow during the lifetime of a user's session, as
long as the user no longer needs the contents of the indexed table.

The memory rules associated with an indexed table's scope still apply; this
method and this procedure, however, allow users to intervene and to explictly
free the memory associated with an indexed table.

The PL/SQL fragment below illustrates the method and the use of procedure

create package foobar
type number_idx_tbl is table of number indexed by binary_integer;
store1_table number_idx_tbl; -- PL/SQL indexed table
store2_table number_idx_tbl; -- PL/SQL indexed table
store3_table number_idx_tbl; -- PL/SQL indexed table
end; -- end of foobar
empty_table number_idx_tbl; -- uninitialized ("empty") version

for i in 1..1000000 loop
store1_table(i) := i; -- load data
end loop;
store1_table := empty_table; -- "truncate" the indexed table
dbms_session.free_unused_user_memory; -- give memory back to system

store1_table(1) := 100; -- index tables still declared;
store2_table(2) := 200; -- but truncated.

Performance Implication:

This routine should be used infrequently and judiciously.

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