7.2 KEEP Buffer Pool

You can use the KEEP buffer pool to keep frequently accessed tables persistent in the buffer pool.

7.2.1 Overview of Using the KEEP Buffer Pool with True Cache

You can assign different objects to the KEEP buffer pool on different True Caches.

At a high level, this involves the following steps:

1. Configure DB_KEEP_CACHE_SIZE on True Cache.
2. Assign objects on True Cache to the KEEP buffer pool.

As with the primary database, when an object is assigned to the KEEP buffer pool on True Cache, the object's blocks are loaded for a query and then kept in the KEEP buffer pool. When new data is inserted into the object on the primary database, that new data is automatically propagated to the KEEP buffer pool on True Cache through the redo apply mechanism.

To propagate direct load data to True Cache, specify the LOGGING clause in the corresponding DDL or DML, or alter the corresponding object property to LOGGING. The LOGGING clause lets you specify whether certain operations will be logged in the redo log file (LOGGING) or not (NOLOGGING).

7.2.1.1 How True Cache Works with the Primary Database Buffer Cache

By default, marking something KEEP on the primary database also marks it KEEP on True Cache.

To prevent objects that are intended to be KEEP objects only on the primary database from filling up the True Cache KEEP buffer pool, you can use the DBMS_CACHEUTIL.TRUE_CACHE_KEEP procedure to mark objects as KEEP on True Cache. DBMS_CACHEUTIL.TRUE_CACHE_KEEP takes precedence and overrides objects that are marked as KEEP on the primary database.

Also consider the following points:

• ALTER TABLE KEEP assignments on the primary database are persistent because ALTER TABLE is a DDL. The DBMS_CACHEUTIL.TRUE_CACHE_KEEP procedure isn't persistent when True Cache restarts.
• You can't use the DBMS_CACHEUTIL.TRUE_CACHE_UNKEEP procedure to unkeep a primary ALTER TABLE KEEP assignment. Instead, either don't configure DB_KEEP_CACHE_SIZE on True Cache or assign a different object with DBMS_CACHEUTIL.TRUE_CACHE_KEEP on True Cache.
• All scans on True Cache use the CACHE path instead of the direct (NOCACHE) path, except temporary tables that are local to True Cache.

7.2.2 Configuring DB_KEEP_CACHE_SIZE on True Cache

To configure the KEEP buffer pool on True Cache, set the DB_KEEP_CACHE_SIZE initialization parameter to a large size (such as 10 GB).

For example:

ALTER SYSTEM SET DB_KEEP_CACHE_SIZE=size SCOPE=BOTH;

7.2.3 Assigning Objects to the KEEP Buffer Pool for True Cache

After configuring the KEEP buffer pool, assign the object to the KEEP buffer pool for True Cache.

To do this, call the DBMS_CACHEUTIL.TRUE_CACHE_KEEP() procedure on True Cache.

Example: Nonpartitioned Object

EXECUTE DBMS_CACHEUTIL.TRUE_CACHE_KEEP('SYS', 'TABLE1');

Example: Partition of a Partitioned Object

EXECUTE DBMS_CACHEUTIL.TRUE_CACHE_KEEP('SYS', 'TABLE2', 'TABLE2_PART1');

7.2.3.1 TRUE_CACHE_KEEP Procedure

When you call this procedure on True Cache, it assigns the object to the KEEP buffer pool on that cache.

To use this procedure, the DB_KEEP_CACHE_SIZE initialization parameter must be configured on True Cache.

Syntax

DBMS_CACHEUTIL.TRUE_CACHE_KEEP(
   schema                 IN VARCHAR2,
   obj                    IN VARCHAR2,
   partition              IN VARCHAR2 := NULL);

Parameters

Table 7-1 TRUE_CACHE_KEEP Procedure Parameters

Parameter	Description
schema	The name of the schema for the object.
obj	The name of the object.
partition	If the object is not partitioned, then this is NULL. If the object is partitioned, use the name of the partition segment. If it's a composited partitioned object, use the name of the subpartition segment.

7.2.4 Removing an Object's KEEP Buffer Pool Assignment for True Cache

You can use DBMS_CACHEUTIL.TRUE_CACHE_UNKEEP() to remove objects from the KEEP buffer pool.

Note that the block is not removed immediately. Instead, it will be naturally aged out as new blocks for other objects are brought into the KEEP buffer pool. Also, the KEEP buffer pool assignment on True Cache is only remembered while True Cache is up.

Example: Nonpartitioned Object

EXECUTE DBMS_CACHEUTIL.TRUE_CACHE_UNKEEP('SYS', 'TABLE1');

Example: Partition of a Partitioned Object

EXECUTE DBMS_CACHEUTIL.TRUE_CACHE_UNKEEP('SYS', 'TABLE2', 'TABLE2_PART1');

7.2.4.1 TRUE_CACHE_UNKEEP Procedure

When an object on True Cache no longer needs to be in the KEEP buffer pool, use this procedure to remove the object's KEEP assignment.

Syntax

DBMS_CACHEUTIL.TRUE_CACHE_UNKEEP(
   schema                 IN VARCHAR2,
   obj                    IN VARCHAR2,
   partition              IN VARCHAR2 := NULL);

Parameters

Table 7-2 TRUE_CACHE_UNKEEP Procedure Parameters

Parameter	Description
schema	The name of the schema for the object.
obj	The name of the object.
partition	If the object is not partitioned, then this is NULL. If the object is partitioned, use the name of the partition segment. If it's a composited partitioned object, use the name of the subpartition segment.

7.2.5 Viewing a List of KEEP Objects on True Cache

Use the V$TRUE_CACHE_KEEP view to see which objects are assigned to the KEEP buffer cache for True Cache.

Example 1

SELECT * FROM v$true_cache_keep;

TS_NUMBER  DATA_OBJECT_ID  CON_ID
---------- --------------  ------
         5          72948       3
         5          72950       3

Example 2

SELECT owner as schema, object_name as keepobj, subobject_name as partition, o.data_object_id
FROM dba_objects o, sys_objects so, v$true_cache_keep vtck
WHERE o.data_object_id = so.object_id
  AND vtck.con_id = sys_context('USERENV', 'CON_ID')
  AND so.ts_number=vtck.ts_number
  AND o.data_object_id = vtck.data_object_id;

SCHEMA  KEEPOBJ  PARTITION     DATA_OBJECT_ID
------  -------  ------------  --------------
SYS     TABLE1                          72948
SYS     TABLE2   TABLE2_PART1           72950

7.2.5.1 V$TRUE_CACHE_KEEP Columns

The following table describes the columns in the V$TRUE_CACHE_KEEP view.

Column	Description
TS_NUMBER	The tablespace number.
DATA_OBJECT_ID	The dictionary object number of the segment that contains the object.
CON_ID	The root container ID.