Table size exceeds Keep Pool Size (db_keep_cache_size)

Question

Hello,
We have a situation where one of our applications started performing bad since last week.
After some analysis, it was found this was due to data increase in a table that was stored in KEEP POOL.
After the data increase, the table size exceeded db_keep_cache_size.
I was of the opinion that in such cases KEEP POOL will still be used but the remaining data will be brought in as needed from the table.
But, I ran some tests and found it is not the case. If the table size exceeds db_keep_cache_size, then KEEP POOL is not used at all.
Is my inference correct here ?
SQL> select * from v$version;
BANNER
Oracle Database 11g Enterprise Edition Release 11.2.0.2.0 - Production
PL/SQL Release 11.2.0.2.0 - Production
CORE    11.2.0.2.0      Production
TNS for Linux: Version 11.2.0.2.0 - Production
NLSRTL Version 11.2.0.2.0 - ProductionSetup
SQL> show parameter keep                    
NAME                                 TYPE        VALUE
buffer_pool_keep                     string
control_file_record_keep_time        integer     7
db_keep_cache_size                   big integer 4M
SQL>
SQL>     
SQL> create table t1 storage (buffer_pool keep) as select * from all_objects union all select * from all_objects;
Table created.
SQL> set autotrace on
SQL>
SQL> exec print_table('select * from user_segments where segment_name = ''T1''');
PL/SQL procedure successfully completed.
SQL> set serveroutput on
SQL> exec print_table('select * from user_segments where segment_name = ''T1''');
SEGMENT_NAME                  : T1
PARTITION_NAME                :
SEGMENT_TYPE                  : TABLE
SEGMENT_SUBTYPE               : ASSM
TABLESPACE_NAME               : HR_TBS
BYTES                         : 16777216
BLOCKS                        : 2048
EXTENTS                       : 31
INITIAL_EXTENT                : 65536
NEXT_EXTENT                   : 1048576
MIN_EXTENTS                   : 1
MAX_EXTENTS                   : 2147483645
MAX_SIZE                      : 2147483645
RETENTION                     :
MINRETENTION                  :
PCT_INCREASE                  :
FREELISTS                     :
FREELIST_GROUPS               :
BUFFER_POOL                   : KEEP
FLASH_CACHE                   : DEFAULT
CELL_FLASH_CACHE              : DEFAULT
PL/SQL procedure successfully completed.DB_KEEP_CACHE_SIZE=4M
SQL> select count(*) from t1;
  COUNT(*)
    135496
Execution Plan
Plan hash value: 3724264953
| Id  | Operation          | Name | Rows  | Cost (%CPU)| Time     |
|   0 | SELECT STATEMENT   |      |     1 |   538   (1)| 00:00:07 |
|   1 |  SORT AGGREGATE    |      |     1 |            |          |
|   2 |   TABLE ACCESS FULL| T1   |   126K|   538   (1)| 00:00:07 |
Note
   - dynamic sampling used for this statement (level=2)
Statistics
          9  recursive calls
          0  db block gets
       2006  consistent gets
       2218  physical reads
          0  redo size
        424  bytes sent via SQL*Net to client
        419  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          1  rows processed
SQL> /
  COUNT(*)
    135496
Execution Plan
Plan hash value: 3724264953
| Id  | Operation          | Name | Rows  | Cost (%CPU)| Time     |
|   0 | SELECT STATEMENT   |      |     1 |   538   (1)| 00:00:07 |
|   1 |  SORT AGGREGATE    |      |     1 |            |          |
|   2 |   TABLE ACCESS FULL| T1   |   126K|   538   (1)| 00:00:07 |
Note
   - dynamic sampling used for this statement (level=2)
Statistics
          0  recursive calls
          0  db block gets
       1940  consistent gets
       1937  physical reads
          0  redo size
        424  bytes sent via SQL*Net to client
        419  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          1  rows processedDB_KEEP_CACHE_SIZE=10M
SQL> connect / as sysdba
Connected.
SQL>
SQL> alter system set db_keep_cache_size=10M scope=both;
System altered.
SQL>
SQL> connect hr/hr@orcl
Connected.
SQL>
SQL> show parameter keep
NAME                                 TYPE        VALUE
buffer_pool_keep                     string
control_file_record_keep_time        integer     7
db_keep_cache_size                   big integer 12M
SQL>
SQL> set autotrace on
SQL>
SQL> select count(*) from t1;
  COUNT(*)
    135496
Execution Plan
Plan hash value: 3724264953
| Id  | Operation          | Name | Rows  | Cost (%CPU)| Time     |
|   0 | SELECT STATEMENT   |      |     1 |   538   (1)| 00:00:07 |
|   1 |  SORT AGGREGATE    |      |     1 |            |          |
|   2 |   TABLE ACCESS FULL| T1   |   126K|   538   (1)| 00:00:07 |
Note
   - dynamic sampling used for this statement (level=2)
Statistics
          0  recursive calls
          0  db block gets
       1940  consistent gets
       1937  physical reads
          0  redo size
        424  bytes sent via SQL*Net to client
        419  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          1  rows processed
SQL> /
  COUNT(*)
    135496
Execution Plan
Plan hash value: 3724264953
| Id  | Operation          | Name | Rows  | Cost (%CPU)| Time     |
|   0 | SELECT STATEMENT   |      |     1 |   538   (1)| 00:00:07 |
|   1 |  SORT AGGREGATE    |      |     1 |            |          |
|   2 |   TABLE ACCESS FULL| T1   |   126K|   538   (1)| 00:00:07 |
Note
   - dynamic sampling used for this statement (level=2)
Statistics
          0  recursive calls
          0  db block gets
       1940  consistent gets
       1937  physical reads
          0  redo size
        424  bytes sent via SQL*Net to client
        419  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          1  rows processedDB_KEEP_CACHE_SIZE=20M
SQL> connect / as sysdba
Connected.
SQL>
SQL> alter system set db_keep_cache_size=20M scope=both;
System altered.
SQL>
SQL> connect hr/hr@orcl
Connected.
SQL>
SQL> show parameter keep
NAME                                 TYPE        VALUE
buffer_pool_keep                     string
control_file_record_keep_time        integer     7
db_keep_cache_size                   big integer 20M
SQL> set autotrace on
SQL> select count(*) from t1;
  COUNT(*)
    135496
Execution Plan
Plan hash value: 3724264953
| Id  | Operation          | Name | Rows  | Cost (%CPU)| Time     |
|   0 | SELECT STATEMENT   |      |     1 |   538   (1)| 00:00:07 |
|   1 |  SORT AGGREGATE    |      |     1 |            |          |
|   2 |   TABLE ACCESS FULL| T1   |   126K|   538   (1)| 00:00:07 |
Note
   - dynamic sampling used for this statement (level=2)
Statistics
          0  recursive calls
          0  db block gets
       1943  consistent gets
       1656  physical reads
          0  redo size
        424  bytes sent via SQL*Net to client
        419  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          1  rows processed
SQL> /
  COUNT(*)
    135496
Execution Plan
Plan hash value: 3724264953
| Id  | Operation          | Name | Rows  | Cost (%CPU)| Time     |
|   0 | SELECT STATEMENT   |      |     1 |   538   (1)| 00:00:07 |
|   1 |  SORT AGGREGATE    |      |     1 |            |          |
|   2 |   TABLE ACCESS FULL| T1   |   126K|   538   (1)| 00:00:07 |
Note
   - dynamic sampling used for this statement (level=2)
Statistics
          0  recursive calls
          0  db block gets
       1943  consistent gets
          0  physical reads
          0  redo size
        424  bytes sent via SQL*Net to client
        419  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          1  rows processedOnly with 20M db_keep_cache_size I see no physical reads.
Does it mean that if the db_keep_cache_size < table size, there is no caching for that table ?
Or am I missing something ?
Rgds,
Gokul

Answer 1

Hello Jonathan,
Many thanks for your response.
Here is the test I ran;
SQL> select buffer_pool,blocks from dba_tables where owner = 'HR' and table_name = 'T1';
BUFFER_     BLOCKS
KEEP          1977
SQL> select count(*) from v$bh where objd = (select data_object_id from dba_objects where owner = 'HR' and object_name = 'T1');
  COUNT(*)
      1939
SQL> show parameter db_keep_cache_size
NAME                                 TYPE        VALUE
db_keep_cache_size                   big integer 20M
SQL>
SQL> alter system set db_keep_cache_size = 5M scope=both;
System altered.
SQL> select count(*) from hr.t1;
  COUNT(*)
    135496
SQL> select count(*) from v$bh where objd = (select data_object_id from dba_objects where owner = 'HR' and object_name = 'T1');
  COUNT(*)
       992I think my inference is wrong and as you said I am indeed seeing the effect of tail end flushing the start of the table.
Rgds,
Gokul