本文通过实验验证了在Oracle数据库中，即使设置了表的存储类型为Keep，如果没有为Keep缓存分配足够的内存，数据仍然会被存储在Default缓存中。通过调整db_keep_cache_size参数，观察到数据如何从Default缓存迁移到Keep缓存。
ocm考试新题中，需要创建keep存储的表，但在该参数是否应该修改上，有一些分歧，有人说asmm会自动给keep分配内存的，该参数就不用设置了。
看文档和asktom，也是云山雾罩，说什么的都有，还是来实际的测试吧：
SQL> col COMPONENT for a30
SQL> select COMPONENT,CURRENT_SIZE/1024/1024||'MB' MB from V_$SGA_DYNAMIC_COMPONENTS where COMPONENT in ('DEFAULT buffer cache','KEEP buffer cache');
COMPONENT MB
------------------------------ ------------------------------------------
DEFAULT buffer cache 352MB
KEEP buffer cache 0MB
SQL> conn hr/hr
Connected.
SQL> drop table t1 purge;
Table dropped.
SQL> create table t1 as select * from employees;
Table created.
SQL> insert into t1 select * from t1;
107 rows created.
SQL> /
214 rows created.
SQL> /
428 rows created.
SQL> /
856 rows created.
SQL> /
1712 rows created.
SQL> /
3424 rows created.
SQL> /
6848 rows created.
SQL> /
13696 rows created.
SQL> commit;
Commit complete.
SQL> col SEGMENT_NAME for a10
SQL> select SEGMENT_NAME,BYTES/1024/1024||'mb' MB from user_segments where SEGMENT_NAME='T1';
SEGMENT_NA MB
---------- ------------------------------------------
T1 3mb
SQL> alter table t1 storage( buffer_pool keep);
Table altered.
--造了一张3m的keep表
SQL> set autot on
SQL> select count(*) from t1;
COUNT(*)
----------
27392
Execution Plan
----------------------------------------------------------
Plan hash value: 3724264953
-------------------------------------------------------------------
| Id | Operation | Name | Rows | Cost (%CPU)| Time |
-------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 85 (0)| 00:00:02 |
| 1 | SORT AGGREGATE | | 1 | | |
| 2 | TABLE ACCESS FULL| T1 | 26589 | 85 (0)| 00:00:02 |
-------------------------------------------------------------------
Note
-----
- dynamic sampling used for this statement
--做全表扫描，没有物理读，说明是从之前的插入语句读取的数据，并做了240次递归
Statistics
----------------------------------------------------------
240 recursive calls
1 db block gets
421 consistent gets
0 physical reads
176 redo size
413 bytes sent via SQL*Net to client
385 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
4 sorts (memory)
0 sorts (disk)
1 rows processed
SQL> select count(*) from t1;
COUNT(*)
----------
27392
Execution Plan
----------------------------------------------------------
Plan hash value: 3724264953
-------------------------------------------------------------------
| Id | Operation | Name | Rows | Cost (%CPU)| Time |
-------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 85 (0)| 00:00:02 |
| 1 | SORT AGGREGATE | | 1 | | |
| 2 | TABLE ACCESS FULL| T1 | 26589 | 85 (0)| 00:00:02 |
-------------------------------------------------------------------
Note
-----
- dynamic sampling used for this statement
--第二次全表扫描已经没有递归了，说明数据已经存入内存，并整齐摆放了
Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
310 consistent gets
0 physical reads
0 redo size
413 bytes sent via SQL*Net to client
385 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
SQL> set autot off
SQL> conn / as sysdba
Connected.
SQL> select COMPONENT,CURRENT_SIZE/1024/1024||'MB' MB from V_$SGA_DYNAMIC_COMPONENTS
2 where COMPONENT in ('DEFAULT buffer cache','KEEP buffer cache');
COMPONENT MB
------------------------------ ------------------------------------------
DEFAULT buffer cache 352MB
KEEP buffer cache 0MB
--查看内存，整齐摆放的数据并未在keep内存里，说明在default里
SQL> alter system set db_keep_cache_size=12m;
System altered.
SQL> select COMPONENT,CURRENT_SIZE/1024/1024||'MB' MB from V_$SGA_DYNAMIC_COMPONENTS
2 where COMPONENT in ('DEFAULT buffer cache','KEEP buffer cache');
COMPONENT MB
------------------------------ ------------------------------------------
DEFAULT buffer cache 340MB
KEEP buffer cache 12MB
--开辟keep内存
SQL> conn hr/hr
Connected.
SQL> set autot on
SQL> select count(*) from t1;
COUNT(*)
----------
27392
Execution Plan
----------------------------------------------------------
Plan hash value: 3724264953
-------------------------------------------------------------------
| Id | Operation | Name | Rows | Cost (%CPU)| Time |
-------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 85 (0)| 00:00:02 |
| 1 | SORT AGGREGATE | | 1 | | |
| 2 | TABLE ACCESS FULL| T1 | 26589 | 85 (0)| 00:00:02 |
-------------------------------------------------------------------
Note
-----
- dynamic sampling used for this statement
--重新做全表扫描，优化器虽然已经发现内存default里有数据，但是keep内存开辟了，表又是keep的，但keep里没找到数据
--所以在此强制做了物理读
--说明keep参数为0的时候，表虽然是keep的，但数据还是在default里的，keep的大小并未被asmm自动分配
Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
310 consistent gets
307 physical reads
0 redo size
413 bytes sent via SQL*Net to client
385 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
SQL> select count(*) from t1;
COUNT(*)
----------
27392
Execution Plan
----------------------------------------------------------
Plan hash value: 3724264953
-------------------------------------------------------------------
| Id | Operation | Name | Rows | Cost (%CPU)| Time |
-------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 85 (0)| 00:00:02 |
| 1 | SORT AGGREGATE | | 1 | | |
| 2 | TABLE ACCESS FULL| T1 | 26589 | 85 (0)| 00:00:02 |
-------------------------------------------------------------------
Note
-----
- dynamic sampling used for this statement
Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
310 consistent gets
0 physical reads
0 redo size
413 bytes sent via SQL*Net to client
385 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
SQL> spool off
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