oracle的内存结构
数据库的内存结构：
Memory structures are allocated to the Oracle instance when the instance is started.
The two major memory structures are known as the System Global Area(Also called the shared
Global Area)and the Program Global Area(also called the Private Global Area or the Process
Global Area)
SGA:
The System Global Area(SGA) is a shared memory area.All users of the database share the
information maintained in this area.The SGA and the backgroud processes constitute an Oracle instance.Oracle allocates memory for the SGA when an Oracle instance is started and de-allocates it when the istance is shut down.The information stored in the SGA is divided into multiple memory structures that are allocated fixed space when the instance started.
DATABASE Buffer Cache:
The database buffer cache is the area of memory that caches the database area,holding blocks
from the data files that have been read recently.The DB buffer cache is shared among all the users connected to the database.
There are three type of buffers.
Dirty buffers:Dirty buffers are the buffer blocks that need to be written to the data files.
The data in these buffers has changed and has not yet been written ti the disk.
Free buffers :Free buffers do not contain any data or are free to be overwritten.When Oracle reads data from disk,free buffers are used to hold this area.
Pinned buffers :Pinned buffers are the ones that are currently being accessed or explicityly retained for futer use.(for example,the KEEP buffer)
Oracle maintains two lists to manage the buffer cache,The write list(dirty buffer list) has the buffers that are modified and need to be written to the disk(the dirty buffers).
The least recently used(LRU) list contains fr
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分页查询格式：
SELECT * from
(
SELECT A.*, ROWNUM RN
from (SELECT * from TABLE_NAME) A
WHERE ROWNUM <= 40
)
WHERE RN >= 21
其中最内层的查询SELECT * from TABLE_NAME表示不进行翻页的原始查询语句。ROWNUM <= 40和RN >= 21控制分页查询的每页的范围。
上面给出的这个分� ......
1、查找表的所有索引（包括索引名，类型，构成列）：
select t.*,i.index_type from user_ind_columns t,user_indexes i where t.index_name = i.index_name and t.table_name = i.table_name and t.table_name = 要查询的表
2、查找表的主键（包括名称，构成列）：
select cu.* from user_cons_columns cu, user_constr ......
相当于alias(别名),比如把user1.table1在user2中建一个同义词table1
create synonym table1 for user1.table1;
这样当你在user2中查select * from table1时就相当于查select * from user1.table1;
优点自己总结吧。
例如：
在oracle中对用户的管理是使用权限的方式来管理的，也就是说，如果我们想使用数据库，我� ......
1、数据库的逻辑结构和物理结构：
Oracle logically divides the database into a smaller units to manage ,store,and retreive
data efficently.
Tablespace\Blocks\Extents\segment\
有四种segment:
data segment、Index segment、Temporay segment、Rollback Segment
temporary segment:
Ar ......
