Logical structure of Oracle Database

Catalog

One 、 summary

  Two 、 Several logical concepts

（ One ）Oracle Data blocks Block

（ Two ） District Extent

（ 3、 ... and ） paragraph （Segment）

 （ Four ） Table space （Tablespace）

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One 、 summary

Oracle The logical structure in includes table space 、 paragraph 、 District 、 block

The database is made up of table spaces

And the table space is made up of segments

And segments are made up of zones

And the district is made up of Oracle data Block make up

Here's the picture ：

  I found a more intuitive picture on the Internet

  Two 、 Several logical concepts

（ One ）Oracle Data blocks Block

Block is the most basic unit used to manage storage space , It is also the smallest logical storage unit . The size of the block is determined by the initialization parameter db_block_size To decide , Different Oracle The size of the version is different , But once the database is created , Its urine and urine cannot be changed . Although each data block can store different types of data , But each data block has the same structure .

  The data block structure includes 5 part ：

1. Block Contains the general attribute information of the data block , Such as the physical address of the data block 、 Segment type, etc
2. Table directory If the data stored in the data block is the data of a table , Then the information of the table is stored here
3. Line directory Used to store valid row information in the data block
4. Free space Index the unused storage space of the data block
5. Row space The data of a table or index is stored in row space , Therefore, row space is the storage space already used in the data block

（ Two ） District Extent

Area is Oracle Minimum unit of storage allocation , It is composed of one or more data blocks . One or more zones form a segment , In other words, the size of the segment is determined by the number of areas . When a segment's space is used up ,Oracle This segment will be automatically assigned a new zone .

The user can also recycle the table by executing the following command 、 An unused area in an object such as an index ：

alter table table_name deallocate unused;

In the district Oracle Data blocks are logically adjacent , But it can be physically distributed in different locations on the disk （RAID Striping and file system implementation can lead to this phenomenon ）

（ 3、 ... and ） paragraph （Segment）

  A segment is a set of extents , It is an independent logical storage structure , Used to store all data of objects with independent storage structure . Segments are generally the smallest storage unit processed by database end users , When the data area of the segment is full ,Oracle Assign it another data area , The data area of the segment may be discontinuous on the disk .

1. Data segment Used to store all data in the table ;
2. Index segment Used to store all the data indexed in the table ;
3. A temporary section Used to store temporary data generated when sorting or summarizing tables
4. LOB paragraph LOB Used to store large data objects in tables , for example CLOB And BLOB
5. Fallback section It is used to store the position and value before the user data is modified ; When you want to rollback the user's data , Use the fallback segment .

 （ Four ） Table space （Tablespace）

The table space is Oracle The largest logical storage structure , Corresponds to a physical data file , But a tablespace can have multiple data files .

1. Data table space Common table space for storing user data .
2. SYSTEM tablespace Default table space , Used to save data dictionaries （ A set of internal system tables and views that hold information about the database itself , And for Oracle Some other objects used internally ）, Save all PL/SQL Program source code and parsing code , Including stored procedures and functions 、 package 、 Database triggers, etc , Save database objects （ surface 、 View 、 Sequence ） The definition of .
3. Rollback table space It is used to store the rollback segment , At most one undo tablespace can be used per instance
4. Temporary table space Storage SQL Temporary data generated during execution

Some words and pictures in this article come from the Internet .