Oracle 11R2 官方文档(中英对照版,PDF)

12 Logical Storage Structures

Introduction to Logical Storage Structures

Oracle Database allocates logical space for all data in the database. The

logical units of database space allocation are data blocks, extents,

segments, and tablespaces. At a physical level, the data is stored in data

files on disk (see Chapter 11, "Physical Storage Structures"). The data in

the data files is stored in operating system blocks.

Figure 12-1 Logical and Physical Storage

Description of "Figure 12-1 Logical and Physical Storage"

A segment is a set of extents allocated for a specific database object, such

Each segment belongs to one and only one tablespace. Thus, all extents

for a segment are stored in the same tablespace. Within a tablespace, a

12-2. For example, one extent for a segment may be stored in

extent can never span data files.

Oracle Database manages space within a tablespace based on the type

that you create. You can create either of the following types of

tablespaces:

Figure 12-3 Logical Space Management

A locally managed tablespace maintains a bitmap in the data file header to

track free and used space in the data file body. Each bit corresponds to a

group of blocks. When space is allocated or freed, Oracle Database

changes the bitmap values to reflect the new status of the blocks.