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Logical database structures provide a conceptual framework for managing data that is somewhat independent of the underlyingphysical hardware. One of the major benefits to using a database is that it frees you from having to remember the physical details of how your data is stored. The database software handles that for you. There are five types of logical structures: Database objects Segments Tablespaces Blocks Extents..
Database objects—A database object is a logical object containing the data that you want to store in a database. A table is such an object, as is an index.Tables and indexes are the two primary examples of database objects. They are the reason you have the database in the first place. You have tables of information that you want to store and retrieve, and you create indexes on those tables to facilitate retrieving that information.
Segments—A segment represents a set of extents that contain data for adatabase object. Segments are stored in tablespaces.Tables, indexes, and clusters are stored in segments. For a nonpartitioned object,you always have exactly one segment. Partitioned objects get one segment per partition.Segments stand between an object and the tablespace or tablespaces used to store that object.
Tablespaces—A tablespace is the logical equivalent of a file.You could consider a tablespace to be the logical equivalent of a data file. Rather than link each table and index directly to the file where the table or index data is stored, Oracle instead has you link each table and index segment to a tablespace.Data for a tablespace is then stored in one or more files.
Data Blocks—A block is the smallest logical unit of storage managed by Oracle.Data blocks are the smallest, most fundamental units of storage within a database. They are the smallest units of storage that Oracle can allocate to an object. Don’t confuse Oracle data blocks with operating system blocks. An operating system block is the smallest unit that the operating system can read from or write to disk. Oracle data blocks are the smallest units that Oracle will read from or write to disk.
Extents—An extent is a contiguous allocation of data blocks within a tablespace.An extent is a contiguous allocation of data blocks. When Oracle needs to allocate space to an object, it doesn’t just add one block. Instead, it allocates a whole group of blocks, and that group is referred to as an extent.
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