Oracle database logical structures mainly comprise of Tablespaces, Segements, Extents and Oracle Datablocks.
I will present the information starting with the finest logical structure Oracle Datablocks.
Oracle Datablocks: Oracle datablocks are at the finest level granularity, all of the oracle datatbase data is stored in oracle datablocks. One oracle datablock corresponds to specific number of bytes which occupy the same number of bytes on the physical disk space. The size of a datablock is determined by the initialization parameter DB_BLOCK_SIZE. In addition to the one specified already you can specify upto 5 additional datablock sizes
Extent: Extent is the next level of oracle logical database space. A extent is comprised specific number of contiguous datablocks, obtained in a single allocation, used to store a specific type of information.
Segment: A segment is a set of extents allocated for a certain logical structure. The segments can be of one of following type data segment,index segment,temporary segment,rollback segment.
Tablespace: Each database is logically divided into one or more tablespaces. One or more datafiles are explicitly created for each tablespace to physically store the data of all logical structures in a tablespace. The combined size of the datafiles in a tablespace is the total storage capacity of the tablespace.
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