Oracle Database Data Blocks and Extents

Oracle Database Data Blocks

At the finest level of granularity, Oracle Database data is stored in data blocks.

One data block corresponds to a specific number of bytes of physical database space on disk.

The standard block size is specified by the DB_BLOCK_SIZE initialization parameter.

A database uses and allocates free database space in Oracle Database data blocks.

Extents

The next level of logical database space is an extent. An extent is a specific number of contiguous data blocks, obtained in a single allocation, used to store a specific type of information.

Segments

Above extents, the level of logical database storage is a segment.

A segment is a set of extents allocated for a table, index, rollback segment, or for temporary use by a session, transaction, or SQL parser.

In relation to physical database structures, all extents belonging to a segment exist in the same tablespace, but they may be in different data files.

When the extents of a segment are full, Oracle Database dynamically allocates another extent for that segment.

Segments Cont’d

Data segment – stores table data

Index segment – stores index data

Temporary segment – temporary space used during SQL execution

Rollback Segment – stores undo