Oracle logical structures

Oracle logical structures

Oracle logical structures can be summarised by the following:
Schema. Tablespace. Datablock. Redo log.


  • collection of objects (table, view, etc)
  • owned by db user
  • name is the same as db user
  • mixed up (user is often used interchangeably with schema)
  • access – user has access to all objects in schema
  • name – user only has access to schema of the same name


  • Logical storage units – oracle divided into one or more logical storage units
  • objects – tablespace stores related database objects
  • access – tablespace can be online (accessible) or offline (not accessible)
  • normally online so users can access data


Tablespace > Segment > Extent > Data block

  • tablespace made of one or more segments.
  • segments made of one or more extents.
  • extents made of one or more data blocks.
  • data block id the smallest logical unit oracle can use or allocate.
  • block size can be specified at db creation.
  • block size should be a multiple of o/s block size.

Redo Log

  • 1 or more redo logs per database
  • record for recovery – records all actions in case of failure so that data can be recovered.
  • duplicate disks – redo logs stored on multiple disks incase og failure due to redo log itsself.
  • crash changes – if data cant be written to datafile due to failure then changes can be recovered from redo log so that no data is lost.

see: Oracle process overview

sources: oracle database documentation

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