ROWID

More explanation at: Overview of ROWID and UROWID Datatypes

Oracle uses rowids internally for the construction of indexes. Each key in an index is associated with a rowid that points to the associated row's address for fast access. End users and application developers can also use rowids for several important functions:

• Rowids are the fastest means of accessing particular rows.
• Rowids can be used to see how a table is organized.
• Rowids are unique identifiers for rows in a given table.

There are Restricted ROWIDs and Extended ROWIDs.

Structure of a Restricted ROWID

An example ROWID is

ROWID ENAME
------------------ ----------
00000DD5.0000.0001 KRISHNAN

As shown, a restricted rowid's VARCHAR2/hexadecimal representation is in a three-piece format, block.row.file:

• The data block that contains the row (block DD5 in the example). Block numbers are relative to their datafile, not tablespace. Two rows with identical block numbers could reside in two different datafiles of the same tablespace.
• The row in the block that contains the row (rows 0, 1, 2 in the example). Row numbers of a given block always start with 0.
• The datafile that contains the row (file 1 in the example). The first datafile of every database is always 1, and file numbers are unique within a database.

Structure of an Extended ROWID

ROWID LAST_NAME
------------------ ----------
AAAAaoAATAAABrXAAA BORTINS
AAAAaoAATAAABrXAAE RUGGLES
AAAAaoAATAAABrXAAG CHEN
AAAAaoAATAAABrXAAN BLUMBERG

An extended rowid has a four-piece format, OOOOOOFFFBBBBBBRRR:

• OOOOOO: The data object number that identifies the database segment (AAAAao in the example). Schema objects in the same segment, such as a cluster of tables, have the same data object number.
• FFF: The tablespace-relative datafile number of the datafile that contains the row (file AAT in the example).
• BBBBBB: The data block that contains the row (block AAABrX in the example). Block numbers are relative to their datafile, not tablespace. Therefore, two rows with identical block numbers could reside in two different datafiles of the same tablespace.
• RRR: The row in the block.