Oracle Database Administration
Database files
Logical database structures
Data storage structures
• Physical structures:
– Data files – disk files that contain database data
• Logical database structures include:
– Tablespaces
– Segments
– Extents
– Blocks
Data files
• Data files:
– all database data is stored there
– data files can automatically extend if required (the
AUTOEXTEND clause)
– one or more datafile form a logical unit of database
storage called a tablespace
– Oracle does not write modified data immediately,
if the database is not closed properly, datafiles can
contain incorrect data
Data files
• Data file:
– Can be regular file on a hard drive (any hard drive
in a computer) (typical case)
– Can be a raw device (on Unix systems). Oracle
writes data directly to disk with minimum
overhead)
– Can use external hard drives (can be used when
configuring Real Application Cluster)
Tablespaces
• Tablespace:
– basic logical storage unit – table data, index data is stored
in a tablespace
– consists of one or more datafile (each datafile belongs to
exactly one tablespace)
– there is no relationship between tablespaces and schemas:
• objects from one schema can exist in different tablespaces
• one tablespace can contain objects from different schemas
– to create tablespace:
CREATE TABLESPACE new_tablespace DATAFILE
'C:\...\new_tab.dbf' SIZE 10M
– to put table in a tablespace:
CREATE TABLE (id number)
TABLESPACE new_tablespace;
Tablespaces
• Each database has special tablespaces:
– SYSTEM tablespace – should only be used by
system objects
– SYSAUX tablespace – Oracle managed internal
data
– temporary tablespace – used by temporary tables
and temporary data – sorting, groupping
– undo tablespace – used to store undo data required
for transaction rollback
Tablespaces
• Each user has default tablespace and
temporary tablespace:
– CREATE USER xxx IDENTIFIED BY yyy DEFAULT
TABLESPACE zzz TEMPORARY TABLESPACE vvv;
• Default tablespace is where user objects are
created unless otherwise specified
• Temporary tablespace is where temporary
objects will be stored:
– user’s temporary table data
– temporary space required by operations like
sorting, grouping, distinct
Tablespaces
• Tablespace size is the size of all its datafiles
• Objects assigned to a tablespace store their
data in tablespace datafiles:
– single object can span multiple datafiles belonging
to the same tablespace
– when all tablespace datafiles become full Oracle:
• tries to extend a datafile created with the
AUTOEXTEND clause with size smaller than
MAXSIZE
• reports an error if no file has AUTOEXTEND clause or
files already have their MAXSIZE
Adding more space to the database
• To add more space do one of:
– add new file to existing tablespace:
• ALTER TABLESPACE name ADD DATAFILE ‘path’ SIZE 1G
AUTOEXTEND OFF
– add new tablespace:
• CREATE TABLESPACE name DATAFILE ‘path’ SIZE 1G
AUTOEXTEND ON NEXT 1G MAXSIZE 10G
– change size of existing datafile:
• ALTER DATABASE DATAFILE ‘path’ SIZE 4G
Adding more space to the database
• Adding datafile to a tablespace
Adding more space to the database
• Adding new tablespace
Adding more space to the database
• Increasing datafile size
Putting objects in a tablespace
• When creating object:
– CREATE TABLE table_name (column definitions)
TABLESPACE tablespace_name
– CREATE INDEX index_name ON table_name(column_list)
TABLESPACE tablespace_name
• When moving object:
– ALTER TABLE table_name MOVE TABLESPACE
tablespace_name
– ALTER INDEX index_name REBUILD TABLESPACE
tablespace_name
• When the table is moved all indexes become
unusable
– unusable index needs to be rebuild before it can be
used: ALTER INDEX index_name REBUILD
Putting objects in a tablespace
Temporary tablespaces
• Creating temporary tablespace requires
different syntax:
CREATE TEMPORARY TABLESPACE temp_demo
TEMPFILE 'temp01.dbf'
SIZE 5M AUTOEXTEND ON;
Information about regular data files is stored in
V$DATAFILE view
 Information about temporary files is stored in
V$TEMPFILE view

Segments
• Segment:
– is a space allocated for a specific data structure,
e.g. each table has its own data segment, each
index has index segment
– segment is always stored in a single tablespace
(possibly in multiple files from that tablespace)
– each segment is divided into extents and extents
are divided into data blocks
Segments
• Example:
– Table with 2 indexes has 1 table segment, and 2
index segments
– Each of those segments can be stored in a different
tablespace
• Information about segments is in the
USER_SEGMENTS and DBA_SEGMENTS
views
Extents
• Extent:
– is a contiguous space in a data file
– when adding data to a segment Oracle tries to fill
existing extents, if all extents are full – Oracle adds
another extent
– extents of a segment may not be contiguous on
disk – multiple extents usually mean that table data
is fragmented
Data blocks
• Data block:
– smallest unit of data allocation
– database block size is usually between 2KB –
32KB
Segments, extents and data blocks
Data blocks
• Data block:
– smallest unit of data allocation
– database block size is usually between 2KB –
32KB
Free space in a data block
• Oracle inserts multiple rows into single data
block (if row size is smaller than block size)
• Sometimes a row will occupy more than one
block:
– if a row size is larger than data block size – data
chaining
– if a row is inserted small and then updated and
there is no space left in the data block – data
migration
• Data chaining is unavoidable, Oracle tries to
prevent data migration
Data migration example
CREATE TABLE test1 (id NUMBER, text VARCHAR2(4000));
INSERT INTO test1 (id, text) VALUES (1, NULL);
INSERT INTO test1 (id, text) VALUES (2, NULL);
INSERT INTO test1 (id, text) VALUES (3, NULL);
INSERT INTO test1 (id, text) VALUES (4, NULL);
INSERT INTO test1 (id, text) VALUES (5, NULL);
-- all rows are inserted into single data block
UPDATE test1 SET text = ‘some long text......’ WHERE id IN
(1, 2, 4, 5, 6);
-- there is no space left in the row for all these rows,
-- Oracle performs data migration
Data migration
• When performing data migration Oracle:
– moves row data to another data block
– keeps pointer to the new data block in the original
data block
• Scanning multiple data blocks to get one row
decreases performance – data migration should
be prevented
• Two parameters are provided to control free
space in the block: PCTFREE and PCTUSED
PCTFREE
• Example:
– CREATE TABLE test (id NUMBER) PCTFREE 20;
• PCTFREE – amount of space in the block reserved
for future updates of the inserted rows
PCTUSED
• Example:
– CREATE TABLE test (id NUMBER) PCTFREE 20 PCTUSED
40;
• PCTUSED – once free space in the block drops
below PCTFREE, oracle uses free space in the block
only for updates until amount of used space drops
below PCTUSED
PCTUSED and PCTFREE
Extent allocation
• Extent allocation strategy is important for
performance and free space management:
– Many small extents decrease performance –
segment is fragmented
– Too large extents waste space
• Extents within a tablespace can be managed:
– LOCALLY – (recommended) information about
extents is stored in tablespace itself
– DICTIONARY – information about extents is
stored in data dictionary in SYSTEM tablespace
Extent allocation
-- Default extent management
CREATE TABLESPACE tsh_data_1
DATAFILE 'c:\Oracle\Oradata\TSH1\tsh101.dbf' SIZE 50M;
-- Explicit dictionary extent management
CREATE TABLESPACE tsh_data_2
DATAFILE 'c:\Oracle\Oradata\TSH1\tsh201.dbf' SIZE 50M
EXTENT MANAGEMENT DICTIONARY;
-- Local extent management using autoallocate
CREATE TABLESPACE tsh_data_3
DATAFILE 'c:\Oracle\Oradata\TSH1\tsh301.dbf' SIZE 50M
EXTENT MANAGEMENT LOCAL AUTOALLOCATE;
-- Local extent management using uniform extents
CREATE TABLESPACE tsh_data_4
DATAFILE 'c:\Oracle\Oradata\TSH1\tsh401.dbf' SIZE 50M
EXTENT MANAGEMENT LOCAL UNIFORM SIZE 1M;