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 exampleCREATE 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_1DATAFILE 'c:\Oracle\Oradata\TSH1\tsh101.dbf' SIZE 50M;

-- Explicit dictionary extent managementCREATE TABLESPACE tsh_data_2DATAFILE 'c:\Oracle\Oradata\TSH1\tsh201.dbf' SIZE 50MEXTENT MANAGEMENT DICTIONARY;

-- Local extent management using autoallocateCREATE TABLESPACE tsh_data_3DATAFILE 'c:\Oracle\Oradata\TSH1\tsh301.dbf' SIZE 50MEXTENT MANAGEMENT LOCAL AUTOALLOCATE;

-- Local extent management using uniform extentsCREATE TABLESPACE tsh_data_4DATAFILE 'c:\Oracle\Oradata\TSH1\tsh401.dbf' SIZE 50MEXTENT MANAGEMENT LOCAL UNIFORM SIZE 1M;