Data hierarchy in Oracle
• Data hierarchy in Oracle is specific (and
demented!) – see figure 3.1
• Normally, unit of storage is file…
– Managed by OS
– Can grow / shrink
– Physical sections logically connected by OS
• In Oracle a DB = one or more tablespaces
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–
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One or more objects, indexes, clusters…
Fixed size – gets full (Extended by DBA)
Many admin functions at that level
One file can support only ONE tablespace but reverse
not true
Terminology
• File: physical entity for storing data on disk
• Segment: collection of extents for a
particular object
• Extent: unit of disk storage made up of
contiguous blocks
• Blocks: sizing of smallest piece of
database data decided upon when db
created
• (Block – OS)
Create a tablespace
• Create tablespace FREDDY datafile
‘path\freddy.dbf’ size 1000K
Default storage (initial 10K next 10K
Minextent 1 maxextents 100
Pctincrease 0)
Permanent;
10 10 10
10 20 30
10
40
10
50
10
60
10
70
10
80
10
90
10
100
10 10 12
10 20 32
14.4
46.4
17.2
63.6
20.7
84.3
25
109.3
30
139.3
36
175.3
42.3
218.5
About tablespaces
• Kind of partitioning
– Easier to deal with small ones
– Inc. monitor perf.
– Oracle very strong at TS level
• Optimal seems to be around 2GB
• Strategy = isolate aged data into one
tablespace => can be made read-only
• Should have same size extents
– Contiguous blocks
Reasoning behind TS creation
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Business relevance
Size / type of objects
Type of activity
Volume of activity
Backup
Transportability
Time-based changes in activity
P 148
Reasoning behind TS creation
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•
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Business relevance e.g. modular design / keep data together
Size / type of objects Object of similar sizes – reuse of space
Type of activity e.g. index VS table / full scan VS rowID
Volume of activity Low I/O VS high I/O objects
Backup TS is smallest unit for Bcup and restore
Transportability Cloning copy is done by TS as well
Time-based changes in activity
Eg: DW versus OLTP systems
Block size
• Smallest unit of oracle DB
• Critical perf. Factor as inefficient space use
leads to increase I/O activity
• Block overhead
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–
–
Integrity section (90 bytes)
Runtime overhead (136 bytes)
Approx. 256 b
Transactional header (24 bytes and growing)
Directory section (Pointers…)
And then………..data
}
• Big is beautiful? Percentage of overhead
decreases when block size increases, but no
right answer here
2 K, 4K or 16K?
• OLTP prefer smaller – small granularity increase
likelihood that data is in buffer (indexed access)
• DW or DSS prefer larger – physically close data
used together + less movement
• In practice constraint on memory will dictate
• Buffer size + now split in 3 areas:
– KEEP pool
– RECYCLE pool
– DEFAULT pool
Then, there was a table
• Tables = one physical space on disk
[segment] – same for any object
• Each segment = space set out in
tablespace [initial extent]
• When table grows allocate next extent –
set of adjacent blocks
• Until tablespace is full
• Look again at figure 20.3
What a segment looks like
• Each segment contains a header block
• Space directory for other blocks
• Properly sized tables = few extents (low
fragmentation)
• Additional extents are never lost until drop
or delete commands are used
• Can also adjust pctfree value (to fill a Null
value column)
Sizing extents
• Try to match the max capacity of your storage
device to limit I/O operations
– eg: minimal read size of OS (64K?)
• For single objects, It is beneficial to keep extents
of same size (deletion)
• Try to limit the number of extents below 50
(measure of the growth of the object)
• If too many, rebuild object with larger size
extents
Roll back segments
• Special case of segment to hold the before
image of DB while transactions take place
• Should be stored in separate TS because
specific behaviour:
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Cyclical rather than random access pattern
Set of segments that gets written and written over
Then written to disk
Best used when “Hot”
• RB Segments grow to be big enough to hold
complete transacs (cannot span segments)
Oracle “space allocation”
• DBA allocates space for table after 2
days of hard thinking
• Oracle ignores it blatantly as follows:
Table name
initial size
next extent
oracle sizing next extent
Smalltab
Mediumtab
7K
103K
7K
103K
20K
120K
Total size after growth smalltab = 40K instead of 14K
mediumtab = 206K instead of 240K
20K
120K
Creating and using tables
• Table creation with CREATE TABLE
statement
• Syntax:
Create table NAME (
field1
datatype special constraint
field2
datatype etc…
);
• Special constraint: e.g. NOT NULL or
PRIMARY KEY or CHECK
Constraints
• Candidate key:
Create table fred (
name
varchar2(10) NOT NULL
firstname
varchar2(10) NOT NULL
age
number
Constraint FRED_UQ unique (name, firstname)
);
Primary Key
Create table fred (
name
varchar2(10)
firstname
varchar2(10)
age
number
Constraint FRED_PK Primary Key (name,
firstname)
);
• Same apart: always NOT NULL + can only have
one of them in a table
• If only one field just list primary key as special
constraint
Foreign key – aka referential
integrity constraint
Create table fred (
ID
number NOT NULL
name
varchar2(10)
firstnamevarchar2(10)
age
number
Constraint FRED_PK Primary Key (ID), foreign
key (age) references OTHERTAB(age)
);
• Use ON DELETE CASCADE to maintain
constraint
• Can use DEFERRAL mode
Check constraint
Create table fred (
ID
number NOT NULL
name
varchar2(10)
firstname
varchar2(10)
age
number CHECK (age
between 18 and 65)
);
Deleting data
• Drop table statement – remove table
• Or truncate table statement – empty it
• Warning: neither can be rolled back
• Otherwise:
Alter table NAME Add (…)
Alter table NAME modify (…)
drop column fieldname
• See rules on handout
Now what? Clusters
• To store different tables in physically close
areas on disk
• Create a cluster to store both objects
• Cluster key is any useful field that links the
tables
• Can keep name of field or pick new one
– e.g. Create cluster fred (name datatype)
– then assign tables: create table…cluster fred
(name)
What this cluster looks like
12 Cork
fred adam
13 Midleton Bob Fitz
14 Bally… Brian O’reilly
Table 1
Table 2
Cluster Key
French
Irish
English
Finally…
• Users can have different logical views on a
database
• These views are called Schemas
• They allow for a re-organisation of
physical DB objects for each users
• Each schema can contain elements from
different tablespaces
• Has limited size allocated to each user
Summary: storage in Oracle
• Keywords: tablespace / datafiles / database
objects / segments / extents
• When you create an object:
– Specify size / tablespace info = storage clause
– Otherwise: default values
• Pctincrease should be used with care (affects reusability of reclaimed space – i.e. extents)
• Wise use of tablespace eases admin (like
partitioning)
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sizing follows sizing of device – e.g. 2GB
Many utilities to work with tablespaces
Isolate processes – e.g. a report running inside a TS
Also for archiving (move to read only – huge perf. gains)
I/O Performance issues
• Extent sizing a key
• Oracle has 2 ways to read data:
– By rowID – typically thru an index
– Full table scan
• In second case, oracle reads several blocks at a
time
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Set with DB_FILE_MULTIBLOCK_READ_COUNT
Limited only by I/O device buffer capacity
E.g. block = 4K, I/O buffer = 64K => ?????
Extent size should be multiple of 64K
example
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Table with 10 extents 64 K each
I/O device buffer = 64K => 10 reads
If extent = 640K => ???? Reads
If 8 extents 80K each => ??? Reads
– Reads cannot span extents => 16 reads or an
increase of 60% in I/O operations
• Cl: either use extents much larger than
buffer or use multiple of buffer size
And for roll back segments
• Should be kept small enough
• Current wisdom is to ensure TS for rollback is
normally 50% free
• Not always possible to use only one RB size
– For different types of processes – e.g. TPS
– Use two different TB to isolate activites
– Easier trouble shooting
• Discarded rollback segments go into the log file
Data utilities
• ORACLE is king of data handling
• Export: to transfer data between DBs
– Extract both table structure and data content
into dump file
• Import: corresponding facility
• SQL*loader automatic import from a
variety of file formats into DB files
– Needs a control file
Control files: using SQLloader
• Data tranfers in and out of DB can be automated
using the loader
– Create a data file with the data(!)
– Create a control file to guide the operation
• Load creates two files
– Log file
– “bad transactions” file
• Also a discard file if control file has selection
criteria in it
Querying the Oracle DB
• 2 main methods:
– Full scan – TABLE ACCESS FULL
– Direct access – TABLE ACCESS BY ROWID
• When spelling out queries – drop HINT
using syntax: /*+ HINT */
• This invokes the OPTIMISER
– RBO
– CBO
Example:
• Accessing the worker field in the worker table:
• By default full scan (+ when no where
statement)
• else
Select /*+ FULL(worker)*/
From worker
Where lodging = ‘Rose Hill’
Select /*+ ROWID(worker)*/
………
• For low hit ratio
• need index or rowid value
Effect of Primary key constraints
• No index by default
• When primary key added = > index created
• Name given will be same as name of primary
key (e.g. TABLE_PK)
• PK indexes are of UNIQUE kind
• Other non-unique indexes can be created on
other fields
• CREATE INDEX name ON table(field)
TABLESPACE indexes
Using the index
• Query with “where” clause on PK
• First the PK index is accessed in INDEX
UNIQUE SCAN mode
• Matching Rowid is retrieved and returned
• Second: access table with ROWID as argument
in TABLE ACCESS BY ROWID mode
• When value sought is in index, no need to read
table at all
Index Range Scan
• If query based on range of values / non-unique
index
– E.g. field like’M%’
• More than one value returned => IRS
• Less efficient
• Especially if values are not in index => IRS
followed by table rowid access
• Another reason why index creation important
Multiple index scans
• When more than one value in WHERE
clause
• Two indexes can be used to produce two
scans of range values
• Then concatenation of scans
• Result of the query
• Broadly similar to cross-reference of
bitmap indexes
Database creation
• Using a script:
Connect internal as sysdba;
create database NAME….;
• Using the OEM GUI
– Parameters stored in a small DB
– Access to other utility pgms – eg: RMAN
– Can accommodate several DBAs
• Key decisions on sizing
– Eg: assign a block size – once and for all
Opening and closing the DB
Startup open NAME;
Or
Startup mount NAME;
Alter database open;
Shutdown;
Else – use OEM menus
When DB is started
page846
1. System Global Area (SGA) created:
a) Data block buffer cache created
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Size is key for performance of DB
typically 2% of whole DB
DB_BLOCK_BUFFERS / DB_BLOCK_SIZE
Least Recently Used (LRU) method
Too small means fight for space between objects
=> increase in misses => increase in I/O
Too big = crippling for other operations
When DB is started (2)
b) Shared SQL Pool created:
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Stores DD cache: info about data structure
Stores library cache: info about SQL statements
run
LRU algorithm
Too small => no re-use and low performance
SHARED_POOL_SIZE in init.ora
Parameter files: INIT.ORA
• Initialisation file for each DB
• MYDB = > initmydb.ora
• Contains sizing of SGA and number of
background processes, DB name and
block size
• Read at startup means changes only
implemented at next startup
The control file
• Each DB also has a control file
• Contains Dbname, name and location of
files and redo logs, timestamp of DB
creation
• Can be mirrored for added security
• Identifies all files that must be opened
when Db is loaded
• Also used in DB recovery
See page 873 – add more stuff on backups
The REDO function
• Very Hot redo is located in buffer – on-line redo
• Semi-Hot redo is kept in redo log files – archived
redo
• Limit to how much can reasonably be held
• So ORACLE organises hot backups of the DB
• Redo files up to that point can then be discarded
• Needs to run in ARCHIVELOG mode
Backups
• When DB is offline
– Datafiles
– Control file
– On-line redo
– Init.ora
• Stores a complete workable DB
• Does not work when DB is running
• Not to be trusted after abort shutdown
On-line backups
• When running in ARCHIVELOG mode
• 3 distinct redo log files
– When one full moves to next one
– After 3, overwrite first one
• Archiver background process backs up each
redo file before overwriting it – to disk
• Automatic recovery from on-line backup
– When DB started all failed transactions rerun or roll
back
• Else, limited back up
Recovery manager
• Takes redo logs on by one and checks
transactions
• Warning redo files cannot be skipped in the
sequence of recovery
• Options:
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Level 0: all blocks ever used
Level 1: all blocks used since last full Bup
Level 2: only those changed last Bup
Huge impact on Bup performance
• See screen shot
Other functions
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Archiving
Locking
Logwriting
Data writing
Recovering
Job queues
System monitor
Process monitor
Summary
Oracle Instance
PMON
SMON
DBWR
SNPn
Shared Pool
LCKn
RECO
Snnn
SGA
Database Buffer Cache
Dnnn
Pnnn
Redo Log
Buffer
CKPT
LGWR
ARCH
Server
Processes
User
Processes
Oracle Database
Parameter File
Control Files
DataFiles
Redo Log Files
After Oracle documentation
Database creation – non-data
objects
• DBs contain number of compulsory objects
• Create / assign tablespaces
• System tablespace
– Data dictionary
– Names and locations of all tablespaces, indexes
and clusters of DB
– Files name / location (multiple instances)
– Rollback segments
• Data tablespaces are separate