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

Block size• Smallest unit of storage in oracle DB• Critical perf. Factor as inefficient space use leads to

increase I/O activity• Block overhead

– Integrity section (90 bytes)– Runtime overhead (136 bytes)– 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

}Approx. 256 b

2 K, 4K, 8K 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 also dictate• Buffer size + now split in 3 areas:

– KEEP pool– RECYCLE pool– DEFAULT pool

Block size and row size

• Table with average row length = 2236 b (10m records)

• Block size 4k = 4006 b => one row per block

• 8 k => 3 rows per block

Opening and closing the DB

Startup open NAME;

Or

Startup mount NAME;

Alter database open;

Shutdown;

Else – use OEM menus

When DB is started

1. System Global Area (SGA) created:a) Data block buffer cache created

• Size is key for performance of DB

• fraction 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

page846

When DB created (2)

b) Shared SQL Pool created:• 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 parameters, 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

Data hierarchy in Oracle• Data hierarchy in Oracle is specific (and demented!)• 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– One or more objects, indexes, clusters…– Fixed size – gets full– Extended by DBA only– One file can support only ONE tablespace

See figure 3.1

About tablespaces

• Kind of partitioning– Except fixed size

• Extended by dba

– 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 store similar objects together

Reasoning behind TS creation

• Business relevance

• Size of objects

• Type of activity

• Volume of activity

• Backup

• Transportability

• Time-based changes in activity

Reasoning behind TS creation

• Business relevance

• Size of objects

• Type of activity

• Volume of activity

• Backup

• Transportability

• Time-based changes in activity

e.g. modular design / keep data togegther

Object of similar sizes – reuse of space

e.g. index VS table / full scan VS rowID

Low I/O VS high I/O objects

TS is smallest unit for Bcup and restore

Cloning copy is done by TS as well

DW env. Aged archived system better – access time relatedBias towards more rather than less = > compromiseAlso perf reading

P 148

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 10 10 10 10 10

1010 20 30 40 50 60 70 80 90

100

10 10 12 14.4 17.2 20.7 25 30 3642.3

10 20 32 46.4 63.6 84.3 109.3 139.3 175.3218.5

• 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• see figure 20.3

Object creation

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

• 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

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 7K 7K 20K 20KMediumtab 103K 103K 120K 120K

Total size after growth smalltab = 40K instead of 14Kmediumtab = 206K instead of 240K

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 KeyCreate table fred (

name varchar2(10)firstname varchar2(10)age numberConstraint 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 NULLname varchar2(10) firstname varchar2(10) age numberConstraint FRED_PK Primary Key (ID), foreign key (age) references OTHERTAB(age));

• Use ON DELETE CASCADE to maintain constraint

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 French13 Midleton Bob Fitz Irish14 Bally… Brian O’reilly English

Cluster Key

Table 1

Table 2

Finally…

• Users can have different logical views on a database

• These views are called…..?• They allow for a re-organisation of physical

DB objects for each users• Each ? 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 re-usability of reclaimed space – i.e. extents)

• Wise use of tablespace eases admin (like partitioning)– 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

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– 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

• 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