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CS 522 - Database Administration

Storage Structure and Manage

Tables

Dr. Dongming Liang

http://class.svuca.edu/~d.liang/(dongming.liang@svuca.edu)

Silicon Valley University

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Agenda

Segment types

Control block space usage

Manage table data

Structure of a row

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Overview

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Segments

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Types of Segments - Table

The most commonly used formof storing user data (Heap table)

DBA has a very limited controlover the distribution of rows

Rows can be stored in any order

depending on the activity on thetable

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Types of Segments Table Partition

Enable the building of scalableapplication

A partitioned table has one or

more partitionsRows can be partitioned using Range partitioning

Hash partitioning, or

List partitioning

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Types of Segments - Cluster

Provides an optional method tostore table data

A cluster is made up of groupof tables that share the samedata blocks

Share common columns andare often used together

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Types of Segments - Index

All the entries for aparticular index are storedwith one index segment.

If a table has three indexes,three index segments are

used

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Types of Segments - IOT

Index-Organized Table (IOT): datais stored within the index based onthe key value.

An IOT does not need a tablelookup, because all the data can beretrieved directly from the index tree

Provides fast key-based access to

table data For queries involving exact matches

and range searches

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Types of Segments Index Partitions

An index can be partitioned andspread across severaltablespaces.

The primary use of apartitioned index is to minimize

contention by spreading indexI/O.

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Types of Segments Undo

Used by a transaction that is makingchanges to a database

Before changing the data or index

blocks, the old value is stored in theundo segment

This allows a user to undo changesmade

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Types of Segments Temporary

When a user executes commands suchas CREATE INDEX, SELECT DISTINCT,and SELECT GROUP BY, the Oracle

server tries to perform sorts in memoryWhen a sort needs more space than the

space available in memory, intermediate

results are written to the diskTemporary segments are used to store

these intermediate results

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Types of Segments - LOB

One or more columns in a table can beused to store large objects (LOBs) Text documents, images, videos

If the column is large, the Oracleserver stores these values in separatesegments known as LOB segments

The table contains only a locator or apointer to the location of thecorresponding LOB data

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Types of Segments - Bootstrap

Created by sql.bsq script when a database iscreatedAlso known as a cache segment

This segments helps to initialize the datadictionary cache when the database isopened by an instance

Bootstrap segment cannot be queried or

updated Does not need any maintenance by DBA

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Question

Given a table created by

Create Table T(ID number Primary Key,

name varchar2(100),phone number,

addr varchar2(50),

video CLOB);

Question:

How many segments do you have for this table?

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Extents

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Extent Allocation andDeallocation

Allocated when the segment is Created

Extended

AlteredDeallocated when the segment is Dropped

Altered

Truncated

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Used and Free Extents

Header: first block inthe file

Contiguous space

used by segment isreferred to as aused extent

Released extents areadded to the pool offree extents

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Blocks

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Database Block

Minimum unit of I/OConsists of one or more OS blocks

Set at tablespace creation

DB_BLOCK_SIZE is the default blocksize

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Database Block Contents

Header: block address,table directory, rowdirectory, etc

Data: row data

Free space: in the

middle of the block

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Manual Data Block Management

Allow you to configure data blocksmanually using parameters such as

PCTFREE

PCTUSED

FREELIST

Only method available in previousOracle versions

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Block Space Usage

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Obtain Storage Information

Data dictionary views DBA_TABLESPACES

DBA_DATA_FILES

DBA_SEGMENTS

DBA_EXTENTS

DBA_FREE_SPACE

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Example: DBA_Segments

Get # of extents and blocks allocatedto a segment

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Example: DBA_Extents

Check the extents for a given segment

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Storing User Data

Use different ways tostore user data

Regular tables

Partitioned tables Index-organized tables

Clustered tables

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Oracle Data Types

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Long and Lob

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ROWID

ROWID is a unique identifier for eachrow in the database

ROWID is not stored explicitly as a

column valueROWID can be queried along with

other columns in a table

ROWID provides the fastest means ofaccessing a row in a table

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ROWID Format

Data object number is assigned to each dataobject, such as table or index (unique withinthe database)

Relative file number is unique to each file

within a tablespaceBlock number represents block positionRow number identifies row position in a block

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ROWID Example

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Structure of a Row

Row data is stored in DBblocks as variable-lengthrecords

Row header: number ofcolumns, chaining info,and row lock status

Row data: columnlength and value

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Create a Table

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Change Storage Parameters

PCTFREE: reserve 30% of each block forupdates and allows inserts of new rows to filla maximum of 70% of each block

PCTUSED: a block becomes a candidate forrow insertion when its used space falls below50%

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Create Temporary Table

Tables retain data only for the duration of atransaction or session

DML locks are not acquired on the data

DMLs do not generate redo logs

Can create indexes, views, and triggers ontemporary tables

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Truncate a Table

Truncate Table employees;

Truncating a table deletes all rows in atable and release used space

Corresponding indexes are truncated

No undo data is generated (it is a DDL)

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Drop a Table

DROP TABLE departments

A table may be dropped if it is nolonger needed or if it is to bereorganized

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Drop a Column

Remove a column from a table

Remove the column length and data fromeach row, freeing space in the data block

Dropping a column in a large table takes aconsiderable amount of time

A checkpoint occurs every 1000 rows

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Obtain Table Information

Information about tables can beobtained by querying the datadictionary

DBA_TABLES

DBA_OBJECTS

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Example: DBA_TABLES

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Example: DBA_OBJECTS

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Summary

Segments typesCreate regular and temporary tables

Outline the structure of a row

Create/drop/truncate table