Databases: Column Stores vs Row Stores
A bird’s eye view
Kameswara Venkatesh Emani
HS 699
Speaking Skills and Oral Presentation
IIT Bombay
April 1, 2014
E. K. Venkatesh, IITB
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Overview of the talk
What is a Database?
Classification of databases
Rows vs Columns - Organization of data
Advantages of Column Stores
Quiz
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What is a Database?
Wikipedia: A database is an organized collection of data.
Why organize?
To easily create, read, update and delete data
Software systems designed for this purpose are called Database
Management Systems
Examples: MySQL, Oracle, PostgreSQL, Microsoft SQL Server, SAP
HANA etc.
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Classification of Databases
Relational : Tables, rows, columns
Most widely used
NoSQL: Other than relational
Graph databases
Key Value Stores
Others
Focus of this talk: Data organization in relational databases - rows
vs columns
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Relational Database - Terminology
Collection of tables
All rows in a table have the same attributes
Example: A table EMPLOYEE has attributes EmpID, LastName,
FirstName, Salary
Columns store specific attribute values
Rows can have keys to specify constraints
Example: primary key uniquely identifies the row
“All employees shall have a unique employee ID”
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Relational Database - Example Table
Table EMPLOYEE:
EmpID
1
2
3
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LastName
Smith
Jones
Johnson
FirstName
Joe
Mary
Cathy
Column Store vs Row Store
Salary
40000
50000
44000
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Data Organization - Rows vs Columns
Column Store
Row store
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Column Store vs Row Store - Example
EmpID
1
2
3
LastName
Smith
Jones
Johnson
Row Store
1,Smith,Joe,40000;
2,Jones,Mary,50000;
3,Johnson,Cathy,44000;
E. K. Venkatesh, IITB
FirstName
Joe
Mary
Cathy
Salary
40000
50000
44000
Column Store
1,2,3;
Smith,Jones,Johnson;
Joe,Mary,Cathy;
40000,50000,44000;
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Column Stores - Advantages
Read less data
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Column Stores - Advantages
Read less data
Example: What is the average salary of an employee?
Row Store
1,Smith,Joe,40000;
2,Jones,Mary,50000;
3,Johnson,Cathy,44000;
Column Store
1,2,3;
Smith,Jones,Johnson;
Joe,Mary,Cathy;
40000,50000,44000;
Row organization
Read all data
From each row, obtain salary
Find average
Column Organization
Read salary data
Find average
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Column Stores - Advantages
Compression - Run length, Bit vector, Dictionary, Heavy-weight schemes
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Column Store vs Row Store
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Column Stores - Advantages
Compression - Run length, Bit vector, Dictionary, Heavy-weight schemes
Run Length encoding
Bit-vector encoding
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Summary
Databases are organized collections of data
There are various kinds of databases
Relational databases are the most widely used
Data in relational databases can be organized as rows or columns
Column stores allow
only the required data to be read
good data compression
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Quiz
When might row stores be preferable over column stores?
Hint: Think data read vs data write
Data compression in column stores is better than in row stores.
Why?
Hint: Think similarity of adjacent data items
Compression scheme A achieves 50% compression. Scheme B
achieves 30% compression. However, Mr. DB Admin chooses
scheme B over scheme A. What could be the reasons?
Hint: Think decompression
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References
http://www.vldb2005.org/program/slides/thu/s553- stonebraker.ppt
Stavros Harizopoulos et. al, Tutorial on Column-Oriented Database Systems, VLDB, 2009.
Wikipedia - The Free Encyclopedia http://en.wikipedia.org
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Thank You!
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