Star schema my sql

Building and Optimizing Data
Warehouse "Star Schemas"
with MySQL

Bert Scalzo, Ph.D.

Bert.Scalzo@Quest.com

About the Author
 Oracle DBA for 20+ years, versions 4 through 10g
 Been doing MySQL work for past year (4.x and 5.x)
 Worked for Oracle Education & Consulting
 Holds several Oracle Masters (DBA & CASE)
 BS, MS, PhD in Computer Science and also an MBA
 LOMA insurance industry designations: FLMI and ACS
 Books
– The TOAD Handbook (Feb 2003)
– Oracle DBA Guide to Data Warehousing and Star Schemas (Jun 2003)
– TOAD Pocket Reference 2nd Edition (May 2005)
 Articles
– Oracle Magazine
– Oracle Technology Network (OTN)
– Oracle Informant
– PC Week (now E-Magazine)
– Linux Journal
– www.linux.com
– www.quest-pipelines.com

Star Schema Design

“Star schema” approach to dimensional data
modeling was pioneered by Ralph Kimball

Dimensions: smaller, de-normalized tables containing
business descriptive columns that users use to query

Facts: very large tables with primary keys formed from
the concatenation of related dimension table foreign key
columns, and also possessing numerically additive, non-
key columns used for calculations during user queries

108th – 1010th

103rd – 105th

The Ad-Hoc Challenge

How much data would a data miner mine,
if a data miner could mine data?

Dimensions: generally queried selectively to find lookup
value matches that are used to query against the fact table

Facts: must be selectively queried, since they generally
have hundreds of millions to billions of rows – even full
table scans utilizing parallel are too big for most systems

Business Intelligence (BI) tools generally offer end-users the
ability to perform projections of group operations on columns
from facts using restrictions on columns from dimensions …

Hardware Not Compensate

Often, people have expectation that using
expensive hardware is only way to obtain
optimal performance for a data warehouse

•CPU •Disk
•SMP •15,000 RPM
•MPP •RAID (EMC)

•OS •MySQL
•UNIX •4.x / 5.x
•64-bit •64-bit

DB Design Paramount

In reality, the database design is the key
factor to optimal query performance for a
data warehouse built as a “Star Schema”

There are certain minimum hardware and
software requirements that once met, play a
very subordinate role to tuning the database

Golden Rule: get the basic database
design and query explain plan correct

Key Tuning Requirements

1. MySQL 5.x

2. MySQL.ini

3. Table Design

4. Index Design

5. Data Loading Architecture

6. Analyze Table

7. Query Style

8. Explain plan

1. MySQL 5.X (help on the way)

•Index Merge Explain
•Prior to 5.x, only one index used per referenced table
•This radically effects both index design and explain plans
•Rename Table for MERGE fixed
•With 4.x, some scenarios could cause table corruption
•New ``greedy search'' optimizer that can significantly reduce the
time spent on query optimization for some many-table joins

•Views
•Useful for pre-canning/forcing query style or syntax (i.e. hints)
•Stored Procedures
•Rudimentary Triggers
•InnoDB
•Compact Record Format
•Fast Truncate Table

2. MySQL.ini

•query_cache_size = 0 (or 13% overhead)
•sort_buffer_size >= 4MB

•bulk_insert_buffer_size >= 16MB
•key_buffer_size >= 25-50% RAM
•myisam_sort_buffer_size >= 16MB

•innodb_additional_mem_pool_size >= 4MB
•innodb_autoextend_increment >= 64MB
•innodb_buffer_pool_size >= 25-50% RAM
•innodb_file_per_table = TRUE
•innodb_log_file_size = 1/N of buffer pool
•innodb_log_buffer_size = 4-8 MB

3. Table Design

SPEED vs. SPACE vs. MANAGEMENT

64 MB / Million Rows (Avg. Fact)
500 Million Rows
===================
32,000 MB (32 GB)

Primary storage engine options:
•MyISAM
•MyISAM + RAID_TYPE
•MERGE
•InnoDB

ENGINE = MyISAM
CREATE TABLE ss.pos_day (
PERIOD_ID decimal(10,0) NOT NULL default '0',
LOCATION_ID decimal(10,0) NOT NULL default '0',
PRODUCT_ID decimal(10,0) NOT NULL default '0',
SALES_UNIT decimal(10,0) NOT NULL default '0',
SALES_RETAIL decimal(10,0) NOT NULL default '0',
GROSS_PROFIT decimal(10,0) NOT NULL default '0‘
PRIMARY KEY(PRODUCT_ID, LOCATION_ID, PERIOD_ID),
ADD INDEX PERIOD(PERIOD_ID),
ADD INDEX LOCATION(LOCATION_ID),
ADD INDEX PRODUCT(PRODUCT_ID)
) ENGINE=MyISAM
PACK_KEYS
DATA_DIRECTORY=‘C:mysqldata’
INDEX_DIRECTORY=‘D:mysqldata’;
Pros:
•Non-transactional – faster, lower disk space usage, and less memory
Cons:
•2/4GB data file limit on operating systems that don't support big files
•2/4GB index file limit on operating systems that don't support big files
•One big table poses data archival and index maintenance challenges
(e.g. drop 1998 data, make 1999 read only, rebuild 2000 indexes, etc)

ENGINE = MyISAM + RAID_TYPE
) ENGINE=MyISAM
PACK_KEYS
RAID_TYPE=STRIPED;
Pros:
•Can help you to exceed the 2GB/4GB limit for the MyISAM data file
•Creates up to 255 subdirectories, each with file named table_name.myd
•Distributed IO – put each table subdirectory and file on a different disk
Cons:
•2/4GB index file limit on operating systems that don't support big files

ENGINE = MERGE
CREATE TABLE ss.pos_merge (
GROSS_PROFIT decimal(10,0) NOT NULL default '0',
INDEX PK(PRODUCT_ID, LOCATION_ID, PERIOD_ID),
INDEX PERIOD(PERIOD_ID),
INDEX LOCATION(LOCATION_ID),
INDEX PRODUCT(PRODUCT_ID)
) ENGINE=MERGE
UNION=(pos_1998,pos_1999,pos_2000) INSERT_METHOD=LAST;
Pros:
•Partitioned tables offer data archival and index maintenance options
•Distributed IO – put individual tables and indexes on different disks
Cons:
•MERGE tables use more file descriptors on database server
•MERGE key lookups are much slower on “eq_ref” searches
•Can use only identical MyISAM tables for a MERGE table

ENGINE = InnoDB
) ENGINE=InnoDB
PACK_KEYS;
Pros:
•Simple yet flexible tablespace datafile configuration
innodb_data_file_path=ibdata1:1G:autoextend:max:2G; ibdata2:1G:autoextend:max:2G
Cons:
•Uses much more disk space – typically 2.5 times as much disk space as MyISAM!!!
•Transaction Safe – not needed, consumes resources (SET AUTOCOMMIT=0)
•Foreign Keys – not needed, consumes resources (SET FOREIGN_KEY_CHECKS=0)
•Prior to MySQL 4.1.1 – no “mutliple tablespaces” feature (i.e. one table per tablespace)

Space Usage 21 Million Records

MERGE
3GB

MyISAM
3GB

InnoDB
8GB

4. Index Design

Index Design must be driven by DW users’ nature: you don’t know what
they’ll query upon, and the more successful they are data mining – the
more they’ll try (which is a actually a really good thing) …

Therefore you don’t know which dimension tables they’ll reference and
which dimension columns they will restrict upon – so:

•Fact tables should have primary keys – for data load integrity
•Fact table dimension reference (i.e. foreign key) columns should each
be individually indexed – for variable fact/dimension joins
•Dimension tables should have primary keys
•Dimension tables should be fully indexed
•MySQL 4.x – only one index per dimension will be used
•If you know that one column will always be used in
conjunction with others, create concatenated indexes
•MySQL 5.x – new index merge will use multiple indexes

Note: Make sure to build indexes based
off cardinality (i.e. leading portion most
selective), so in this case the index was
built backwards

MyISAM Key Cache Magic

1. Utilize two key caches:
•Default Key Cache – for fact table indexes
•Hot Key Cache – for dimension key indexes
command-line option:
shell> mysqld --hot_cache.key_buffer_size=16M

option file:
[mysqld]
hot_cache.key_buffer_size=16M

CACHE INDEX t1, t2, t3 IN hot_cache;

2. Pre-Load Dimension Indexes:
LOAD INDEX INTO CACHE t1, t2, t3 IGNORE LEAVES;

5. Data Loading Architecture
Archive:
•ALTER TABLE fact_table UNION=(mt2, mt3, mt4)
•DROP TABLE mt1

Load:
•TRUNCATE TABLE staging_table
•Run nightly/weekly data load into staging_table
•ALTER TABLE merge_table_4 DROP PRIMARY KEY
•ALTER TABLE merge_table_4 DROP INDEX
•INSERT INTO merge_table_4 SELECT * FROM staging_table
•ALTER TABLE merge_table_4 ADD PRIMARY KEY(…)
•ALTER TABLE merge_table_4 ADD INDEX(…)
•ANALYZE TABLE merge_table_4

6. Analyze Table

ANALYZE TABLE ss.pos_merge;

•Analyze Table statement analyzes and stores the
key distribution for a table

•MySQL uses the stored key distribution to decide
the order in which tables should be joined

• If you have a problem with incorrect index usage,
you should run ANALYZE TABLE to update table
statistics such as cardinality of keys, which can
affect the choices the optimizer makes

7. Query Style

Many Business Intelligence (BI) and Report Writing
tools offer initialization parameters or global settings
which control the style of SQL code they generate.

Options often include:
•Simple N-Way Join
•Sub-Selects
•Derived Tables
•Reporting Engine does Join operations

For now – only the first options make sense…
(see following section regarding explain plans)

8. Explain Plan

The EXPLAIN statement can be used either as a synonym for
DESCRIBE or as a way to obtain information about how the
MySQL query optimizer will execute a SELECT statement.

You can get a good indication of how good a join is by taking
the product of the values in the rows column of the
EXPLAIN output. This should tell you roughly how many
rows MySQL must examine to execute the query.

We’ll refer to this calculation as the explain plan cost – and
use this as our primary comparative measure (along with of
course the actual run time) …

Method: Derived Tables

Huge Explain Cost, and
statement ran forever!!! Cost = 1.8 x 1016th

Method: Sub-Selects

Sub-Select better than
Derived Table – but not
as good as Simple Join Cost = 2.1 x 107th

Method: Simple Joins and
Single-Column Indexes

Join better than Sub-
Select and much better
than Derived Table Cost = 7.1 x 106th

Method: Merge Table and
Single-Column Indexes

Same Explain Cost, but
statement ran 2X faster Cost = 7.1 x 106th

Multi-Column Indexes

Concatenated Index
yielded best run time Cost = 3.2 x 105th

Merge Indexes

MySQL 5.0 new Index
Merge = best run time Cost = 5.2 x 105th

Conclusion …

•MySQL can easily be used to build large and
effective “Star Schema” Data Warehouses

•MySQL Version 5.x will offer even more useful index
and join query optimizations

•MySQL can be better configured for DW use through
effective mysql.ini option settings

•Table and Index designs are paramount to success

•Query style and resulting explain plans are critical to
achieving the fastest query run times

Star schema my sql

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