The Thinking Person’s Guide to Data Warehouse Design

The Thinking Person’s Guide to Data Warehouse

Design

Robin SchumacherDirector of Product Strategy

www.enterprisedb.com


Building a Logical DesignTransitioning to the PhysicalMonitoring for Success


Building a Logical Design


Why Care About

Design…?



What is the key component for success?

In other words, what you do with your PostgreSQL Server– in terms of physical design, schema design, and

performance design – will be the biggest factor on whether a BI system hits the mark…

* Philip Russom, “Next Generation Data Warehouse Platforms”, TDWI, 2009.

*

The #1 Cause of Database

Downtime…?

Bad Design…


Source: Oracle Corporation

Your Database


First Recommendation – Use a Modeling Tool

*


A logical design for OLTP Isn’t For Data Warehouse


Simple reporting databases

OLTP Database Read Shard OneReporting Database

Application Servers

End Users

ETL

Just use the same design on a different box…

Replication

Data Warehouse Horror Story

Number One…



The logical design for analytics/data warehousing


• Datatypes are more generally defined, not directed toward a database engine. Still choose carefully

• Entities aren’t designed for performance necessarily• Redundancy is avoided, but simplicity is still a goal• Bottom line: you want to make sure your data is

correctly represented and is easily understood (new class of user today)


Manual horizontal partitioningModeling technique to overcome large data volumes


Manual Vertical Partitioning


Pro’s/con’s to manual partitioning

• More tables to manage• More referential integrity to

manage• More indexes to manage• Joins oftentimes needed to

accomplish query requests• Oftentimes, a redesign is needed

because the rows / columns you thought you’d be accessing together change; it’s hard to predict ad-hoc query traffic

• Less I/O if design holds up• Easy to prune obsolete data• Possibly less object contention

Pro’s

Con’s

Use a modeling toolDon’t use 3rd normal formManual partition but…Let the DB do the heavy lifting


Transitioning to a Physical Design



SQL or NoSQL…?

Row or Column database…?

How to scale…?

Should I worry about High availability…?

Index or no…?

How should I partition my data…?

Is sharding a good idea…?


What technologies you should be looking at


*


• Whether you choose to go NoSQL, Shard, use MPP databases, or something similar, divide & conquer is your best friend

• You can scale-up and divide & conquer to a point, but you will hit disk, memory, or other limitations

• Scaling up and out is the best future proof methodology


Divide & Conquer via Sharding


Row or Column-Based Database?A column-oriented architecture looks the same on the surface, but stores data differently than legacy/row-based databases…


Yes, Row-based database! Yes, Column-based database!• Will need most columns in a table for query

• Only need subset of columns for query

• Will be doing lots of single inserts/deletes • Need very fast loads; little DML

• Small-medium data • Medium-very large data

• Know exactly what to index; won’t change • Very dynamic; query patterns change

Row or Column-Based Database…?


Example: Column DB vs. “Leading” row DBInfiniDB takes up 22% less space InfiniDB loaded data 22% faster

InfiniDB total query times were 65% less InfiniDB average query times were 59% less

Notice not only are

the queries faster, but also more

predictable

* Tests run on standalone machine: 16 CPU, 16GB RAM, CentOS 5.4 with 2TB of raw data


Hybrid Row / Column Databases

Some vendors now give you a choice of table format – row or column – based on expected access patterns.


What about NoSQL options?

• Standard model is not relational• Typically don’t use SQL to access the data• Take up more space than column databases• Lack special optimizers / features to reduce I/O


What about NoSQL options?

• Really are row-oriented architectures that store data in ‘column families, which are expected to be accessed together (remember logical vertical partitioning?) Individual columns cannot be accessed independently

• Will be faster with individual insert and delete operations• Will normally be faster with single row requests• Will lag in typical analytic / data warehouse use cases


PostgreSQL Specific - Partitioning

• Main goal: reduce I/O via partitioning• Partitioning in PostgreSQL is somewhat more

laborious than other RDBMS’s• Consider when table size exceeds memory capacity• Partitioning key is ‘key’ for many reasons• Have seen > 90% response time reductions when

done right • Partitioning also assists in more efficient data

pruning activities than typical DELETE operations


PostgreSQL Specific - GridSQL

• One option for divide-and-conquer strategy• Does have limitations with respect to PostgreSQL

feature and syntax support• One customer of EnterpriseDB is running well with

8TB


What About Indexing? • If query patterns are known and predictable, and data is

relatively static, then indexing isn’t that difficult• If the situation is a very ad-hoc environment, indexing

becomes more difficult. Must analyze SQL traffic and index the best you can

• Over-indexing a table that is frequently loaded / refreshed / updated can severely impact load and DML performance. Test dropping and re-creating indexes vs. doing in-place loads and DML. Realize, though, any queries will be impacted from dropped indexes

• Remember that a benefit of (most) column databases is that they do not need or use indexes


Optimizing for Data Loads

• The two biggest killers of load performance are (1) very wide tables for row-based tables; (2) many indexes on a table / foreign keys;

• Column-based tables typically load faster than row-based tables with load utilities, however they will experience slower insert/delete rates than row-based tables

• Move the data as close to the database as possible; avoid having applications on remote machines do data manipulations and send data across the wire a row at a time – perhaps the worst way to load data

• Oftentimes good to create staging tables then use procedural language to do data modifications and/or create flat files for high speed loaders

• PostgreSQL COPY much faster than INSERT; EnterpriseDB’s EDB*Loader faster than COPY


Optimizing for Data Loads

• Increasing maintenance_work_mem to a large values (e.g. > 1GB) helps speed index and foreign key constraint creations

• Turning autovacuum off can help speed load operations • Turning off synchronous commit (synchronous_commit )can

help improve load efficiency, but utilize generally only for all-or-nothing use cases

• Minimizing checkpoint I/O is a good idea (checkpoint_segments to 100-200 and checkpoint_timeout to a higher value such as 1 hour or so).

Monitoring and Tuning the Design


1. Bottleneck Analysis

2. Workload Analysis

3. Ratio Analysis



Bottleneck Analysis

• The focus of this methodology is the answer to the question “what am I waiting on?”

• With general PostgreSQL, unfortunately, it can be difficult to determine latency in the database server

• Lock contention rarely an issue in data warehouses• Can use EnterpriseDB’s wait interface• Problems found in bottleneck analysis translate into better

lock handling in the app, partitioning improvements, better indexing, or storage engine replacement


Workload Analysis

• The focus of this methodology is the answer to three questions: (1) Who’s logged on?; (2) What are they doing?; (3) How is my machine handing it?

• Monitor active and inactive sessions. Keep in mind idle connections do take up resources

• I/O and ‘hot objects’ a key area of analysis• Key focus should be on SQL statement monitoring and

collection; something that goes beyond standard pre-production EXPLAIN analysis


The Pain of Slow SQL


Data Warehouse Horror Story

Number Two…



Workload Analysis

• SQL analysis basically becomes bottleneck analysis, because you’re asking where your SQL statement is spending its time

• Once you have collected and identified your ‘top SQL’, the next step is to do tracing and interrogation into each SQL statement to understand its execution

• Historical analysis is important too; a query that ran fine with 5 million rows may tank with 50 million or with more concurrent users

• Design changes usually involve data file striping, indexing, partitioning, or parallel processing additions


Workload Analysis

• The pgstatspack utility available to the community can provide some statistics for workload analysis

• The pgfouine log analyzer tool can help you identify bad SQL• EnterpriseDB packages a utility that duplicates Oracle Automatic

Workload Repository (AWR) reports, and shows hot objects, top wait events, and much more

• A new SQL Profiler utility will be available from EnterpriseDB in the first half of 2011 that will help in tracing and analyzing SQL statement execution


Ratio Analysis

• Least useful of all the performance analysis methods• May be OK to get a general rule of thumb as to how various

resources are being used• Do not be misled by ratios; for example, a high cache hit ratio

is sometimes meaningless. Databases can be brought to their knees by excessive logical I/O

• Design changes from ratios typically include the altering of configuration parameters and sometimes indexing

Conclusions



Ratio Analysis

• Design is the #1 contributor to the overall performance and availability of a system

• With PostgreSQL, you have greater flexibility and opportunity than ever before to build well-designed data warehouses

• With PostgreSQL, you now have more options and features available than ever before

• The above translates into you being able to design data warehouses that can be future proofed: they can run as fast as you’d like (hopefully) and store as much data as you need (ditto)

The Thinking Person’s Guide to Data Warehouse

Design

Thanks…!


Documents

The Thinking Person’s Guide to Data Warehouse Design