PG West Thinking Persons Guide to Data Warehouse Design

Report
The Thinking
Person’s Guide to
Data Warehouse
Design
Robin Schumacher
Director of Product Strategy
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Building a Logical Design
Transitioning to the Physical
Monitoring for Success
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Building a Logical
Design
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Why Care
About
Design…?
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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.
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Your Database
The #1 Cause of
Database
Downtime…?
Bad Design…
Source: Oracle Corporation
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First Recommendation – Use a Modeling Tool
*
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A logical design for OLTP Isn’t For Data Warehouse
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Simple reporting databases
End Users
Application Servers
Reporting
Read Shard
Database
One
OLTP Database
ETL
Replication
Just use the same design
on a different box…
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Data Warehouse
Horror Story
Number One…
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The logical design for analytics/data warehousing
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• 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)
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Manual horizontal partitioning
Modeling technique to overcome large data volumes
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Manual Vertical Partitioning
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Pro’s/con’s to manual partitioning
Pro’s
• Less I/O if design holds up
• Easy to prune obsolete data
• Possibly less object contention
Con’s
•
•
•
•
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
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Use a modeling tool
Don’t use 3rd normal form
Manual partition but…
Let the DB do the heavy lifting
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Transitioning to a
Physical Design
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SQL or NoSQL…?
How to scale…?
Should I worry about High availability…?
Index or no…?
Is sharding a good idea…?
How should I partition my data…?
Row or Column database…?
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What technologies you should be looking at
*
* Philip Russom, “Next Generation Data Warehouse Platforms”, TDWI, 2009.
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• 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
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Divide & Conquer via Sharding
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Row or Column-Based Database?
A column-oriented architecture looks the same on the surface, but stores data
differently than legacy/row-based databases…
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Row or Column-Based Database…?
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
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Example: Column DB vs. “Leading” row DB
InfiniDB takes up 22% less space
InfiniDB total query times were 65% less
InfiniDB loaded data 22% faster
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
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Hybrid Row / Column Databases
Some vendors now give you a choice of table format –
row or column – based on expected access patterns.
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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
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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
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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
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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
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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
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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 rowbased 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
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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 allor-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).
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Monitoring and
Tuning the Design
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1. Bottleneck
Analysis
2. Workload
Analysis
3. Ratio Analysis
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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
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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 preproduction EXPLAIN analysis
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The Pain of Slow SQL
* Philip Russom, “Next Generation Data
Warehouse Platforms”, TDWI, 2009.
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Data Warehouse
Horror Story
Number Two…
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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
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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
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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
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Conclusions
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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)
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The Thinking
Person’s Guide to
Data Warehouse
Design
Thanks…!
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