practical problems of transforming data in data warehouse - data-warehouse

i need to explain the practical problems that might be encountered when transforming their transactional (and other) data from their diverse sources into the Data Warehouse. according to my knowledge this is about cleansing and scrubbing data. if anyone knows about any practical problem please help me.thanks for your help

That's a broad topic, but I'll offer a few good starting points.
For starters, think about history. If a transaction updates some data point, do you need to apply that retroactively, or do you need to remember what the value was at any given point in time. For example, suppose you have a monthly report of customers by city, and one of your customers moves. How should the DW reflect that.
Think about data acceptance. Is every input row a good input? For example, if you're dealing with web data, there are crawlers and spammers that you might not want to count the same as you count user traffic.
Think about data synchronization. Do all your inputs use the same keys? Do you know how to translate between them? Does Team A mean the same thing by "cust_id" as Team B does? A project glossary is very helpful here.
Think about localization. Are you inputs all in the same time zone? Do they all use the same calendar system? Do you need to handle unicode?
Think about reporting. Are the data you're capturing able to answer the questions people will ask of the DW? If not, how can you capture data that can?
Think about presentation. Should you be showing customers the same data you're using for internal reporting? Does finance need to see a different slice of the data than marketing?
This really only scratches the surface of the issues that come up on a major DW project. I would refer you to Ralph Kimball's assorted books on Data Warehousing for a more in depth discussion of problems and solutions. Hope this helps you get started.

You give the answer in your question.
According to my knowledge this is about cleansing and scrubbing data.
And you are correct. Cleansing data means that you have a company-wide list of clean element attributes, and a mapping that changes the unclean elements into clean elements.
Processing the data against the clean element attributes is a piece of cake compared to creating the company-wide list of clean element attributes.
You have to get people from different departments to agree on what data to warehouse, and to agree on what each element means. This is a difficult sociological problem. It's not a terribly hard technical problem.
Good luck getting your company-wide list of clean element attributes.

Related

How to integrate various data marts?

I recently joined a healthcare company and they have separate datamarts for each type of each type of diseases. Lets say I have three different DM's as follows:
HIV
HepC
Respiratory
How would I go on to integrate these into one Data-warehouse?
From what I have read, this is a Kimball Aprroach.
And I should look for similar dimensions and try to build on that.
Any other recommendations ?
Your question is too vague. Without knowing what you want to do with a data warehouse, and how the data marts are structured, it's hard to comment on how you should go about it. You might want to step back and think about two things, and explain: what do I want to do? and what do I have?
Talk with the stake holders to settle on what you they to have on a data warehouse. How do they want to use a data warehouse? Is it for internal analytics is for simple aggregation reports? If so, what kind of metrics need to be aggregated? If they are doing complex analytics, what kind of metrics do they need for it? I recommend identifying a list of "needs", and prioritize them, so you can think about what dimensions need to be delivered first.
After that, research what you have closely. What does each disease data mart have? Does it have information about disease? taxonomy? patients who have that disease? procedures done for that disease? Identify the structure of the data marts, and make a list of attributes that can be derived from them.
After that, you might have a more fruitful conversation on the methods of integration.

Core Data Model Design - Attributes vs Entities

I've been developing a very basic core data application for over a year now (Toy Collector, http://bit.ly/tocapp), and I'm looking at doing a redesign so that I can build in iCloud support. I figured while I'm doing that, I might as well update my core data model (if needed), and I'm having a heck of a time tracking down "best practices" for the following:
Currently, I have 2 entities:
Toy, Keywords
Toy has all the information about the object: Name, Year, Set, imageName, Owned, Wanted, Manufacturer, etc, (18 attributes in all)
Keywords has the normalized words to help speed up the search
My question is whether or not there is any advantage to breaking out some of the Toy attributes into their own entities. For example, I could have a manufacturer entity that stores the dozen or so manufacturers, instead of keeping that information in the Toy object. My gut tells me this could reduce the memory footprint (instead of 50,000 objects storing a manufacturer string, there would simple be 12 manufacturer strings in an entity with a relationship to the main Toy entity). Does that kind of organization really matter? Am I trying to overcomplicate things? I just feel like my entity has a lot of attributes, and I'm not sure if taking the time to break it apart into multiple entities would make a difference.
Any advice or pointers would be appreciated!
Zack
Your question is pretty broad, since it addresses the topic of database design. Let me say upfront that it is almost impossible to give you any sensible suggestions, since I would need to know a lot more about your app, use cases, etc. than it is possible through a S.O. question.
Coming to your concrete questions, I would say that you correctly identify one of advantages of splitting a table into multiple ones; actually, the advantage of doing that is not just reducing the database footprint, rather keep data redundancy to a minimum. Redundancy not only affects memory footprint but also manageability and modifiability of your data, and lack of redundancy could even cause anomalies or corruption. There is even a whole database theory topic which is known as database normalisation that addresses this king of concerns.
On the other hand, as it is always the case, redundancy can help performance, and this is actually the case when you can fetch your data through a simple query instead of multiple queries or table joins. There is a technique to improving a database performance which is known as database denormalization and is the exact opposite to normalisation. Your current scheme is fully denormalized.
Using Core Data, which is a relational object graph manager running often on top of SQLite, which is a relational database manager, you have also to take into account the fact that Core Data will automatically build your object graph and fetch into memory the data when you need it. This means that if you can take a smaller memory footprint on disk for granted, this might not be the case when it comes to RAM footprint of your query results (Core Data will "explode", so to say, at some moment your data from multiple tables into one object plus its attributes).
In your specific case, you should also possibly take into account the cost of migrating your existing user base (if the database is not read-only).
All in all, I would say that if your app does not have any database footprint issues at the moment; if you do not feel that creating new tables might be useful, e.g., in order to add new functionality, such as listing all manufacturers; and, finally, if you do not foresee tasks like renaming a manufacturer or such at some point, then maybe refactoring your database will not add much benefit. But, as I say, without knowing your app in detail and your roadmap for it, it is difficult to say anything really on spot. In any case, I hope this general considerations will help you take a decision.
EDIT:
If you want to investigate your core data performance and try to understand where the bottlenecks are, give a try to Instruments/Core Data tool (Product/Profile menu). There are a lot of things that can go bad.
On the other hand, it is really hard to help you further without having more details about the type of searches your app allows to do. One thing that is not clear to me is if your searches are slow only when they return a lot of results or they are slow even when returning a few results.
Normalizing might help performance if you only use (say, after doing a search) just one normalized entity (e.g., to display the toy name in a table). In this case all of the attributes referring to other entities would be faults (hence would not occupy memory nor take) and this might speed up things. But, if you do a search and then display the information from the other tables as well, then there might not be any advantage, quite the opposite, since the faults would have to be resolved immediately and this would produce more accesses to the database.
Also it is true that depending on how you use it, core data could not be the best way to handle your data. Have a look at this Brent Simmons' post relating his experience.

Large datasets for a project

Not sure if Stack Overflow is the right site for it, but since there are many DW developers here...
I'm going to build a data warehouse for a graduation project, and to do so I need a good dataset, and by good I mean bad :) I need a dataset which requires a lot of transformations, is contained in many files (with various or weird formatting if possible). It should also have a lot of columns so a moderately large cube can be built on it. Most of the datasets available on the internet are too simple for this. Can anyone recommend something?
Perhaps you could use US Census Data? There's lots of different kinds of data available. Maybe focus on a specific state? Your cube could allow roll ups across various political or geographic areas, or by various demographics.
http://www.census.gov/population/www/cen2010/glance/
It doesn't appear that all the data's available yet, so you can always use the 2000 census instead.

What's a succinct, useful and efficient way to store large time-series in F#?

I'm currently learning F# and I'm exploring using it to analyse financial time-series. Can anyone recommend a good data structure to store time-series data in?
F# offers a rich selection of native types and I'm looking for a some simple combination that would provide an elegant, succinct and efficient solution.
I'm looking store tick data, which consists of millions of records each with a time stamp, and several (~5-20) fields of numerical and textual data, with possible missing values.
My first thoughts are perhaps a sequence of tuples or records, but I was wondering if someone could kindly suggest something that has worked well in the real world.
EDIT:
A few extra points for clarification:
The common operations that I'm likely to require are:
Time based lookup - i.e. find the most recent data point at a given time
Time based joins
Appends
(Updates and deletes are going to be rare. )
I should make it clear I'm exploring using F# primarily as an interactive tool for research, with the ability to compile as a (really big) added bonus.
ANOTHER EDIT:
I should also have mentioned, my role/use of F# and this data is purely within research not development. The intention being that once we understand the data (and what we want to do with it) better then we can later specify tools that our developers would build. Such as data warehouses etc. at which we'd start using their data structures etc.
Although, I am concerned that our models are computationally intensive, use a lot of memory and can't always be coded in a recursive manner. So we many end up having to query out large chunks anyway.
I should also say that I've always used Matlab or R for these sorts of tasks before but I'm now interested in F# as it offers that interactive, high level flexibility for Research but the same code can be used in production.
My apologies for not giving this context information at the start (It's my first question), I can see now that it helps people form their answers.
My thanks again to everyone that's taken the time to help me.
It really sounds like your data should be stored and queried in a relational database (where is it currently stored?: loading millions of records with several fields into memory must be an expensive operation, and could leave you with stale data and difficulty persisting changes). And then you could use the F# LINQ to SQL implementation (which I believe you can find in the Power Pack) to have F# expressions translated to SQL expressions.
Here's a link from Don Syme about LINQ Support in F# Power Pack: http://blogs.msdn.com/b/dsyme/archive/2009/10/23/a-quick-refresh-on-query-support-in-the-f-power-pack.aspx
The best choice of data structure depends upon what operations you want to do on it.
The simplest would be an array of structs. This has the advantages of fast random lookup, good space efficiency for an uncompressed representation and good locality. If there is sharing between substructures (like the strings) then intern them to make sure they get shared.
Alternatives might be a seq that is loaded from disk on-demand, a singly-linked list that allows you to prepend elements quickly or a balanced binary trees that allows operations like insertion at random locations efficiently.

Free data warehousing systems--specifically, for data storage

I am building out some reporting stuff for our website (a decent sized site that gets several million pageviews a day), and am wondering if there are any good free/open source data warehousing systems out there.
Specifically, I am looking for only something to store the data--I plan to build a custom front end/UI to it so that it shows the information we care about. However, I don't want to have to build a customized database for this, and while I'm pretty sure an SQL database would not work here, I'm not sure what to use exactly. Any pointers to helpful articles would also be appreciated.
Edit: I should mention--one DB I have looked at briefly was MongoDB. It seems like it might work, but their "Use Cases" specifically mention data warehousing as "Less Well Suited": http://www.mongodb.org/display/DOCS/Use+Cases . Also, it doesn't seem to be specifically targeted towards data warehousing.
http://www.hypertable.org/ might be what you are looking for is (and I'm going by your descriptions above here) something to store large amounts of logged data with normalization. i.e. a visitor log.
Hypertable is based on google's bigTable project.
see http://code.google.com/p/hypertable/wiki/PerformanceTestAOLQueryLog for benchmarks
you lose the relational capabilities of SQL based dbs but you gain a lot in performance. you could easily use hypertable to store millions of rows per hour (hard drive space withstanding).
hope that helps
I may not understand the problem correctly -- however, if you find some time to (re)visit Kimball’s “The Data Warehouse Toolkit”, you will find that all it takes for a basic DW is a plain-vanilla SQL database, in other words you could build a decent DW with MySQL using MyISAM for the storage engine. The question is only in desired granularity of information – what you want to keep and for how long. If your reports are mostly periodic, and you implement a report storage or cache, than you don’t need to store pre-calculated aggregations (no need for cubes). In other words, Kimball star with cached reporting can provide decent performance in many cases.
You could also look at the community edition of “Pentaho BI Suite” (open source) to get a quick start with ETL, analytics and reporting -- and experiment a bit to evaluate the performance before diving into custom development.
Although this may not be what you were expecting, it may be worth considering.
Pentaho Mondrian
Open source
Uses standard relational database
MDX (think pivot table)
ETL ( via Kettle )
I use this.
In addition to Mike's answer of hypertable, you may want to take a look at Apache's Hadoop project:
http://hadoop.apache.org/
They provide a number of tools which may be useful for your application, including HBase, another implementation of the BigTable concept. I'd imagine for reporting, you might find their mapreduce implementation useful as well.
It all depends on the data and how you plan to access it. MonetDB is a column-oriented database engine from the most revolutionary team on database technologies. They just got VLDB's 10-year best paper award. The DB is open source and there are plenty of reviews online praising them.
Perhaps you should have a look at TPC and see which of their test problem datasets match best your case and work from there.
Also consider the need for concurrency, it adds a big overhead for any kind of approach and sometimes is not really required. For example, you can pre-digest some summary or index data and only have that protected for high concurrency. Profiling your data queries is the following step.
About SQL, I don't like it either but I don't think it's smart ruling out an engine just because of the front-end language.
I see a similar problem and thinking of using plain MyISAM with http://www.jitterbit.com/ as data access layer. Jitterbit (or another free tool alike) seems very nice for this sort of transformations.
Hope this helps a bit.
A lot of people just use Mysql or Postgres :)

Resources