Question about ROLAP and MOLAP

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Question about ROLAP and MOLAP

Post  juanvg1972 on Tue May 05, 2015 12:36 pm

I want to know the differences between ROLAP and MOLAP.

I know that ROLAP is when you store the data ina relational database and MOLAP in a multidimensional database., but I have a doubt: you can create a cube structure (dimensions, hierarchies, measures, etc..) with a B.I. tool (for example SSAS of Microsoft). Whew the cube structure is created there are two options:
1) build the physical cube calculating all the necesary agregates
2) don't build the physical cube and send the SQL (a join of dimensions, hierarchies, fact tables...) to relational database where the data is.

In the 2) option there is a cube structure, not physical but logical, this is ROLAP??, I suppose 1) option is clearly MOLAP. And then HOLAP, hybrid'?, can you give me an example?

Do you have examplesof tools using ROLAP and MOLAP. I have used Pentaho Mondrian which seems to ne ROLAP (if I am not mistaken)

Thanks in advance

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Re: Question about ROLAP and MOLAP

Post  ngalemmo on Tue May 05, 2015 4:15 pm

SQL Server is an example of HOLAP. It has both a relational store and a multidimensional store and you can move between the two.

Multidimensional databases store the data in a non-relational structure (proprietary to the particular MDDB product). BI applications based solely on such a source are considered MOLAP.

BI applications based solely on a relational DB are ROLAP. It doesn't really matter what the BI tool does with it (many will build cube structures internally), it is a matter of where the tool gets it's data.
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Another question about ROLAP/MOLAP

Post  juanvg1972 on Tue May 05, 2015 4:44 pm

Thanks for your answer.

Then in a ROLAP model, yo get your data in a relational-database. I think there can be two situations in ROLAP:
1) the data of the cube (all the agregations) calculated and saved in a relational database
2) There is a cube structure defined, but the agregations are not precalculated, when a MDX ask for data, the data is retrieved on-line from the relational database.

But if the difference between ROLAP and MOLAP is only if you get the data in multidimensional database or relational database, I don't undesrstand tha caracteristic that I have read in many places:

"MOLAP takes more time than ROLAP calculatings the cube, but it takes less time resolving the query of the report because the data is precalculated and in ROLAP is no"

There are two concepts: 1) Where do yo keep the data (MDDB or RDB) 2) If you build the cube or not (calculation of all the agregates)

That's what confuse me....




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Re: Question about ROLAP and MOLAP

Post  ron.dunn on Tue May 05, 2015 10:18 pm

If you are using SSAS, the following comments apply.

Regardless of which cube technology you use, you must keep your data in a separate source so that you can rebuild the cube if necessary. I've seen too many SSAS users unable to recover from a failure, because they didn't keep the data they used to build the cube.

For MOLAP, the data is aggregated and cached in the SSAS database. A load process extracts data from the source (ie, data warehouse), moves it into facts and dimensions in the cube, then performs the calculations and aggregations defined in the cube.

For ROLAP, only the cube metadata is kept in the SSAS database, and the data is retrieved from the underlying source database with calculations and aggregations applied in real time.

MOLAP has traditionally been the solution to providing fast access for drill-down, drill-across, etc., but it does have down-time when the cube is being rebuilt.

Users of super-fast data sources, such as Microsoft's APS, often find that they can get near real-time access to their source using ROLAP, and I expect that ROLAP will increasingly replace MOLAP over time.

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Re: Question about ROLAP and MOLAP

Post  ngalemmo on Wed May 06, 2015 12:04 am

The terms describe architectures and not software solutions.

The term 'OLAP' itself is incredibly vague. I met Codd once a long time ago. He coined the term OLAP. In the conversation he stated it simply meant being able to do something on a computer in an interactive, ad-hoc fashion. He said a spreadsheet fits into that definition.

So, OLAP means you have some piece of software that lets you play with the data. How it internally works with the data and how you interact with it doesn't matter, as long as it is interactive in nature.

How you physically implement it is the architecture, hence the prefixes R, M and H were added to describe the architecture. They indicate you were using a relational database, a multidimensional database (MDDB), or a mix respectively.
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ROLAP and MOLAP

Post  juanvg1972 on Thu May 07, 2015 4:05 am

Thanks ron.dunn and ngalemno for your answers are usefull for em.

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Tools which uses ROLAP or MOLAP

Post  juanvg1972 on Thu May 07, 2015 4:10 am


Can you give me examples of tools with one or both of the architectures.

is this right?

ROLAP: Microstrategy, IBM Cognos, Business Objects, Oracle B.I.
MOLAP: Oracle’s Hyperion Essbase, Microsoft Analysis Services(also option ROLAP), SAS OLAP, Cognos PowerCubes.

An example of HOLAP?

Thanks in advance

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Re: Question about ROLAP and MOLAP

Post  ngalemmo on Thu May 07, 2015 5:33 pm

Business Objects is more HOLAP today than it used to be. It used to be relational only, but since SAP took it over, they added MDX support to deal with BW.
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