How much memory uses a null value in a table?

[jmvilaplanap]

Hi all,

In my models, normally I create a Fact Table with several facts, concatenating individual fact tables (later is easy to use, using set analysis expressions to filter the data set)

Then finally I have a Fact Table with a lot of fields with null values for example:

DataSet	Dim1 - ID	Dim2 - ID	Dim3 - ID	Value1	Value2	Value3
1	1	-	-	10	-	-
1	2	-	-	12	-	-
2	-	1	-	-	34	-
2	-	2	-	-	33	-
3	-	-	1	-	-	21
3	-	-	2	-	-	53

From time to time, some datasets have shared dimensions but I still have a lot of nulls. For example, in a model I have a Fact Table with 8 distinct datasets, 1.5M rows and 60 Fields (between dimension keys and values).

There are better way to do this? Is this efficient?

Thanks

[Oleg_Troyansky]

Hi Jose,

I think you might be using the term "snowflake" loosely... Snowflake, strictly speaking, requires a single Fact and a number of Dimensions, when some of the Dimensions are linked to other Dimensions and not to the Star.

For example: Fact table contains Sales data. Dimensions: Customer, Product, Supplier, SalesPerson.

In a Star model, all 4 dimensions should be linked to the Fact. In a Snowflake, Supplier might be linked to Products and SalesPerson might be linked to Customer. Both models are valid. If the original data is organized closer to the latter option, then you need to add some data modelling logic to bring it to the Star format. That's the extra effort that may be necessary or may be unnecessary - depending on your preference.

What you described as a "snowflake" sounds like an Entity-Relationship model that's coming straight from a transnational system. That's not what we usually call "snowflake".

These different models are described very well in my book QlikView Your Business - check it out!..

cheers,

Oleg Troyansky

[jmvilaplanap]

Thanks Oleg

You're right I was using the term "snowflake" in a non correct way.

I was speaking about models with more than a Fact Table and linked tables in between.

By the way, this book looks good, have several topic I'm interested in.

Regards