Type II dimension has been popularized by R Kimball. it has become so popular that in any interview related to data warehouse, the interviewer will surely ask to explain the concepts. And chances are that if you don’t know; they will laugh at your ignorance and reject you.
Here’s your chance to laugh at them..
If you read this article, probably you will end up knowing something more than them. This is not because you will find the definition of type II dimension, but for an entirely different reason.
To be continued…
To clearly explain the pitfalls of Type II dimension, let’s take an example. In the example, there are three tables, that is, DIM_Instrument, FACT_Trade, and FACT_Settlement. Each of the tables contains data as shown below:
DIM_Instrument table
In the Dim_Instrument table, the property of instrument IBM changes from X to Y. So to maintain type II dimensions, a new entry is added in the table by updating the status of current entry to obsolete (denoted by ‘O’) and adding date in the TODT column as well. In the new entry, the ToDT column is NULL and the status is current (denoted by ‘C’).
FACT_Trade table
The trade table contains information about just one trade and that was executed on April 29th, 2011, which means it was processed for InstrumentKey ‘1’ as Instrumentkey ‘2’ did not exist on April 29th, 2011.
FACT_Settlement table
Generally, it takes three days for a trade to settle. So the trade that was executed on 29th April 2011, got settled only on 2-MAY-2011. During this period the property of instrument ‘IBM’ changed on May 1st 2011, a new entry was made in the DIM_Instrument table for Instrument IBM, which incremented the instrumentKey to 2.
Now in the settlement table, the instrument key against the same trade is different, which can cause the issues/concerns that are described using various scenarios.
Scenario 1:
Get data for each trade and settlement for the current instrument, using the following query:
SELECT T.INSTRUMENTKEY, T.TradeDate as Sec_Date
From
DIM_INSTRUMENT I, FACT_TRADE T
WHERE
T.InstrumentKey=I.InstrumentKey
and I.Status=’C’
UNION ALL
SELECT S.INSTRUMENTKEY, S.SettlementDt as Sec_Date
From
DIM_INSTRUMENT I, FACT_SETTLEMENT S
WHERE
S.InstrumentKey=I.InstrumentKey
and I.Status=’C';
The output does not show any data from Fact_Trade table. Because the current record in Dim_instrument is withInstrumentKey as ’2′ and the Fact_Trade table does not contain InstrumentKey ’2′.
It happens because the instrumentkey is changed between trade execution and trade settlement. Though, the settlement is done against the trade processed on April 29th, 2011 but with the change in InstrumentKey in between, there is no way to retrieve it.
Scenario 2:
Rather than querying data for current instrument, get data for all the instrument records. You can do so by using the following query:
SELECT T.INSTRUMENTKEY, T.TradeDate as Sec_Date
From
DIM_INSTRUMENT I, FACT_TRADE T
WHERE
T.InstrumentKey=I.InstrumentKey
UNION ALL
SELECT S.INSTRUMENTKEY, S.SettlementDt as Sec_Date
From
DIM_INSTRUMENT I, FACT_SETTLEMENT S
WHERE
S.InstrumentKey=I.InstrumentKey
The above query returns the following output:
If you analyze the output closely, it returns the trade information for InstrumentKey ’1′ but no settlement information forInstrumentKey ’1′. Similarly for InstrumentKey ’2′ there is no trade information but the settlement information. This output can be best viewed as the following:
For an instrument when trade exists, settlement does not exist and vice versa.
Scenario 3:
Maintain a relationship between two fact tables by maintaining a common key (i.e. TradeID) and join both the tables based on this common key. In this case, the TradeID uniquely identifies the trade and settlement data that refer to the same trade.
But this scenario results in fact-to-fact relationship, which is not recommended because of the following reasons:
Fact tables being the largest tables adversely impact the performance of the join.
It is not a practical scenario when multiple fact tables need to be joined because doing so complicates the design and is not always possible to achieve.
All the above scenarios highlight various pitfalls of Type II dimension. All the scenarios and examples used throughout this article are purely for demonstrating the highlighted pitfalls.