A Case of Factless fact

 

Challenge

http://www.kimballgroup.com/1996/09/factless-fact-tables/

When we  group by Doctor to get the total amount and Total patient count which are under  the doctor the total amount is correct because there is a entry in fact table for this. Consider the case of total patient under doctor that is incorrect because not all patients have a bill amount. For this we need to design a factless fact table  

Factless  facts  are  of  two  types --  EVENT TRACKING  and COVERAGE  a event tacking only says who took part in the event while a coverage allows you to answer who did not take part  

Answer is  Simple -- Create a Factless fact table which has entry for each doctor and patient . Sum up the values for Provider you will get number of patients . Then sum up the billing fact details based on provider . Then join both facts based on provider keys and then join them with dimension.

The reason for joining with dimensions later is size of the tables for joining would increase and it would require additional hash joins. Hash joins are CPU intensive and will slow down your system also consider your dimension is 1GB with 50000 doctors and 200 million records for fact you are repeating the doctor name for each of 200 million records and doctor name is 100 KB field then you are adding that much size so your temp size the amount of memory for PGA everything will increase this will cause a slowdown.