Fan and Chasm Traps

building semantic layer
1

Hi,

This may seem like a repeat question, but I always get confused as to when does a Chasm Trap or a Fan Trap occur.

I know the basics (Fan Trap - 1>N>N and Chasm Trap - N>1>N) but i dont seem to remember by any logic as to when they occur.

So, is it like that whenever we have tables in that relationship, we have one of the two traps? Does it depend of whether we select a measure or a dimension? And from which table?

May sound like a stupid question, but just the fact that we can choose so many combinations of objects (measure from some, dimension from some, all measures, all dimensions etc) leaves me confused :slight_smile:

Smith85 (BOB member since 2009-12-10)

2

Logical Trap #1 – Chasm Traps
Many to one to many relationship
No relationship from left to right
Usually resolved with aliases

Logical Trap #2 – Fan Traps
One to many to many relationships
Also known as master-detail relationships
Trouble when aggregating on the master side
Several ways of resolving fan traps
Don’t aggregate master columns
2009 SAP BusinessObjects User Conference 51
Use contexts to provide master and detail paths

assist (BOB member since 2006-04-17)

3

Chasm traps are resolved with contexts, not aliases. Creating aliases does not remove the chasm, so we use contexts to split it into two queries.

Dave Rathbun :us: (BOB member since 2002-06-06)

4

Just to confirm you got this wrong by mistake … Chasm occurs when we have N > 1 < N not the one you mentioned.

sulabhagra :uk: (BOB member since 2007-03-05)