Fan trap and Chasm trap
Relational database problem where join path returns more data than expected. A Fan Trap occurs in one – many – many relationship and Chasm trap occurs in many – one relationship.
A fan trap is a less common problem than chasm traps but has the same effect of returning more data than expected.
A Fan Trap is many – many relationship 3 or more then three tables are involved.
It can cause numbers to multiply (It gives Inflated results when u pull data from these tables). Business Objects resolves this transparently by using 2 queries.
It is a one – many – one relationship.
The chasm trap causes the query to return every possible combination of rows from facttable 1 – measure A to factable 2 – measure B i.e cartision product.
Unlike loops, chasm traps are not detected automatically by Designer,
however, you can use Detect Contexts. You can also detect chasm traps graphically by analyzing the one-to-many join paths in your schema. If you do not run Detect Contexts, nor spot the chasm trap in the schema,the only way to see the problem is to look at the detail rows. Otherwise there is nothing to alert you to the situation.
You can use the following methods to resolve a chasm trap:
• Create a context for each fact table. This solution works in all cases.
• Modify the SQL parameters for the universe so you can generate separate SQL queries for each measure. This solution only works for measure objects. It does not generate separate queries for dimension or detail objects.
Creating contexts will always solve a chasm trap in a universe. When you have dimension objects in one or both fact tables, you should always use a context.
For more detail :