Look Up Error

Question

Hi All,
Can anyone tell me as to how I can guess by seeing the mapping itself (that uses a lookup) that it will suffer from the following runtime error:
"A table can be outer joined with at most one other table".
After developing a long enough mapping I discover it suffers from the above error in runtime. Then the obviuos workaround is to replace the look ups with functions or joiners.
If I can guess the above error beforehand, it enables me to structure my mapping logic to avoid this rather than to do it and then keep on changing the structure of it.
I have tried to produce the above error in some simple mappings by using some lookups but it does not occur.
How is it that I can expect such errors to occur. What I feel is OWB deploys code whose structure is very much different from the way it looks in the mapping editor.
Any hints/suggestions are welcome.
Regards
-AP

Answer 1

Hello,
Functions are definitely is not best choice as far as using them involve SQL – PLSQL context switching which is very expensive. This will influence overall mapping performance, especially on large data volumes. So to perform lookups joins are the most straight forward and best working approach.
Views (inline or presented by an database object) are nice way of “splitting” mapping. But, currently OWB does not provide the INLINE VIEW operator. Actually this is the thing that I need most in OWB. Currently I use ORDER BY operator to facilitate this functionality. On other hand, if used properly, ORDER operator would not lead to heavy performance degradation – at the end we need ALL records to be inserted into the target. Anyway, this is a price for using the tool consistently - to accept it's approach.
Sergey