I know this question has been asked before at [PostgreSQL to Data-Warehouse: Best approach for near-real-time ETL / extraction of data](https://stackoverflow.com/questions/2519985/postgresql-to-data-warehouse-best-approach-for-near-real-time-etl-extraction).
But I want to rephrase this question.

I am attempting a real-time data warehouse. The difference between real-time and near real-time is huge.
I find real-time data warehouse to be event-driven and transactional in approach.
While near real-time would do the same batch mode application but would poll data more frequently. 
It would put so much extra load on the production server and would certainly kill the production system. 
Being a batch approach it would scan through all the tables for changes and would take rows which have changed from 
a cut-off time stamp.
I mean by event driven, it would be specific to tables which have undergone changes are focus only on transaction 
which are happening currently.

But the source system is an elephant of system, SAP, assuming which has 25,000 tables. It is not easy to model that, 
not easy to write database triggers on each table to capture each change. I want impact on the production server to be minimal.

Is there any trigger at database level so that I could capture all changes happening in database in one trigger.
Is there any way to write that database trigger on a different database server so that production server goes untouched.
I have not been keeping pace with changes happening to database technology and am sure some nice new technologies would have come by to capture these changes easily.

I know of Log miners and Change data captures but it would be difficult to filter out the information which I need from redo logs.
Alternate ways to capture database write operations on the go.

Just for completeness sake let us assume databases are a heterogeneous mix of Oracle, SQL Server and DB2. But my contention is
the concepts we want to develop. 

This is a universal problem, every company is looking for easy to implement solution. So a good discussion would benefit all.