In order to generate the target file names from the mapping, we should make use of the special "FileName" port in the target file. You can't create this special port from the usual New port button. There is a special button with label "F" on it to the right most corner of the target flat file when viewed in "Target Designer".
Below two screen-shots tell you how to create the special port in your target file.
Once this is done, the job is done. When you want to create the file name with a timestamp attached to it, just use a port from an Expression transformation before the target to pass a value of Output Port with expression $$FILE_NAMEto_char(sessstarttime, 'YYYYMMDDHH24:MISS')'.csv'.
Please note that $$FILE_NAME is a parameter to the mapping and I've used sessstarttime because it will be constant through out the session run.
If you use sysdate, it will change if you have 100s of millions of records and if the session may run for an hour, each second a new file will get created.
Please note that a new file gets created with the current value of the port when the port value which maps to the FileName changes.
We'll come to the mapping again. This mapping generates two files. One is a dummy file with zero bytes size and the file name is what is given in the Session properties under 'Mappings' tab for target file name. The other file is the actual file created with the desired file name and data.
When you have different sets of input data with different target files created, use the same instance, but with a Transaction Control transformation which defines the boundary for the source sets. A demonstration of this is available in the PowerCenter documentation.
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