Setting the Case Search Scenario 
You can choose the search engine type based on your requirements. The two types are Standard and SAF/TREX, where SAF/TREX is faster and allows the system to handle more complicated search requests. In Customizing choose .
Before the SAF/TREX search can be used, you need to install TREX and customize SAF accordingly. In Customizing choose .
After selecting the SAF/TREX search option, case indexes must be built and compiled using the SAF tool for this function to work properly.
Also refer to SAP Note 880200 for more information.