The variable IV_SEARCH_STRING is not being filled

Hi Friends,

I am using the following URL's

http://xxxxx/xxx/xxx/sdata/sap/ZSALESORDER2/SalesorderheaderCollection?$select=SO_NUMBER_KEY,SO_NUMBER,CUSTOMER_NAME

http://xxxx/xxx/xxx/sdata/sap/ZSALESORDER2/SalesorderheaderCollection?$select=*

In both the cases the variable IV_SEARCH_STRING is not being filled. Please help me in resolving this.

Thanks and Regards,

Reshma Nadella.