When creating a base view on top of an operation of a web service data source, the base view can be created using the option “Do not stream output” or “Stream output at specified level”. In some cases when the stream output option is used, the queries executed on the base view will not return any result.
This problem occurs when the message returned by the web service contains 'multirefs' elements. Multirefs are references to other elements in XML files. The streaming option for web services does not support the use of 'multirefs' elements.
To solve this scenario the web service will have to be modified to avoid using 'multirefs' elements or the option 'Do not stream output' selected to create the base view. When using this option, in order to obtain the fields from a particular level of the web service output, a Flatten view can be created on top of the base view to obtain the desired output schema.
Virtual DataPort Administration Guide: Creating Flatten Views.
Advanced VQL Guide: Web Services Wrappers.