If you are talking of the DICOM Q/R feature, there is a warning message "Do you really want to execute a query with empty parameters ?" in case the query is on a large data set.
In modern PACS (like dcm4chee 5) you can separate collections of examens with an AE Title, so sometimes questioning an entire collection is necessary. In addition, you can configure the max returning elements of a DICOM query in dcm4chee 5.