In DICOM Query, when the SCU requests matching on optional Key Attributes that are not supported for matching by the SCP, the baseline response behavior is for the SCP to treat them as "universal match", i.e., no filtering is performed by the SCP. In the Repository Query or Inventory Creation SOP Classes, such behavior may result in a substantial number of records not desired by the SCU being returned. For example, the SCU may request inventory of Studies updated in the last year by specifying a date range match on Study Update DateTime. If the SCP does not support matching on that Attribute, the baseline behavior would be to return inventory of all Studies in the repository. This could have significant performance impacts on both the SCU and SCP.

The Inventory Creation SOP Class specifies a Warning response to an Initiate N-ACTION request, B010 - "One or more of Key Attributes are not supported for matching", with the list of unsupported Attributes provided in the N-ACTION response field Attribute Identifier List (0000,1005). The SCU can evaluate the Warning and, if desired, send a Cancel N-ACTION.

The Repository Query SOP Class does not provide such a warning. However, the SCP's Conformance Statement is required to identify Attributes supported for matching, although if that list is site-configurable the Conformance Statement may not provide the requisite information. The SCU could, however, request a relatively small Maximum Number of Records (0008,041E) in the initial Query, evaluate the Query responses, and check that responses do not exceed the requested match values before continuing with a subsequent Query.