Issue
The SQL service does not work on the system "_signals" collection due to the signals collection not having the "random" field configured OOTB.
Environment
Fusion versions before Fusion 5.6. The SQL service was removed in Fusion 5.6.
Resolution
You need to add the following to the "_signals" managed-schema.xml:
<dynamicField name="random_*" type="random"/>
Cause
Essentially the problem here is the SQL service uses streaming expressions in Solr to complete it's query. In this case it uses the stream source called "random", which requires the above to be a field in the schema.xml, else it fails.