SPARQL query engines are tools to query over RDF-based Knowledge Graphs.
Such will convert SPARQL query strings into an executable query plan after a query planning phase.
Many query engines allow query hints to be given to the query engine to force the query plan to take a specific form, in case the query writer has more information than the query planner.
So far, such query hints have only been designed to work for Centralized Knowledge Graphs, and it is not known yet how these will behave for the more recently upcoming Decentralized Knowledge Graphs.
These are Knowledge Graphs that are spread over the Web in different data sources, and are linked to each other.