  The Web is a ubiquitous economic, educational, and collaborative space.
However, it also serves as a haven for personal information harvesting.
Existing decentralised Web-based ecosystems, such as Solid, aim to combat
personal data exploitation on the Web by enabling individuals to manage their
data in the personal data store of their choice. Since personal data in these
decentralised ecosystems are distributed across many sources, there is a need
for techniques to support efficient privacy-preserving query execution over
personal data stores. Towards this end, in this position paper we present a
framework for efficient privacy preserving federated querying, and highlight
open research challenges and opportunities. The overarching goal being to
provide a means to position future research into privacy-preserving querying
within decentralised environments.
