Preface We, the chairs, are happy to present this volume of the proceedings of the 2021 edition of the VLDB Ph.D. Workshop. The workshop was co-located with the 47th International Conference on Very Large Database (VLDB 2021) in Copenhagen and was held in a hybrid on-line and in-person setup on August 16, 2021. We assembled the technical program to give Ph.D. students an opportunity to present their research ideas in a premier international research venue. The resulting program, consisting of 12 papers accepted out of the 18 submissions we received, presents a representative sample of relevant topics in database research. It included contributions from young researchers from America, Asia, and Europe. This year’s program also included a keynote talk by Tim Kraska (MIT). The keynote provided guidance to Ph.D. students on successful strategies for pursuing a career in research and discussed novel issues and challenges in machine learning in and for database systems. Due to the pandemic situation, only three of the 12 Ph.D. students were able to attend the conference and workshop in person. To accommodate speakers in their time zones, we split the schedule into virtual and local sessions. Additionally, we offered two forms of mentorship. We generated a mentor-mentee mapping between authors and PC members for individual feedback sessions based on the submitted paper and review. We further connected the attendees with well established members of the VLDB community for general questions about research and the community. We thank Felix Naumann and Luna Dong, the VLDB 2021 PC chairs, and Pinar Tözün and Philippe Bonnet, the general chairs, ​who entrusted us with this role. We also acknowledge the great support of the whole local and technical organizing team. Finally, we thank the workshop program committee, who did a great job evaluating the papers and writing constructive feedback for the authors. We believe the event gave Ph.D. students an excellent opportunity to share and exchange research ideas with experienced researchers and become members of the friendly and welcoming VLDB community. We wish all participants the very best for their research! The VLDB 2021 Ph.D. Workshop Chairs Philip Bernstein, Microsoft Research Tilmann Rabl, Hasso Plattner Institute, University of Potsdam Program Committee PC Chairs Philip Bernstein, Microsoft Research Tilmann Rabl, Hasso Plattner Institute, University of Potsdam Program Committee Members Alessandro Margara, Politecnico di Milano Boris Glavic, IIT Chicago Ce Zhang, ETH Zurich Chen Xu, East China Normal University Fatemeh Nargesian, University of Rochester Hannes Mühleisen, CWI Jana Giceva, TU Munich Jaroslaw Szlichta, Ontario Tech University Kaiwen Zhang, ETS Montreal Matthias Boehm, TU Graz Oliver Kennedy, University of Buffalo Raul Castro Fernandez, University of Chicago Sebastian Schelter, University of Amsterdam Yogesh Simmhan, IISc Bangalore Community Mentors Angela Bonifati, Lyon 1 University Christopher Jermaine, Rice University Divesh Srivastava, AT&T Felix Naumann, HPI Goetz Graefe, Google Marcello Arenas, PUC Chile Norman Paton, Manchester University Peter Pietzuch, Imperial College Tim Kraska, MIT Timos Sellis, Swinburne Thomas Neumann, TU Munich Volker Markl, TU Berlin