On the Expressiveness of Languages for Querying Property Graphs in Relational Databases
Abstract
SQL/PGQ is the emerging ISO standard for querying property graphs defined as views over relational data. We formalize its expressive power across three fragments: the read-only core, the read-write extension, and an extended variant with richer view definitions. Our results show that graph creation plays a central role in determining the expressiveness. The read-only fragment is strictly weaker than the read-write fragment, and the latter is still below the complexity class NL. Extending view definitions with arbitrary arity identifiers closes this gap: the extended fragment captures exactly NL. This yields a strict hierarchy of SQL/PGQ fragments, whose union covers all NL queries. On ordered structures the hierarchy collapses: once arity-2 identifiers are allowed, higher arities add no power, mirroring the classical transitive-closure collapse and underscoring the central role of view construction in property graph querying.
Cite
@article{arxiv.2510.07062,
title = {On the Expressiveness of Languages for Querying Property Graphs in Relational Databases},
author = {Hadar Rotschield and Liat Peterfreund},
journal= {arXiv preprint arXiv:2510.07062},
year = {2025}
}