Abstract
Property graphs have reached a high level of maturity, witnessed by multiplerobust graph database systems as well as the ongoing ISO standardization effortaiming at creating a new standard Graph Query Language (GQL). Yet, despitedocumented demand, schema support is limited both in existing systems and inthe first version of the GQL Standard. It is anticipated that the secondversion of the GQL Standard will include a rich DDL. Aiming to inspire thedevelopment of GQL and enhance the capabilities of graph database systems, wepropose PG-Schema, a simple yet powerful formalism for specifying propertygraph schemas. It features PG-Types with flexible type definitions supportingmulti-inheritance, as well as expressive constraints based on the recentlyproposed PG-Keys formalism. We provide the formal syntax and semantics ofPG-Schema, which meet principled design requirements grounded in contemporaryproperty graph management scenarios, and offer a detailed comparison of itsfeatures with those of existing schema languages and graph database systems.