Not exactly what you are asking for but this sits on top of PostgresQL and uses non-standard indexing to improve graph queries. https://github.com/bitnine-oss/

Not exactly what you are asking for but this sits on top of PostgresQL and uses non-standard indexing to improve graph queries. https://github.com/bitnine-oss/agens-graph https://github.com/bitnine-oss/agens-graph