I presume you used B-Tree indexing in PostgreSQL vs hash-indexing in BDB & your own code? PostgreSQL does support hash-indexing, but the documentation suggests

I presume you used B-Tree indexing in PostgreSQL vs hash-indexing in BDB & your own code? PostgreSQL does support hash-indexing, but the documentation suggests it's not faster than B-trees anyway (probably because it would involve a second disk access). Let's be honest though - this is an incredibly niche use-case. There's nothing in the relational model that precludes supporting this use case optimally, but in practice engineering resources get focused on more mainstream uses. You did your due-diligence and found that support for your scenario was lacking in existing RDBMS systems. That's great, but I think you should be qualifying your support for NoSQL lest others with traditional use cases (best served by RDBMS systems) simply copy your conclusions where they don't apply.