> The reason PostgreSQL results were omitted is no matter what kind of optimization we throw at it, the benchmark always took more than 2 hours, regardless of partitioning, whereas MongoDB and Elasticsearch took a couple of minutes.

This seems like something was done incorrectly the comparison shouldn't be that drastic.

> Just one PostgreSQL 9.6.10 instance (shared_buffers = 128MB)

This looks way too low. The postgresql docs say a good starting point for shared_buffers is 25% of the server's memory. In this case that would be 32GB.

https://www.postgresql.org/docs/9.1/runtime-config-resource....