> How else could you solve what Vespa does using Rust, Go, or C/C++ libraries?

Let me try myself answering my own question, I hope someone hops in and tells me where I'm wrong or how else to improve :)

     1) Get PostgresSQL exntensions via "package manager" pgxnclient
     1.1) pg_bouncer - For connetion pooling
     1.2) yoke - As a high-availability cluster manager with auto-failover and automated cluster recovery
     1.3) prestodb.io - Distributed SQL query engine for pgsql
     1.4) pglogical - Logical streaming replication for using a publish/subscribe model
     1.5) pg_lambda - To create your own AWS (meta) Lambda
     1.6) pg_strom - To offload tasks to the GPU
     1.7) zombodb - To utilize full-text searching via indexes backed by Elasticsearch
     2) Put all together with pglogical and presto to seperate GPU/CPU intensive tasks.
     2.1) "Build Missing Middleware" - To design/fuse a query visually that combines multiple backends
     2.1.1) Create a binary data-stream by integrating pg_lambda, pg_strom, presto and zombodb
     2.1.2) "Build Missing Middleware" - A tensor processing extension to use ML Model evaluations
     2.1.3) "Use Missing Middleware" - For data-processing via Machine-Learning models
     2.1.4) "Use Missing Middleware"- To output ML processed results into the database
     2.2) Partition these queries using "pg_lambda + middleware" to create accelerated and fused query results

So what's missing to create a Vespa alternative using existing technologies is everything in Point 2) if I'm not mistaken. Torrent based replication isn't exactly neccessary, except at Twitter/Facebook scale, but if you reach that stage you can hire a libtorrent author.