alt Hacker News> Threads does not offer any major performance advantage
This is very not true. When it comes to parallel queries, a process model adds a ton of overhead. You can't pass pointers between processes because the address space is different. This adds a ton of overhead in a bunch of different places. For example when doing a parallel hash join, Postgres will have each worker build a local hash table. Then it will take all the tuples out of the local hash table and copy them through shared memory to the leader who will then construct a new hash table. This duplicates a lot of work as you have to hash the tuples multiple times.
A lot of getting to Clickhouse level performance was making better use of parallelism.
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Ok ... you know PostgreSQL supports hash tables in shared memory, right? PostgreSQL could in theory share those if we wanted to. The issue is just that coding anything which uses shared memory is a lot of work.
Additionally the reasons PostgreSQL does not offer Clickhouse performance has very little to do with parallelism. PostgreSQL plans to move to threading but the efforts around imporving OLAP performance are almost entirely unrelated.
Passing pointers is not significantly faster than passing offsets into a shared memory pool.