CitusDB is not relevant here, I believe, as it still uses Postgres' table storage, so it's not columnar. It might be good for analytical workloads, but I very much doubt it will perform anywhere close to ClickHouse.

Greenplum: I've not used it, but it does support columnar tables, so maybe it's comparable.

Greenplum seems to like a way to throw more hardware at a problem. Queries are too slow with postgres? Shard your data across machines and distribute queries to speed things up by running in parallel. It's using scale as a means to compensate for low efficiency.

On the opposite side of the spectrum you have other open source projects like questDB that have full focus on core performance: constantly optimise to get as much as possible from a single processor core. You can't scale out (at least yet), but given how fast it is on single core, it will be pretty powerful if they chose to go this route.

Note that Greenplum supports column-oriented tables. I haven't used it myself, so I can't comment on whether it's slower than ClickHouse.