zheap is an alternative storage engine proposed by EnterpriseDB intended - in combination with pluggable storage - to minimize the need for tuple vacuuming inherent in PostgreSQL's native MVCC implementation.
The design of
zheap is centred around the concept of UNDO logs, by which redundant tuples are written to storage outside of the main heap, and can easily be removed when no longer required. This also means the main heap is no longer prone to bloat caused by the presence of dead tuples, and vacuuming becomes much less of an issue.
As of 2020, development is being continued out by CyberTec with funding by Heroic Labs (source).