Currently there are only two ways to decided how much memory to reserve for each query in memory-based admission control:
- Using the memory estimates, which often makes bad decisions (e.g. huge overestimates) and doesn't have any enforcement on the backend
- Using a static pool or user-set mem_limit, which is very difficult to set to a reasonable value for all queries.
The memory reservation work will allow us to come up with more powerful and flexible polices