Investigate what is the actual bottleneck in hash agg processing for dirty groups

[kwannoel]

It doesn't seem to be heap or cpu bottleneck. So what is the actual bottleneck, is it IO cost, due to lookups? If so we need a metric for it.

Or is it skew? because in some scenarios, the workload peaks at 1600%. But we have 32 cores.

Needs further investigation.

[kwannoel]

Some workloads to test:

1. What happens when a large number of existing agg groups get updated.
2. What happens when a large number of new agg groups are created.
3. Does it change according to cache configurations.
4. Test first_value agg.
5. Make sure to use minio rate limit configuration, to simulate latency when fetching from aws s3.