I see that other systems often forgot many small details. Imagine you are implementing COUNT(DISTINCT) function. And imagine it is used in GROUP BY something,

I see that other systems often forgot many small details. Imagine you are implementing COUNT(DISTINCT) function. And imagine it is used in GROUP BY something, like GROUP BY region_id. And on realistic datasets, you will have a large number of different region_id and something like power law distribution of their frequency: https://en.wikipedia.org/wiki/Zipf%27s_law https://en.wikipedia.org/wiki/Zipf%27s_law Then you will expect that most of the values of COUNT(DISTINCT) will be small. And the natural optimization will be - using a linear array before using a hash table. Or: place smaller hash tables in something like a memory pool. This is about a ton of optimizations on distributions of real datasets.