performance regression in groupby: mean(v1), mean(v2), mean(v3)

One type of queries we are running in dbb seems to be much slower than before, here is 1e8 rows timings in seconds
```
question 20190708_d3b8f8b 20190906_911eb32 20191031_2c34c60 20191124_f05652e 20200124_7569c1e
1: sum v1 by id1 2.575 2.601 2.509 4.117 3.974
2: sum v1 by id1:id2 3.568 5.462 4.305 4.714 4.674
3: sum v1 mean v3 by id3 9.621 11.083 10.833 10.554 11.179
4: mean v1:v3 by id4 1.808 1.921 1.945 10.194 10.904
5: sum v1:v3 by id6 3.392 3.387 3.981 4.534 3.657
```