As part of GSOC 2020, this is one of the two projects I am interested in pursuing. As guided by last years' comments by Varun and Vicențiu, I have set up a debugger and corresponding breakpoints in the Item_sum_sum::add and Item_sum_sp::add for a custom sum aggregate function to understand the code flow. 

I had a couple of queries regarding the same: 

1. In do_add from decimal.c, there are three parts with comments -              /* part 1 - MY_MAX(frac) ... min (frac) */, /* part 2 - MY_MIN(frac) ... MY_MIN(intg) */. Can someone please elaborate on what do the comments mean ? 
2. In Item_sum_sum::add_helper, there is an unlikely branch for variable direct_added. Can someone please give an idea about when will direct added be true ? In fact in all the uses for direct_added, it is always in an unlikely branch in Item_sum.cc.

Regards

Tavneet