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GSOC 2020 - MDEV-11263 Aggregate Window Functions


Hi everyone,

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.


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