As GSOC is nearing its end, I have written a blog which contains how can aggregate functions be written and executed to get the desired results.In the blog I also have added the technical details of this project on aggregate functions.

Here are the links:

Jira issue: https://jira.mariadb.org/browse/MDEV-7773

Blog Link: http://varunraiko.github.io/aggregate-functions/

Disclaimer:  I hereby declare that all my work done so far (and future) by me on the Aggregate Functions’ project, is released under the NEW BSD licence.