Aggregate Window Functions

With a few exceptions, most native aggregate functions are supported as window functions.
https://mariadb.com/kb/en/library/aggregate-functions-as-window-functions/

In MDEV-7773, support for creating of custom aggregate functions was added.
This task proposes to extend that feature and allow custom aggregate functions to be used as window functions

An example of a creating a custom aggregate function is given below:

create aggregate function agg_sum(x INT) returns double

begin

  declare z double default 0;

  declare continue handler for not found return z;

  loop

    fetch group next row;

    set z = z + x;

  end loop;

end|

This functions can be used in the following query:

create table balances (id int, amount int);

insert into balances values (1, 10), (2, 20), (3, 30);

select agg_sum(amount) from balances;

After this task is complete the following must also work:

select agg_sum(amount) over (order by id);