Implementation of aggregate functions

Hello all,
I am doing a project as a part of a course work in
database implementation in which we are required to
implement some feature in postGres or extend some of
the existing functionality.
I am keen in implementing certain additional aggregate
functions like percentage, standard deviation etc. I
would like to know the complexity of this and also the
parts of the code which would have to be modified for
the same,

Thanks in advance for all the help.

Best regards,
Sayali

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The degree of technical competence is
inversely proportional to the level of management.
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