Parser - Query Analyser

My name is Michail Giannakopoulos and I am a graduate student at University of Toronto. I have no previous experience in developing a system like postgreSQL before.

What I am trying to explore is if it is possible to extend postgreSQL in order to accept queries of the form:

Select function(att1, att2, att3) AS output(out1, out2, ..., outk) FROM [database_name];

where att1, att2, att3 are attributes of the relation [database_name] while output(out1, out2, out3) expresses the output that comes from 'function' and the fields that this output should have are (out1, out2, out3). What I mean is that this kind of query should return tuples that obay to the 'output(...)' schema and are produced by processing the original tuples of 'database_name' relation in attributes att1, att2 and att3.

From what I have seen I need to teak both the parser and the query analyser in order to accept this form of query. However, I do not know how to tweak these systems. Even worse I do not know where to begin from.

The manual of extending SQL did not helped me and currently I am debugging postgreSQL source code. I would appreciate if you could provide me with more resources and correct ways of how to hack inside postgreSQL files and system. For example, where do I add a new parse node, where should I change the the grammar. 

Thank you very much for all your time and all your help,

Michail