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Update

As Evan Carrol has pointed out on him comments you can avoid use CTE and change function language from plpgsql to plain SQL.

CREATE OR REPLACE FUNCTION GroupIntervals(low int, high int, step int) RETURNS TABLE (amount int, interv int) AS $$ SELECT COALESCE(SUM(T.amount),0)::int AS amount, CTS.Serie as interv FROM generate_series(low, high, step) AS CTS(Serie) LEFT JOIN T ON index >= CTS.Serie AND index < CTS.Serie + step GROUP BY CTS.Serie ORDER BY CTS.Serie; $$ LANGUAGE sql; 

SELECT * FROM GroupIntervals(0, 14, 4); 

 amount | interv -----: | -----: 55 | 0 219 | 4 175 | 8 0 | 12 

SELECT * FROM GroupIntervals(5, 14, 5); 

 amount | interv -----: | -----: 219 | 5 175 | 10 

dbfiddle here

SELECT generate_series(0, 14, 4) Serie; | serie | | ----: | | 0 | | 4 | | 8 | | 12 | WITH CTS AS ( SELECT generate_series(0, 14, 4) Serie ) SELECT COALESCE(SUM(amount),0) AS amount, CTS.Serie FROM CTS LEFT JOIN T ON index >= CTS.Serie AND index < CTS.Serie + 4 GROUP BY CTS.Serie ORDER BY CTS.Serie; amount | serie -----: | ----: 55 | 0 219 | 4 175 | 8 0 | 12 

CREATE OR REPLACE FUNCTION GroupIntervals(low int, high int, step int) RETURNS TABLE (amount int, interv int) AS $$ BEGIN RETURN QUERY WITH CTS AS ( SELECT generate_series(low, high, step) Serie ) SELECT COALESCE(SUM(T.amount),0)::int AS amount, CTS.Serie as interv FROM CTS LEFT JOIN T ON index >= CTS.Serie AND index < CTS.Serie + step GROUP BY CTS.Serie ORDER BY CTS.Serie; END; $$ LANGUAGE plpgsql; 

 ✓ 

SELECT generate_series(0, 14, 4) Serie; | serie | | ----: | | 0 | | 4 | | 8 | | 12 | WITH CTS AS ( SELECT generate_series(0, 14, 4) Serie ) SELECT COALESCE(SUM(T.amount),0) AS amount, CTS.Serie FROM CTS LEFT JOIN T ON T.index >= CTS.Serie AND T.index < CTS.Serie + 4 GROUP BY CTS.Serie ORDER BY CTS.Serie; amount | serie -----: | ----: 55 | 0 219 | 4 175 | 8 0 | 12 

CREATE OR REPLACE FUNCTION GroupIntervals(low int, high int, step int) RETURNS TABLE (amount int, interv int) AS $$ BEGIN RETURN QUERY WITH CTS AS ( SELECT generate_series(low, high, step) Serie ) SELECT COALESCE(SUM(T.amount),0)::int AS amount, CTS.Serie as interv FROM CTS LEFT JOIN T ON T.index >= CTS.Serie AND T.index < CTS.Serie + step GROUP BY CTS.Serie ORDER BY CTS.Serie; END; $$ LANGUAGE plpgsql; 

 ✓ 

dbfiddle here

You can use a user defined function that allows you to use parameters

CREATE OR REPLACE FUNCTION GroupIntervals(low int, high int, step int) RETURNS TABLE (amount int, interv int) AS $$ BEGIN RETURN QUERY WITH CTS AS ( SELECT generate_series(low, high, step) Serie ) SELECT COALESCE(SUM(T.amount),0)::int AS amount, CTS.Serie as interv FROM CTS LEFT JOIN T ON index >= CTS.Serie AND index < CTS.Serie + step GROUP BY CTS.Serie ORDER BY CTS.Serie; END; $$ LANGUAGE plpgsql; 

 ✓ 

SELECT * FROM GroupIntervals(0, 14, 4); 

 amount | interv -----: | -----: 55 | 0 219 | 4 175 | 8 0 | 12 

SELECT * FROM GroupIntervals(5, 14, 5); 

 amount | interv -----: | -----: 219 | 5 175 | 10 

dbfiddle here