Use a function to handle a negative offset - sql

Question

I am creating a Postgresql function in order to handle a negative offset. When the offset is negative then my function should return 0. However, when I try to run this function, there is an error near "row", I am not sure why. What is wrong with my conditional branch?
CREATE OR REPLACE FUNCTION
calc_offset(row integer, padding integer) RETURNS integer AS $$
BEGIN
if (row-1-padding) < 0 then return 0;
else return (row-1-padding);
END;
$$ LANGUAGE plpgsql;
WITH all_the_trimmings AS (
SELECT ui.id, ui.name, ui.time,
row_number() over(order by name asc) as rownumber
FROM user_infos ui
), my_row AS (
SELECT * FROM all_the_trimmings WHERE id=1
), the_slice AS (
SELECT * FROM all_the_trimmings LIMIT 5
OFFSET calc_offset((SELECT rownumber FROM my_row)::int,2)
)
SELECT * from my_row
UNION ALL
SELECT * from the_slice;
SQL fiddle for the full picture

Answer 1

Your function calc_offset() can be replaced with this simple SQL function:
CREATE OR REPLACE FUNCTION calc_offset(p_row int, padding int)
RETURNS int AS 'SELECT GREATEST ($1 - $2 - 1, 0)' LANGUAGE sql;
And you don't need the function at all. Just use GREATEST with a subselect:
WITH all_the_trimmings AS (
SELECT ui.id, ui.name, ui.time, row_number() OVER (ORDER BY name) AS rn
FROM user_infos ui
)
, my_row AS (
SELECT * FROM all_the_trimmings WHERE id=1
)
SELECT * FROM my_row
UNION ALL
SELECT * FROM all_the_trimmings LIMIT 5
OFFSET GREATEST ((SELECT rn - 3 FROM my_row), 0);
Or simpler:
WITH cte AS (
SELECT ui.id, ui.name, ui.time, row_number() OVER (ORDER BY name) AS rn
FROM user_infos ui
)
SELECT * FROM cte WHERE id = 1
UNION ALL
SELECT * FROM cte LIMIT 5
OFFSET GREATEST ((SELECT rn - 3 FROM cte WHERE id = 1), 0);
SQL Fiddle showing both.

Answer 2

The row parameter is a reserved word. Try replacing it with another name. You were also missing an end if;
CREATE OR REPLACE FUNCTION
calc_offset(p_row integer, padding integer) RETURNS integer AS $$
BEGIN
if (p_row-1-padding) < 0 then
return 0;
else
return (p_row-1-padding);
end if;
END;
$$ LANGUAGE plpgsql;