Dynamic Limit in PostgreSQL - postgresql

Question

I have function returning list of Employees, My requirement is if i pass Limit to function than i should get result with limit and offset, If i don't pass limit than all the rows should be returned
for example
When Limit is greater than 0(I am passing Limit as 10)
Select * from Employees
Limit 10 offset 0
When Limit is equal to 0 than
Select * from Employees
Is their any way to do such logic in function?

Answer 1

Yes, you can pass an expression for the LIMIT and OFFSET clauses, which includes using a parameter passed in to a function.
CREATE FUNCTION employees_limited(limit integer) RETURNS SET OF employees AS $$
BEGIN
IF limit = 0 THEN
RETURN QUERY SELECT * FROM employees;
ELSE
RETURN QUERY SELECT * FROM employees LIMIT (limit)
END IF;
RETURN;
END; $$ LANGUAGE plpgsql STRICT;
Note the parentheses around the LIMIT clause. You can similarly pass in an OFFSET value.
This example is very trivial, though. You could achieve the same effect by doing the LIMIT outside of the function:
SELECT * FROM my_function() LIMIT 10;
Doing this inside of a function would really only be useful for a complex query, potentially involving a large amount of data.
Also note that a LIMIT clause without an ORDER BY produces unpredictable results.

Answer 2

Sorry I can't comment. Solution is almost provided by Patrick.
First we should write function to return result without limitation.
CREATE FUNCTION test ()
RETURNS TABLE (val1 varchar, val2 integer) AS $$
BEGIN
RETURN QUERY SELECT val1, val2 FROM test_table;
END;
$$ LANGUAGE plpgsql;
Then we have to write wrapper function, which will process limitation.
CREATE FUNCTION test_wrapper (l integer DEFAULT 0)
RETURNS TABLE (name varchar, id integer) AS $$
BEGIN
IF l = 0 THEN
RETURN QUERY SELECT * FROM test(); -- returns everything
ELSE
RETURN QUERY SELECT * FROM test() LIMIT (l); -- returns accordingly
END IF;
END;
$$ LANGUAGE plpgsql;
In my case I needed to return tables as final result, but one can get anything required as return from wrapper function.

Answer 3

Please note that a SELECT with a LIMIT should always include an ORDER BY, because if not explicitly specified the order of the returned rows can be undefined.
Insead of LIMIT you could use ROW_NUMBER() like in the following query:
SELECT *
FROM (
SELECT *, row_number() OVER (ORDER BY id) AS rn
FROM Employees
) AS s
WHERE
(rn>:offset AND rn<=:limit+:offset) OR :limit=0

Answer 4

I used the below approach in postgres
Create Function employees_limited(limit integer, offset interger, pagination boolean) RETURNS SET OF employees AS $$
BEGIN
if pagination = false then --skip offset and limit
offset = 0;
limit = 2147483647; -- int max value in postgres
end if;
RETURN QUERY
SELECT * FROM employees order by createddate
LIMIT (limit) Offset offset;
RETURN;
END; $$ LANGUAGE plpgsql STRICT;