Postgres stored procedure/function - postgresql

Question

New to Stored Procedures , have a requirement where I need to execute multiple queries inside stored procedure and return results. I would like to know whether this is possible or not ..
Ex :
Query 1 returns a list of userid ..
Select userid from user where username = ?
For each userid from the above query , I need to execute three different queries like
Query 2 select session_details from session where userid = ?
Query 3 select location from location where userid = ?
The return value should be a collection of , session_details and location.
Is this possible,can you provide some hints?

Answer 1

You can loop through query results like so:
FOR id IN Select userid from user where username = ?
LOOP
...
END LOOP;

Answer 2

As #Fahad Anjum says in his comment, its better if you can do it in a query. But if that's not posible, you have tree posibilities to achive what you want.
SETOF
TABLE
refcursor
1. SETOF
You can return a set of values. The set can be an existing table, a temporal table, or a TYPE you define.
TYPE example:
-- In your case the type could be (userid integer, session integer, location text)
CREATE TYPE tester AS (id integer);
-- The pl returns a SETOF the created type.
CREATE OR REPLACE FUNCTION test() RETURNS SETOF tester
AS $$
BEGIN
RETURN QUERY SELECT generate_series(1, 3) as id;
END;
$$ LANGUAGE plpgsql
-- Then, you get the set by selecting the PL as if it were a table.
SELECT * FROM test();
Table and Temp Table examples:
-- Create a temporal table o a regular table:
CREATE TEMP TABLE test_table(id integer);
-- or CREATE TABLE test_table(id integer);
-- or use an existing table in your schema(s);
-- The pl returns a SETOF the table you need
CREATE OR REPLACE FUNCTION test() RETURNS SETOF test_table
AS $$
BEGIN
RETURN QUERY SELECT generate_series(1, 3) as id;
END;
$$ LANGUAGE plpgsql
-- Then, you get the set by selecting the PL as if it were a table.
SELECT * FROM test();
-- NOTE: Since you are only returning a SETOF the table,
-- you don't insert any data into the table.
-- So, if you select the 'temp' table you won't see any changes.
SELECT * FROM test_table
2. TABLE
A PL can return a table, it would be similar to create a temporal table and then return a SETOF, but, in this case you declare de 'temp' table on the 'returns' sentence of the PL.
-- Next to TABLE you define the columns of the table the PL will return
CREATE OR REPLACE FUNCTION test() RETURNS TABLE (id integer)
AS $$
BEGIN
RETURN QUERY SELECT generate_series(1, 3) as id;
END;
$$ LANGUAGE plpgsql
-- As the other examples, you select the PL to get the data.
SELECT * FROM test();
3. refcursor
This one is the more complex solution. You return a cursor, not the actual data. If you need 'dynamic' values for your returning set, this is the solution.
But since you need static data, you won't need this option.
The use of any of these ways depends on any specific case, if you use regularly the userid,session,location in different ways and PLs, it would be better to Use the SETOF with a type.
If you have a table that has the userid,session,location columns, it's better to return a SETOF table.
If you just use the userid,session,location for one case, then it would be better to use a 'RETURNS TABLE' approach.
If you need to return a dynamic set you would have to use cursors... but that solution is really more advanced.

Answer 3

Based solely on your example, here's probably the easiest way to do it:
CREATE FUNCTION my_func(user_id INTEGER)
RETURNS TABLE (userid INTEGER, session INTEGER, location TEXT) AS
$$
SELECT u.userid, s.session, l.location
FROM -- etc... your query here
$$
LANGUAGE SQL STABLE;
Addressing comment:
That's a bit of a different question. One question is how to return multiple records containing multiple fields in a stored procedure. One way is as above.
The other question is how to write a query that gets data from multiple tables. Again, there are many ways to do it. One way is (again, based on my interpretation of your requirements in the examples):
SELECT userid
, ARRAY_AGG(SELECT session_details FROM session s WHERE s.userid = u.userid)
, ARRAY_AGG(SELECT l.location FROM location l WHERE l.userid = u.userid)
FROM user u
WHERE username = user_name
This will return one record containing the user_id, an array of session_details for that user, and an array of locations for that user.
Then the function can be changed to:
CREATE FUNCTION my_func(user_name TEXT, OUT userid INTEGER, OUT session_details TEXT[], OUT locations TEXT[])
AS $$
SELECT userid
, ARRAY(SELECT session_details FROM session s WHERE s.userid = u.userid)
, ARRAY(SELECT l.location FROM location l WHERE l.userid = u.userid)
FROM user u
WHERE username = user_name;
$$ LANGUAGE SQL STABLE;