Temporary table postgresql function - sql

Question

I can't find a clear explanation of the syntax to create (and use) tables just for the inside calculations of a function. Could anyone give me a syntax exemple please ?
From what I've found, I have tried this (with and without # before temp_table) :
CREATE FUNCTION test.myfunction()
RETURNS SETOF test.out_table
AS $$
DECLARE #temp_table TABLE
(
id int,
value text
)
BEGIN
INSERT INTO #temp_table
SELECT id, value
FROM test.another_table;
INSERT INTO test.out_table
SELECT id, value
FROM #temp_table;
RETURN END
$$ LANGUAGE SQL;
I get :
ERROR: syntax error at or near "DECLARE"
LINE 5: DECLARE #temp_table TABLE
-
I also tried the CREATE TABLE approach suggested here, this way :
CREATE FUNCTION test.myfunction()
RETURNS SETOF test.out_table
AS $$
CREATE TABLE temp_table AS
SELECT id, value
FROM test.another_table;
INSERT INTO test.out_table
SELECT id, value
FROM temp_table;
$$ LANGUAGE SQL;
And I get this :
ERROR: relation "temp_table " does not exist
LINE 11: FROM temp_table
(Obviously, I'm aware the temp_table is not necessary for what I'm doing in the code above, but that's not the point :) => I want to understand the syntax to get it to work)

Answer 1

The appropriate syntax for creating a temp table is
create temp table...
but you have to be sure to drop the temp table before existing out of the function. Also, I'd suggest this syntax instead:
CREATE TEMP TABLE IF NOT EXISTS temp_table AS
SELECT id, value
FROM test.another_table;
Thus your function will be like this:
CREATE FUNCTION test.myfunction()
RETURNS SETOF test.out_table
AS $$
CREATE TEMP TABLE IF NOT EXISTS temp_table AS
SELECT id, value
FROM test.another_table;
INSERT INTO test.out_table
SELECT id, value
FROM temp_table;
DROP TABLE temp_table;
$$ LANGUAGE SQL;
But if I can be so kind, I'd like to rewrite this function so it is more correct:
CREATE FUNCTION test.myfunction()
RETURNS TABLE (id int, value varchar) -- change your datatype as needed
AS $$
BEGIN;
CREATE TEMP TABLE IF NOT EXISTS temp_table AS
SELECT id, value
FROM test.another_table;
INSERT INTO test.out_table
SELECT id, value
FROM temp_table;
DROP TABLE temp_table;
RETURN QUERY
SELECT id, value
from temp_table;
END;
$$ LANGUAGE plpgsql;
Untested; let me know if this fails.