"stable or volatile function is used as if it is immutable" warning in Postgresql function - sql

Question

I have a Postgresql function in which there is only a SELECT statement:
CREATE OR REPLACE FUNCTION fun_test()
RETURNS INTEGER AS $$
DECLARE size INTEGER;
BEGIN
SELECT COUNT(*) INTO size FROM tab;
RETURN size;
END;
$$ LANGUAGE plpgsql STABLE;
When I call the function with:
SELECT fun_test()
Although the result are correct, there will be a warning too:
WARNING: A stable or volatile function is used as if it is immutable
HINT: The function should be declared as stable or volatile in create function statement.
I found in Postgresql document that STABLE is a appropriate selection for functions whose results depend on database lookups, parameter variables (such as the current time zone), etc. http://www.postgresql.org/docs/8.2/static/sql-createfunction.html
My question is where the warning comes from? It seems that I am doing what the document requires to do. Any help is appreciated.
EDIT:
I am using postgresql server 8.2.15
The whole story:
CREATE TABLE algo.chengb_tmp
(
userid INT,
username varchar(100)
)
CREATE OR REPLACE FUNCTION algo.chengb_fun_test()
RETURNS INTEGER AS $$
DECLARE size INTEGER;
BEGIN
SELECT COUNT(*) INTO size FROM algo.chengb_tmp;
RETURN size;
END;
$$ LANGUAGE plpgsql STABLE;
SELECT algo.chengb_fun_test()
cheng

Answer 1

Obviously, your question does not show the whole story. I tested your function in PostgreSQL 9.1 and it works for me, as expected. No warning.
Possible explanations include:
A RULE on SELECT on the involved table tab that calls another function. (There are no triggers for SELECT)
A bug in the outdated PostgreSQL version 8.2.15, which might go away with an upgrade.
You over-simplified the question and abstracted the actual cause of the problem away.
Add more details in the question: your Version of PostgreSQL, the complete definition of table tab, the complete error message and its context.
As an aside, could be simplified:
CREATE OR REPLACE FUNCTION fun_test()
RETURNS INTEGER AS
$$
BEGIN
RETURN
(SELECT COUNT(*) FROM tab);
END;
$$ LANGUAGE plpgsql STABLE;
Ore even:
CREATE OR REPLACE FUNCTION fun_test()
RETURNS INTEGER AS
$$
SELECT count(*) FROM tab;
$$ LANGUAGE sql STABLE;
But that's probably not the point here.