Shouldn't this PostgreSQL function return zero

问题:

Given the schema

CREATE TABLE users (
    id bigserial PRIMARY KEY,
    email varchar(254) NOT NULL
);
CREATE UNIQUE INDEX on users (lower(email));

CREATE FUNCTION all_users() RETURNS users AS $$
    SELECT * FROM users;
$$ LANGUAGE SQL STABLE;

, shouldn't SELECT * FROM all_users() (assuming the users table is empty) return no rows, not a row with all null values?

See the SQL Fiddle here: http://sqlfiddle.com/#!15/b5ba8/2

回答1:

That's because your function is broken by design. It should be:

CREATE FUNCTION all_users() RETURNS SETOF users AS
'SELECT * FROM users' LANGUAGE sql STABLE;

Or alternatively, the more flexible form RETURNS TABLE (...) like @Clodoaldo posted. But it's generally wiser to use RETURNS SETOF users for a query with SELECT * FROM users.

Your original function always returns a single value (a composite type), it has been declared that way. It will break in a more spectacular fashion if you insert some rows.

Consider this SQL Fiddle demo.

For better understanding, your function call does the same as this plain SELECT query:

SELECT (SELECT u from users u).*;

Returns:

id     | email
-------+------
<NULL> | <NULL>

The difference: Plain SQL will raise an exception if the subquery returns more than one row, while a function will just return the first row and discard the rest.

As always, details in the manual.

回答2:

Your function returns records. So it must return at least one record. If you want an empty result set do return a table:

CREATE or replace FUNCTION all_users()
RETURNS table (id bigint, email varchar(254)) AS $$
    SELECT id, email FROM users;
$$ LANGUAGE SQL STABLE;