Postgres 11

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CREATE TABLE t(tag_id int, tag text unique);

CREATE OR REPLACE FUNCTION f_tag_id(_tag_id int, _tag text, OUT _tag_id_ int, OUT _tag_ text) AS
$func$
BEGIN
LOOP
   SELECT t.tag_id, t.tag
   FROM   t
   WHERE  t.tag = _tag
   INTO   _tag_id_, _tag_;

   EXIT WHEN FOUND;

   INSERT INTO t (tag_id, tag)
   VALUES (_tag_id, _tag)
   ON     CONFLICT (tag) DO NOTHING
   RETURNING t.tag_id, tag
   INTO   _tag_id_, _tag_;

   EXIT WHEN FOUND;
END LOOP;
END
$func$ LANGUAGE plpgsql;
SELECT * FROM f_tag_id(1, 'foo');


_tag_id_
_tag_


1
foo


SELECT * FROM f_tag_id(1, 'foo');


_tag_id_
_tag_


1
foo


TABLE t;


tag_id
tag


1
foo