No hay repeat declaración en plpgsql. Utilice loop simples.
.
CREATE OR REPLACE FUNCTION random_token(_table TEXT, _column TEXT, _length INTEGER) RETURNS text AS $$
DECLARE
alphanum CONSTANT text := 'abcdefghijkmnopqrstuvwxyz23456789';
range_head CONSTANT integer := 25;
range_tail CONSTANT integer := 33;
random_string text;
ct int;
BEGIN
LOOP
SELECT substring(alphanum from trunc(random() * range_head + 1)::integer for 1) ||
array_to_string(array_agg(substring(alphanum from trunc(random() * range_tail + 1)::integer for 1)), '')
INTO random_string FROM generate_series(1, _length - 1);
EXECUTE FORMAT('SELECT count(*) FROM %I WHERE %I = %L', _table, _column, random_string) INTO ct;
EXIT WHEN ct = 0;
END LOOP;
RETURN random_string;
END
$$ LANGUAGE plpgsql;
Nota, random_string debe ser un parámetro para format() .
Actualizar. De acuerdo con la sugerencia precisa de Abelisto, esto debería ser más rápido para una tabla grande:
DECLARE
dup boolean;
...
EXECUTE FORMAT('SELECT EXISTS(SELECT 1 FROM %I WHERE %I = %L)', _table, _column, random_string) INTO dup;
EXIT WHEN NOT dup;
...
