test=> \c - postgres
You are now connected as new user "postgres".
test=> create table testtrunc(i int);
CREATE TABLE
test=> grant select on testtrunc to user1;
GRANT
test=> insert into testtrunc values ( 1 );
INSERT  0   1 
test=> CREATE OR REPLACE FUNCTION truncate_testtrunc() RETURNS void AS
test-> 'truncate table testtrunc' LANGUAGE sql SECURITY DEFINER;
CREATE FUNCTION
test=> grant execute on function truncate_testtrunc() to user1;
GRANT
test=> \c - user1
You are now connected as new user "user1".
test=> select * from testtrunc;
 i 
---
  1 
( 1  row)
test=> truncate table testtrunc;
ERROR:  must be owner of relation testtrunc
test=> select truncate_testtrunc();
 truncate_testtrunc 
--------------------
 
( 1  row)
test=> select * from testtrunc;
 i 
---
( 0  rows)