How to know which row in a table is changed using java and postgresql -

i have following class listens notifications postgresql database.

class listener extends thread {         private connection conn;         private org.postgresql.pgconnection pgconn;  listener(connection conn) throws exception {     this.setconn(conn);     this.pgconn = (org.postgresql.pgconnection) conn;     statement stmt = conn.createstatement();     stmt.execute("listen userdata");     stmt.close(); }  public connection getconn() {     return conn; }  public void setconn(connection conn) {     this.conn = conn; } public static void main(string[] args) {         try {             class.forname("org.postgresql.driver");             string url = "jdbc:postgresql://localhost/users";             string user = "admin";             string password = "password";             connection listenercon = drivermanager.getconnection(url, user,                     password);             listener listener = new listener(listenercon);             listener.start();         } catch (exception e) {             e.printstacktrace();         }      }  public void run() {     while (true) {         try {             statement stmt = conn.createstatement();             resultset rs = stmt                     .executequery("select * userdata id='1'");             rs.close();             stmt.close();             org.postgresql.pgnotification notifications[] = pgconn                     .getnotifications();              if (notifications != null) {                 (int = 0; < notifications.length; i++) {                     system.out.println("got notification: "                             + notifications[i].getparameter());                 }             }             thread.sleep(500);         } catch (exception e) {             e.printstacktrace();         }     } } } 

these following columns in userdata table:

id cn sn givenname c l st street o ou title description postaladdress telephonenumber email userpassword adorldap 

here stored procedure in users database

    ccreate or replace function get_effected_row_primarykey() returns trigger $$ declare     primarykey text; begin     if (tg_op='update')         if old.cn <> new.cn             select pg_notify('userdata', 'cn');         end if;         if old.sn <> new.sn             select pg_notify('userdata', 'sn');         end if;         if old.givenname <> new.givenname             select pg_notify('userdata', 'givenname');         end if;         if old.c <> new.c             select pg_notify('userdata', 'c');         end if;         if old.l <> new.l             select pg_notify('userdata', 'l');         end if;         if old.st <> new.st             select pg_notify('userdata', 'st');         end if;         if old.street <> new.street             select pg_notify('userdata', 'street');         end if;         if old.o <> new.o             select pg_notify('userdata', 'o');         end if;         if old.ou <> new.ou             select pg_notify('userdata', 'ou');         end if;         if old.title <> new.title             select pg_notify('userdata', 'title');         end if;         if old.description <> new.description             select pg_notify('userdata', 'description');         end if;         if old.postaladdress <> new.postaladdress             select pg_notify('userdata', 'postaladdress');         end if;         if old.telephonenumber <> new.telephonenumber             select pg_notify('userdata', 'telephonenumber');         end if;         if old.email <> new.email             select pg_notify('userdata', 'email');         end if;         if old.userpassword <> new.userpassword             select pg_notify('userdata', 'userpassword');         end if;         if old.adorldap <> new.adorldap             select pg_notify('userdata', 'adorldap');         end if;         return new;     elsif (tg_op='delete')         select pg_notify('userdata', old.id::text);         return old;     elsif (tg_op='insert')         select pg_notify('userdata', new.id::text);         return new;     end if;     return null; end; $$ language plpgsql; 

here trigger

    create trigger create_data after insert or update or delete  on userdata  each row execute procedure get_effected_row_primarykey(); 

java program above works fine, gets notified whenever there insert, update, or delete in table. questions are:

updated stored procedure. when insert table, stored procedure returning following error:

error: query has no destination result data hint: if want discard results of select, use perform instead. context: pl/pgsql function get_effected_row_primarykey() line 59 @ sql statement 

can resolve it.any appreciated

in pl/pgsql function, when issue select command, need specify put result, even when no rows returned.

so in case replace all

select pg_notify('userdata', ...); 

with

perform pg_notify('userdata', ...);