Trigger to update table info

Hello friends,

I have a table with the following structure

Create table postablestatus

(

tablename
varchar(30) NOT NULL,

updatetime timestamp,

reccount
int,

size
int,

CONSTRAINT postablestatus_pkey PRIMARY KEY(tablename)

)
WITH (oids = false);

where
a) tablename - is the name of table where a record is inserted,updated or deleted
b) updatetime - the time when table was updated (w/o timezone)
c) reccount - records in table
d) size - bytesize of the table

--
for example, if a table called item_master had 1 record and a
new record is inserted and it's size is 2000 bytes

> postablestatus must have a entry like

item_master 08/12/2013 12:40:00 2 2000
--

I request to know how to write a trigger in PLSQL so that the postablestatus
is updated whenever a record is inserted,updated or deleted in item_master

PLEASE NOTE : a new record must be created if the item_master is not present in postablestatus

thanks in advance
arvind