after update the table insert data like arrays - postgresql

Question

I have two tables
Table A
col1 col2 col3
Table B
table_name column_name new_value old_value
if any update happened on table A it will insert the data on table B
the output of table B is==>
table_name column_name new_value old_value
---------------- ------------------ -------------- -----------
A {col1}
A {col1,col2} {col1.new_value, {col1.old_value,
col2.new_value} col2.old_value},
so anyone can tell me how to capture the column_names and it store data target table like arrays

Answer 1

Try this
Use Trigger Function
CREATE OR REPLACE FUNCTION update_history()
RETURNS trigger AS
$BODY$
DECLARE col_name VARCHAR[];
DECLARE od_value VARCHAR[];
DECLARE ne_value VARCHAR[];
DECLARE each_column RECORD;
DECLARE each_entity RECORD;
DECLARE column_name VARCHAR;
DECLARE old_value VARCHAR;
DECLARE new_value VARCHAR;
FOR each_column IN
select c.column_name --- Get the all column names in affected table
from information_schema.columns c
where(table_name = tg_relname And c.TABLE_SCHEMA = TG_TABLE_SCHEMA)
LOOP
FOR each_entity IN --- Its used to get old and new columns value
EXECUTE 'SELECT text((' || quote_literal(OLD.*) || '::"' || tg_table_schema || '"."' || tg_relname || '")."' || each_column.column_name || '") as old_val,
text((' || quote_literal(NEW.*) || '::"' || tg_table_schema || '"."' || tg_relname || '")."' || each_column.column_name || '")
AS new_val
FROM "' || tg_table_schema || '"."' || tg_relname || '";'
LOOP
old_value = each_entity.old_val;
new_value = each_entity.new_val;
IF old_value != new_value THEN
i=i+1;
col_name[i]=each_column.column_name;
od_value[i]=old_value;
ne_value[i]=new_value;
END IF;
END LOOP;
END LOOP;
INSERT INTO B
(
tablename,
columnnames,
oldvalues,
newvalues
)
VALUES
(
tg_relname,
col_name,
od_value,
ne_value
);
End if;
RETURN NEW;
END;
$BODY$
LANGUAGE plpgsql VOLATILE

Answer 2

I thinks it's a good case to use hstore extension of PostgreSQL:
create or replace function history_trigger_func()
returns trigger AS
$$
begin
insert into TableB
select
tg_relname,
case when tg_op in ('UPDATE', 'INSERT') then hstore(new) end,
case when tg_op in ('UPDATE', 'DELETE') then hstore(old) end;
return null;
end;
$$
language plpgsql;
create trigger tr_history_trigger after insert or update or delete on TableA
for each row execute procedure history_trigger_func();
sql fiddle demo
You can extend this further by removing columns which are not changing, or, if you're using PostgreSQL 9.3, you can use JSON instead of hstore.