user aggregate function ( calculate the average value of each index of an array column )

Hi,

I have a table with several lines as following;

- Create table mytable (type number , values integer [2]) ;

- Insert into mytable values (1, '{ 10, 0 }' );

- Insert into mytable values (1, '{ 20, 30 }' );

- Insert into mytable values (2, '{30, 60}' );

(In fact, the array size is very big (ex. values [10000]) but the size is
fix. In order to simplify the example, I used an array integer [2]).

I would like to obtain the average value of each index of values column.

Is it possible to create an aggregate function which can works as following
? :

(Suppose that avg_mytable is the aggregation function name.)

Ex1) Select avg_mytable (values) from mytable ;

avg_mytable (values)

------------------------

{ 20, 30}

(- Explication of the results: 20 because (10+20+30)/3 , 30 because
(0+30+60)/3)

Ex2) Select type, avg_mytable (values) from mytable Group by type ;

Type | avg_mytable (values)

---------------------------------------------

1 | { 15, 15}

2 | { 30, 60}

I searched in the documentation for "array functions" but I could not find
functions useful for me...

Thank you so much,

Lea