How to split an array into columns

Say if I have an float8 array:

id| data
--|---------------
a | {1,2}
b | {2,4}

If I could using query to make it looks like this:

id| data[1] | data[2]
--|----------|-----------
a | 1 | 2
b | 2 | 4

Since I would have around 200,000 rows, I would prefer it having enough capacity to carry out the calculation such as sum().

Thank you so much!