How to split an array into columns

[a]

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!