Array access performance

Hi,

I'm looking for a hint how array access performs in PostgreSQL in respect to performance. Normally I would expect
accessof a 1-dimensional Array at slot i (array[i]) to perform in constant time (random access). 

Is this also true for postgres' arrays?

May concrete example is a 1-dimensional array d of length <= 600 (which will grow at a rate of 1 entry/day) stored in a
table'scolumn. I need to access this array two times per tuple, i.e. d[a], d[b]. Therefore I hope access is not linear.
Isthis correct? 

Also I'm having some performance issues building this array. I'm doing this with a used-defined aggregate function,
startingwith an empty array and using array_append and some calculation for each new entry. I assume this involves some
copying/memoryallocation on each call, but I could not find the implementation of array_append in postgres-source/git.  

Is there an efficient way to append to an array? I could also start with a pre-initialized array of the required
length,but this involves some complexity. 

Thank you

Regards,
Andreas