Reading all tuples in Index Access Method

I'm trying to create an Index Access Method Roting.

Building the index requires iterating over all rows and calculating, 

which is then used during index construction.

The methods in the IndexAmRoutine seem to deal with insertion / index build one row at a time.

Is there any workaround you can suggest that would allow me to calculate the median of a column, 

store it someplace and then use it during Inserts to the index?

Thanks,

Sushrut