Hierarchical rollups.

I am looking to model a hierarchical structure where the parent nodes on
each level will contain calculations based on immediate children's values.
For example the parent row may have an average of all the child rows (the
calculations will be more complicated than that of course).

So every time a record changes this is going to possibly trigger an
aggregation function on the parent which will trigger an aggregation on the
parent of that record etc until the root node is calculated.

I am wondering if it's more efficient to build very complicated selects
which do the calculations on the fly or whether to put the calculation
results in the rows to make selects more efficient but make the writes more
inefficient.

At this time I am not planning on using ltree but I would be interested in
hearing your experiences with ltree as well.

TIA