Calculates reduction of tree node where self is value of the current row and list represents values passed from children (it
may be or be not values of children rows depending on strategy type).
Reduces the list of values to one value. Describes 'flat' reduction of elements without hierarchy. Called reducing direct children values or
from merge methods if not overridden. Can be called from convert method
Calculates reduction of tree node where self is value of the current row, and reduced is result of flat reduction. Value
modifications can be done here if one wants to build reduction that computes value based on depth in the structure. Called from strict subtree
reduction strategy or from other merge method if not optimized.
Calculates reduction of tree node where self is value of the current row and list represents values passed from children (it
may be or be not values of children rows depending on strategy type). Value modifications can be done here if one wants to build reduction
that computes value based on depth in the structure. Called from full subtree reduction strategy, that is default and the most frequently
used strategy for most reducing aggregates (it makes this method the most probable target for optimization).