Also called: Volcano model, iterator model, pull-based execution
In plain English
Demand-pull execution means rows flow because the top of the plan asks for them. Each node, when asked for a tuple, asks its own children for tuples, and so on down to the scans. Nothing is pushed; everything is pulled from above, one row at a time.
Why it matters
This model needs no central scheduler and naturally supports early stopping: a LIMIT simply stops pulling. It also explains why PostgreSQL streams results instead of building the whole answer first — except where a blocking node has to. The classic name comes from the 1990s Volcano research system.