1.4.6 Aggregate Cost: Choosing HashAgg vs GroupAgg (opens in new tab)

A query like SELECT region, COUNT(*) FROM sales GROUP BY region folds many rows together, collapsing each group into a single value. This folding of many rows into one is aggregation, and COUNT, SUM, AVG are the familiar examples. PostgreSQL handles aggregation in the execution plan with one of two nodes: HashAggregate or GroupAggregate. Both do the same job, grouped aggregation, but they go about it differently, and the planner picks one or the other to nail into the plan tree for a given qu...