GROUPBY HASH Query Plan

Here's an example of how GROUPBY HASH operations look in EXPLAIN output. 

=> EXPLAIN SELECT COUNT(DISTINCT annual_income) 
     FROM customer_dimension 
     WHERE customer_region='NorthWest';

The output shows that the optimizer chose the less efficient GROUPBY HASH path, which means the projection was not presorted on the annual_income column. If such a projection is available, the optimizer would choose the GROUPBY PIPELINED algorithm.

Access Path:
+-GROUPBY NOTHING [Cost: 256, Rows: 1 (NO STATISTICS)] (PATH ID: 1)
|  Aggregates: count(DISTINCT customer_dimension.annual_income)
| +---> GROUPBY HASH (LOCAL RESEGMENT GROUPS) [Cost: 253, Rows: 10K (NO STATISTICS)] (PATH ID: 2)
| |      Group By: customer_dimension.annual_income
| | +---> STORAGE ACCESS for customer_dimension [Cost: 227, Rows: 50K (NO STATISTICS)] (PATH ID: 3)
| | |      Projection: public.customer_dimension_super
| | |      Materialize: customer_dimension.annual_income
| | |      Filter: (customer_dimension.customer_region = 'NorthWest'
...