Mastering PostgreSQL 12 - Third Edition

Using grouping sets

Grouping sets are very useful if we want to run more than one aggregate at the same time. Using grouping sets can speed up aggregation because we don't have to process the data more than once.

Here is an example of this:

test=# SELECT x % 2, array_agg(x)          FROM generate_series(1, 4) AS x          GROUP BY ROLLUP (1);  ?column? | array_agg ----------+-----------         0 | {2,4}         1 | {1,3}           | {2,4,1,3} (3 rows) 

PostgreSQL offers more than just the ROLLUP clause. The CUBE and GROUPING SETS clauses are also supported.