This query generated a single result set with the aggregates for all grouping sets.
Even though the above query works as you expected, it has two main problems.
- First, it is quite lengthy.
- Second, it has a performance issue because PostgreSQL has to scan the
salestable separately for each query.
To make it more efficient, PostgreSQL provides the GROUPING SETS clause which is the subclause of the GROUP BY clause.
The GROUPING SETS allows you to define multiple grouping sets in the same query.
The general syntax of the GROUPING SETS is as follows:
SELECT c1, c2, aggregate_function(c3) FROM table_name GROUP BY GROUPING SETS ( (c1, c2), (c1), (c2), () );Code language: SQL (Structured Query Language) (sql)
In this syntax, we have four grouping sets (c1,c2), (c1), (c2), and ().
To apply this syntax to the above example, you can use GROUPING SETS clause instead of the UNION ALL clause like this:
SELECT brand, segment, SUM (quantity) FROM sales GROUP BY GROUPING SETS ( (brand, segment), (brand), (segment), () );Code language: SQL (Structured Query Language) (sql)