CLUSTER BY clause (SELECT)
Applies to: Databricks SQL Databricks Runtime
Repartitions the data based on the input expressions and then sorts the data within each partition. This is semantically equivalent to performing a DISTRIBUTE BY followed by a SORT BY. This clause only ensures that the resultant rows are sorted within each partition and does not guarantee a total order of output.
Parameters
-
Specifies combination of one or more values, operators and SQL functions that results in a value.
Examples
> CREATE TEMP VIEW person (name, age)
AS VALUES ('Zen Hui', 25),
('Anil B', 18),
('Shone S', 16),
('Mike A', 25),
('John A', 18),
('Jack N', 16);
-- Reduce the number of shuffle partitions to 2 to illustrate the behavior of `CLUSTER BY`.
-- It's easier to see the clustering and sorting behavior with less number of partitions.
> SET spark.sql.shuffle.partitions = 2;
-- Select the rows with no ordering. Please note that without any sort directive, the results
-- of the query is not deterministic. It's included here to show the difference in behavior
-- of a query when `CLUSTER BY` is not used vs when it's used. The query below produces rows
-- where age column is not sorted.
> SELECT age, name FROM person;
16 Shone S
25 Zen Hui
16 Jack N
25 Mike A
18 John A
18 Anil B
-- Produces rows clustered by age. Persons with same age are clustered together.
-- In the query below, persons with age 18 and 25 are in first partition and the
-- persons with age 16 are in the second partition. The rows are sorted based
-- on age within each partition.
> SELECT age, name FROM person CLUSTER BY age;
18 John A
18 Anil B
25 Zen Hui
25 Mike A
16 Shone S
16 Jack N