Klausa CLUSTER BY (SELECT)
Berlaku untuk:
Databricks SQL Databricks Runtime
Mempartisi ulang data berdasarkan ekspresi input dan kemudian mengurutkan data dalam setiap partisi. Ini secara semantik setara dengan melakukan DISTRIBUTE BY diikuti oleh SORT BY. Klausul ini hanya memastikan bahwa baris yang dihasilkan diurutkan dalam setiap partisi dan tidak menjamin urutan total output.
Sintaks
CLUSTER BY expression [, ...]
Parameter
-
Menentukan kombinasi satu atau lebih nilai, operator, dan fungsi SQL yang menghasilkan nilai.
Contoh
> 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