Clausola CLUSTER BY (SELECT)

Si applica a: Databricks SQL Databricks Runtime

Ripartiziona i dati in base alle espressioni di input e quindi ordina i dati all'interno di ogni partizione. Ciò equivale semanticamente all'esecuzione di un oggetto DISTRIBUTE BY seguito da un SORT BY. Questa clausola garantisce solo l'ordinamento delle righe risultanti all'interno di ogni partizione e non garantisce un ordine totale di output.

Sintassi

CLUSTER BY expression [, ...]

Parametri

• expression

  Specifica la combinazione di uno o più valori, operatori e funzioni SQL che generano un valore.

Esempi

> 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

Articoli correlati

• Query
• DISTRIBUTE BY
• SORT BY