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cláusula CLUSTER BY (SELECT)

  • Artigo

Aplica-se a:marca de seleção positiva SQL do Databricks marca de seleção positiva Runtime do Databricks

Reparticiona os dados com base nas expressões de entrada e, em seguida, classifica os dados dentro de cada partition. Isso é semanticamente equivalente à execução de um DISTRIBUTE BY seguido por um SORT BY. Essa cláusula garante apenas que as linhas resultantes sejam classificadas dentro de cada partition e não garante uma ordem total de saída.

Sintaxe

CLUSTER BY expression [, ...]

Parameters

  • expression

    Especifica a combinação de um ou mais values, operadores e funções SQL que resultam em um valor.

Exemplos

> 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