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cláusula CLUSTER BY (SELECT)

  • Artículo

Se aplica a:casilla marcada como Sí Databricks SQL casilla marcada como Sí Databricks Runtime

Reparticiona los datos basadas en las expresiones de entrada y, a continuación, ordena los datos dentro de cada partition. Esto es semánticamente equivalente a realizar un DISTRIBUTE BY seguido de un SORT BY. Esta cláusula solo garantiza que las filas resultantes se ordenan dentro de cada partition y no garantiza un orden total de salida.

Sintaxis

CLUSTER BY expression [, ...]

Parameters

  • expression

    Especifica la combinación de una o varias values, operadores y funciones SQL que dan como resultado un valor.

Ejemplos

> 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