CLUSTER BY-sats (SELECT)

  • Artikel

Gäller för:check marked yes Databricks SQL check marked yes Databricks Runtime

Partitionerar om data baserat på indatauttrycken och sorterar sedan data inom varje partition. Detta är semantiskt likvärdigt med att utföra en DISTRIBUTE BY följt av en SORT BY. Den här satsen säkerställer bara att de resulterande raderna sorteras inom varje partition och inte garanterar en total ordning på utdata.

Syntax

CLUSTER BY expression [, ...]

Parametrar

  • Uttryck

    Anger en kombination av ett eller flera värden, operatorer och SQL-funktioner som resulterar i ett värde.

Exempel

> 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