Categories:: Aggregate functions (Percentile Estimation) , Window function syntax and usage

APPROX_PERCENTILE_COMBINE¶

Combines (merges) percentile input states into a single output state.

This allows scenarios where APPROX_PERCENTILE_ACCUMULATE is run over horizontal partitions of the same table, producing an algorithm state for each table partition. These states can later be combined using APPROX_PERCENTILE_COMBINE, producing the same output state as a single run of APPROX_PERCENTILE_ACCUMULATE over the entire table.

See also:: APPROX_PERCENTILE_ACCUMULATE , APPROX_PERCENTILE_ESTIMATE

Syntax¶

APPROX_PERCENTILE_COMBINE( <state> )

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Arguments¶

state: An expression that contains state information generated by a call to APPROX_PERCENTILE_ACCUMULATE.

Example¶

Return an approximation for the median of numbers in the testtable.c2 column (0.5 means the 50th percentile):

CREATE OR REPLACE TABLE mytesttable AS
  SELECT APPROX_PERCENTILE_COMBINE(td) s FROM
    (
      (SELECT APPROX_PERCENTILE_ACCUMULATE(c2) td FROM testtable WHERE c2 <= 0)
        UNION ALL
      (SELECT APPROX_PERCENTILE_ACCUMULATE(c2) td FROM testtable WHERE c2 > 0 AND c2 <= 0.5)
        UNION ALL
      (SELECT APPROX_PERCENTILE_ACCUMULATE(C2) td FROM testtable WHERE c2 > 0.5)
    );

SELECT APPROX_PERCENTILE_ESTIMATE(s , 0.5) FROM mytesttable;

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Return an approximate value for the 2nd percentile of numbers in mytest.s1 union mytest2.s2.

CREATE OR REPLACE TABLE mytest AS (SELECT APPROX_PERCENTILE_ACCUMULATE(c2) s1 FROM testtable WHERE c2 < 0);

CREATE OR REPLACE TABLE mytest2 AS (SELECT APPROX_PERCENTILE_ACCUMULATE(c2) s1 FROM testtable WHERE c2 >= 0);

CREATE OR REPLACE TABLE combinedtable AS
  SELECT APPROX_PERCENTILE_COMBINE(s) combinedstate FROM
    (
      (SELECT s1 s FROM mytest)
        UNION ALL
      (SELECT s1 s FROM mytest2)
    );

SELECT APPROX_PERCENTILE_ESTIMATE(combinedstate , 0.02) FROM combinedtable;

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For a more extensive example, see the Examples section in APPROX_PERCENTILE_ACCUMULATE.