HLL_COMBINE_SKETCHES function - Amazon Redshift
SyntaxArgumentReturn typeExamples
Services or capabilities described in Amazon Web Services documentation might vary by Region. To see the differences applicable to the China Regions, see Getting Started with Amazon Web Services in China (PDF).

HLL_COMBINE_SKETCHES function

The HLL_COMBINE_SKETCHES is a scalar function that takes as input two HLLSKETCH values and combines them into a single HLLSKETCH.

The combination of two or more HyperLogLog sketches is a new HLLSKETCH that encapsulates information about the union of the distinct values that each input sketch represents.

Syntax

HLL_COMBINE_SKETCHES (hllsketch_expression1, hllsketch_expression2)

Argument

hllsketch_expression1 and hllsketch_expression2

Any valid expression that evaluates to an HLLSKETCH type, such as a column name.

Return type

The HLL_COMBINE_SKETCHES function returns an HLLSKETCH type.

Examples

The following example returns the combined HLLSKETCH values in the table hll_table.

WITH tbl1(x, y)
     AS (SELECT Hll_create_sketch(1),
                Hll_create_sketch(2)
         UNION ALL
         SELECT Hll_create_sketch(3),
                Hll_create_sketch(4)
         UNION ALL
         SELECT Hll_create_sketch(5),
                Hll_create_sketch(6)
         UNION ALL
         SELECT Hll_create_sketch(7),
                Hll_create_sketch(8)),
     tbl2(x, y)
     AS (SELECT Hll_create_sketch(9),
                Hll_create_sketch(10)
         UNION ALL
         SELECT Hll_create_sketch(11),
                Hll_create_sketch(12)
         UNION ALL
         SELECT Hll_create_sketch(13),
                Hll_create_sketch(14)
         UNION ALL
         SELECT Hll_create_sketch(15),
                Hll_create_sketch(16)
         UNION ALL
         SELECT Hll_create_sketch(NULL),
                Hll_create_sketch(NULL)),
     tbl3(x, y)
     AS (SELECT *
         FROM   tbl1
         UNION ALL
         SELECT *
         FROM   tbl2)
SELECT Hll_combine_sketches(x, y)
FROM   tbl3;