HLL_COMBINE_SKETCHES function - Amazon Redshift
SyntaxArgumentReturn typeExamples

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;