HLL_ COMBINE _ SKETCHES 功能

該 HLL COMBINE _ SKETCHES 是一個標量函數，它接受作為輸入兩個HLLSKETCH值，並將它們組合成一個單HLLSKETCH一的。

兩個或多個 HyperLogLog 草圖的組合是一個新的HLLSKETCH，它封裝有關每個輸入草圖所代表的不同值的聯集的信息。

語法

HLL_COMBINE_SKETCHES (hllsketch_expression1, hllsketch_expression2)

引數

hllsketch_expression1 和 hllsketch_expression2

評估為HLLSKETCH類型的任何有效運算式，例如欄名稱。

傳回類型

該 HLL COMBINE _ SKETCHES 函數返回一個HLLSKETCH類型。

範例

下列範例會傳回資料表中的組合HLLSKETCH值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;