类别：

:doc:`/sql-reference/functions-aggregation`（基数估算）、:doc:`窗口函数 </sql-reference/functions-window-syntax>`（基数估算）

HLL_ACCUMULATE

在聚合结束时返回 HyperLogLog 状态。

有关 HyperLogLog 的更多信息，请参阅 估计非重复值的数量。

HLL 在返回最终基数估计值时放弃其中间状态。在高级用例中，例如批量加载期间的增量基数估计，可能希望保持中间状态。中间状态稍后可以与其他中间状态组合（合并），也可以导出到外部工具。

与 HLL 相反，HLL_ACCUMULATE 不返回基数估计值。相反，它会跳过最终的估计步骤并返回算法状态本身。状态是最多 4096 个字节的二进制值。有关更多信息，请参阅 估计非重复值的数量。

另请参阅：

HLL_COMBINE、 HLL_ESTIMATE

语法

聚合函数

HLL_ACCUMULATE( [ DISTINCT ] <expr> )

HLL_ACCUMULATE(*)

窗口函数

HLL_ACCUMULATE( [ DISTINCT ] <expr> ) OVER ( [ PARTITION BY <expr1> ] )

HLL_ACCUMULATE(*) OVER ( [ PARTITION BY <expr1> ] )

有关 OVER 子句的详细信息，请参阅 窗口函数的语法和用法。

实参

expr

要估计基数（非重复值数）的表达式。这通常是列名，但也可以是更通用的表达式。

使用说明

示例

这显示了估计加拿大各省不同邮政编码数量的一步。在此步骤中，我们计算了马尼托巴省不同邮政编码的近似数量，并存储了计算“状态”的内部表示，稍后我们可以将其与其他省份的类似信息相结合：

CREATE TABLE temporary_hll_state_for_manitoba AS
  SELECT HLL_ACCUMULATE(postal_code) AS h_a_p_c
    FROM postal_data
    WHERE province = 'Manitoba';

Here is another example. This example shows how to use the three related functions HLL_ACCUMULATE, HLL_ESTIMATE, and HLL_COMBINE.

创建简单的表和数据：

CREATE OR REPLACE SEQUENCE seq92;
CREATE OR REPLACE TABLE sequence_demo (c1 INTEGER DEFAULT seq92.nextval, dummy SMALLINT);
INSERT INTO sequence_demo (dummy) VALUES (0);

INSERT INTO sequence_demo (dummy) SELECT dummy FROM sequence_demo;
INSERT INTO sequence_demo (dummy) SELECT dummy FROM sequence_demo;
INSERT INTO sequence_demo (dummy) SELECT dummy FROM sequence_demo;

Create a table that contains the "state" that represents the current approximate cardinality information for the table named sequence_demo:

CREATE OR REPLACE TABLE resultstate1 AS (
  SELECT HLL_ACCUMULATE(c1) AS rs1
    FROM sequence_demo);

Now create a second table and add data. (In a more realistic situation, the user could have loaded more data into the first table and divided the data into non-overlapping sets based on the time that the data was loaded.)

CREATE OR REPLACE TABLE test_table2 (c1 INTEGER);
INSERT INTO test_table2 (c1) SELECT c1 + 4 FROM sequence_demo;

仅获取新数据的“状态”信息。

CREATE OR REPLACE TABLE resultstate2 AS
  (SELECT HLL_ACCUMULATE(c1) AS rs1
     FROM test_table2);

合并两批行的“状态”信息：

CREATE OR REPLACE TABLE combined_resultstate (c1) AS
  SELECT HLL_COMBINE(rs1) AS apc1
    FROM (
      SELECT rs1 FROM resultstate1
      UNION ALL
      SELECT rs1 FROM resultstate2
    );

获取行组合集的近似基数：

SELECT HLL_ESTIMATE(c1)
  FROM combined_resultstate;

+------------------+
| HLL_ESTIMATE(C1) |
|------------------|
|               12 |
+------------------+