近似直方图函数（APPROX_HISTOGRAM）

功能描述

approx_histogram 函数用于根据给定的数值型列 col 和直方图的桶数 n，生成一个近似的直方图。该函数返回一个数组，数组中的每个元素是一个结构体（STRUCT），包含三个字段：最小值（min）、最大值（max）和计数（count）。该函数在统计数据分布时非常有用，尤其是在处理大量数据时，可以快速了解数据的分布情况。

参数说明

• col：需要生成直方图的数值型列。
• n：直方图的桶数，必须是一个大于 0 的整数常量。

返回结果

函数返回一个结构体数组，每个结构体（代表一个桶）包含三个字段：

• min：直方图桶的最小值。
• max：直方图桶的最大值。
• count：落在该桶内的数据数量。

使用示例

以下示例展示了如何使用 approx_histogram 函数生成不同桶数的直方图。

示例 1：生成 2 个桶的直方图

SELECT approx_histogram(col, 2) FROM VALUES (0), (1), (2), (3), (4) AS t(col);
+-------------------------------------------------------------------+
|                     approx_histogram(col, 2)                      |
+-------------------------------------------------------------------+
| [{"min":0.0,"max":0.5,"count":1},{"min":0.5,"max":4.0,"count":4}] |
+-------------------------------------------------------------------+

示例 2：生成 5 个桶的直方图

SELECT approx_histogram(col, 5) FROM VALUES (0), (1), (2), (3), (4) AS t(col);
+-------------------------------------------------------------------------------------------------------------------------------------------------------------------+
|                                                                     approx_histogram(col, 5)                                                                      |
+-------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| [{"min":0.0,"max":0.0,"count":1},{"min":0.0,"max":1.0,"count":1},{"min":1.0,"max":2.0,"count":1},{"min":2.0,"max":3.0,"count":1},{"min":3.0,"max":4.0,"count":2}] |
+-------------------------------------------------------------------------------------------------------------------------------------------------------------------+

示例 3：生成 10 个桶的直方图

SELECT approx_histogram(col, 10)
FROM VALUES (0), (0.1), (0.2), (0.3), (0.4), (0.5), (0.6), (0.7), (0.8), (0.9) AS t(col);
+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
|                                                                                                                                    approx_histogram(col, 10)                                                                                                     |
+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| [{"min":0.0,"max":0.0,"count":1},{"min":0.0,"max":0.1,"count":1},{"min":0.1,"max":0.2,"count":1},{"min":0.2,"max":0.3,"count":1},{"min":0.3,"max":0.4,"count":1},{"min":0.4,"max":0.5,"count":1},{"min":0.5,"max":0.6,"count":1},{"min":0.6,"max":0.7,"count":1}] |
+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+