arrays - BigQuery 参数最大值 : Is array order maintained when doing CROSS JOIN UNNEST

Question

问题:

在 BigQuery 中，标准 SQL，如果我运行

SELECT *
FROM mytable
CROSS JOIN UNNEST(mytable.array)

我可以确定生成的行顺序与数组顺序相同吗？

示例:

假设我有下表mytable:

Row | id   | prediction
1   | abcd | [0.2, 0.5, 0.3]

如果我运行 SELECT * FROM mytable CROSS JOIN UNNEST(mytable.prediction)，我可以确定行顺序与数组顺序相同吗？ IE。结果表是否总是:

Row | id   | unnested_prediction
1   | abcd | 0.2
2   | abcd | 0.5
3   | abcd | 0.3

更多用例背景(argmax):

我正在尝试在每行 (argmax) 中查找数组中具有最大值的数组索引，即上面数组中的第二个元素 (0.5)。因此，我的目标输出是这样的:

Row | id   | argmax
1   | abcd | 2

使用 CROSS JOIN、一个由 prediction 值排序的 DENSE_RANK 窗口函数和一个 ROW_NUMBER 窗口函数来找到 argmax，我可以使用一些测试数据来完成这项工作。您可以使用此查询进行验证:

WITH predictions AS (
  SELECT 'abcd' AS id, [0.2, 0.5, 0.3] AS prediction
  UNION ALL
  SELECT 'efgh' AS id, [0.7, 0.2, 0.1] AS prediction
),
ranked_predictions AS (
  SELECT 
    id,
    ROW_NUMBER() OVER (PARTITION BY id) AS rownum, -- This is the ordering I'm curious about
    DENSE_RANK() OVER (PARTITION BY id ORDER BY flattened_prediction DESC) AS array_rank
  FROM
     predictions P
  CROSS JOIN
    UNNEST(P.prediction) AS flattened_prediction
)
SELECT
  id,
  rownum AS argmax
FROM
  ranked_predictions
WHERE array_rank = 1

ROW_NUMBER 在我的测试中表现良好(即它根据未嵌套数组排序)可能只是巧合，所以很高兴能确定。

Answer 1

Can I be certain that the resulting row order is the same as the array order?

您应该使用 WITH OFFSET 来获取数组中元素的位置，这样您就可以使用它们在您的进一步逻辑中进行排序

#standardSQL
WITH `project.dataset.table` AS (
  SELECT 'abcd' id, [0.2, 0.5, 0.3] prediction
)
SELECT id, unnested_prediction
FROM `project.dataset.table`, 
UNNEST(prediction) unnested_prediction WITH OFFSET pos
ORDER BY id, pos