2个数组中具有相同索引的元素的乘积答案

【问题标题】：Product of elements with same index in 2 arrays2个数组中具有相同索引的元素的乘积
【发布时间】：2017-11-05 18:24:53
【问题描述】：

我需要将 2 个数组的每个元素相乘并投影一列，该列是一个数组，每个元素都是乘积结果。

例子：

select * from vetor_query;

query_id |pesos                                                                                               |
---------|----------------------------------------------------------------------------------------------------|
1        |{2.0000,0.4150,2.0000,0.0000,0.0000,0.0000,0.0000,0.0000,0.0000,0.0000,0.0000,0.0000,0.0000,0.0000} |

还有查询：

select * from vetor_documento;

doc    |pesos                                                                                               |
-------|----------------------------------------------------------------------------------------------------|
d1.txt |{3.0000,0.8301,4.0000,0.0000,0.0000,0.0000,0.0000,0.0000,0.0000,0.0000,0.0000,0.0000,0.0000,0.0000} |
d2.txt |{2.0000,0.0000,0.0000,0.0000,2.0000,2.0000,2.0000,2.0000,2.0000,0.0000,0.0000,0.0000,0.0000,0.0000} |
d3.txt |{0.0000,1.0729,0.0000,0.0000,0.0000,0.0000,2.0000,1.0000,0.0000,2.0000,2.0000,0.0000,0.0000,0.0000} |
d4.txt |{0.0000,1.0729,0.0000,0.0000,0.0000,0.0000,0.0000,0.0000,0.0000,0.0000,0.0000,5.1699,4.0000,4.0000} |

我需要结合两个查询（交叉连接）并为每个 doc 和 query_id 生成内部产品的结果数组。

我的第一次尝试是这个：

select vq.query_id, vd.doc, unnest(vq.pesos) * unnest(vd.pesos)
from vetor_query vq
cross join vetor_documento vd;

但是，它会产生这个错误：

函数和运算符最多可以接受一个集合参数

【问题讨论】：

标签： sql arrays postgresql unnest

【解决方案1】：

您可以使用unnest() 函数的便捷功能来并行取消嵌套多个数组。

在 LATERAL 连接中执行此操作，将每个结果行相乘并将其提供给 ARRAY 构造函数：

SELECT q.query_id, d.doc, qd.prod
FROM   vetor_query          q
CROSS  JOIN vetor_documento d
CROSS  JOIN LATERAL (
   SELECT ARRAY(SELECT x*y FROM unnest(q.pesos, d.pesos) t(x, y)) AS prod
   ) qd;

这是假设所有数组都具有相同的长度，否则多余的元素用 NULL 填充。