Postgres中的向量（数组）加法 [英] Vector (array) addition in Postgres

问题描述

我有一列具有 numeric [] 值的列，它们的大小均相同。我想按元素进行平均。我的意思是平均

I have a column with numeric[] values which all have the same size. I'd like to take their element-wise average. By this I mean that the average of

{1, 2, 3}, {-1, -2, -3}, and {3, 3, 3}

应为 {1、1 ，1} 。同样有趣的是如何对这些元素进行求和，尽管我希望一个解决方案将是另一个解决方案。

should be {1, 1, 1}. Also of interest is how to sum these element-wise, although I expect that any solution for one will be a solution for the other.

（NB：数组是固定在单个表中的，但是在表之间可能会有所不同。因此，我需要一个没有一定长度的解决方案。）

(NB: The length of the arrays is fixed within a single table, but may vary between tables. So I need a solution which doesn't assume a certain length.)

我最初的猜测是应该以某种方式使用 unnest ，因为 unnest 应用于数字[] 列展平所有数组。因此，我想认为有一种很好的方法将其与某种窗口功能+ group by 一起使用，以挑选出每个数组的各个分量并将其求和。

My initial guess is that I should be using unnest somehow, since unnest applied to a numeric[] column flattens out all the arrays. So I'd like to think that there's a nice way to use this with some sort of windowing function + group by to pick out the individual components of each array and sum them.

-- EXAMPLE DATA
CREATE TABLE A
  (vector numeric[])
;

INSERT INTO A
  VALUES
    ('{1, 2, 3}'::numeric[])
    ,('{-1, -2, -3}'::numeric[])
    ,('{3, 3, 3}'::numeric[])
;

推荐答案

我自己发现了一个解决方案，可能是我将使用一个。

I discovered a solution on my own which is probably the one I will use.

首先，我们可以定义一个用于添加两个向量的函数：

First, we can define a function for adding two vectors:

CREATE OR REPLACE FUNCTION vec_add(arr1 numeric[], arr2 numeric[])
RETURNS numeric[] AS
$$
SELECT array_agg(result)
FROM (SELECT tuple.val1 + tuple.val2 AS result
      FROM (SELECT UNNEST($1) AS val1
                   ,UNNEST($2) AS val2
                   ,generate_subscripts($1, 1) AS ix) tuple
      ORDER BY ix) inn;
$$ LANGUAGE SQL IMMUTABLE STRICT;

和一个乘以常数的函数：

and a function for multiplying by a constant:

CREATE OR REPLACE FUNCTION vec_mult(arr numeric[], mul numeric)
RETURNS numeric[] AS
$$
SELECT array_agg(result)
FROM (SELECT val * $2 AS result
      FROM (SELECT UNNEST($1) AS val
                   ,generate_subscripts($1, 1) as ix) t
      ORDER BY ix) inn;
$$ LANGUAGE SQL IMMUTABLE STRICT;

然后我们可以使用PostgreSQL语句 CREATE AGGREGATE 直接创建 vec_sum 函数：

Then we can use the PostgreSQL statement CREATE AGGREGATE to create the vec_sum function directly:

CREATE AGGREGATE vec_sum(numeric[]) (
    SFUNC = vec_add
    ,STYPE = numeric[]
);

最后，我们可以找到平均值：

And finally, we can find the average as:

SELECT vec_mult(vec_sum(vector), 1 / count(vector)) FROM A;

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