在这种情况下不理解 json_agg

Question

参考：

begin;
CREATE TEMP TABLE segments (segments_id serial PRIMARY KEY, payload jsonb);
INSERT INTO segments (payload)
VALUES ('[{"kind": "person", "limit": "1"}, {"kind": "B", "filter_term": "fin"}]');
INSERT INTO segments (payload)
VALUES ('[{"kind": "person", "limit": "3"}, {"kind": "A", "filter_term": "abc"}]');
INSERT INTO segments (payload)
VALUES ('[{"kind": "person", "limit": "2"}, {"kind": "C", "filter_term": "def"}]');
commit;

CTE 查询：

 with a  as (select jsonb_array_elements(s.payload) j from segments s)
    SELECT  json_agg(a.j) AS filtered_payload from a
    where j @> '{"kind":"person"}';

Return: [{"kind": "person", "limit": "1"}, {"kind": "person", "limit": "3"}, {"kind": "person", "limit": "2"}]

这个查询A：

SELECT a.filtered_payload,
         a.ct_elem_row
        , sum(ct_elem_row) OVER () AS ct_elem_total
     , count(*)         OVER () AS ct_rows
FROM   segments s
           JOIN   LATERAL (
    SELECT json_agg(j.elem) AS filtered_payload, count(*) AS ct_elem_row
    FROM   jsonb_array_elements(s.payload) j(elem)
    WHERE  j.elem @> '{"kind":"person"}'
        ) a ON ct_elem_row > 0
WHERE  s.payload @> '[{"kind":"person"}]';

return :

在 QueryA 中，结构如下：select ... from segments s join lateral filtered_payload.... segments 是 3 行横向连接与一行 (filtered_payload)。 filtered_payload return 只有根据 CTE 查询的行，作为合并 JSON 数组。所以总的来说，我对 QueryA 中的 json_agg 很困惑。

编辑于 2021-10-05 16:36 +5:30：即使是下面的代码，a.filtered_payload return 3 jsonb 数组，而不是一个 arrgregate json 数组。我不知道什么时候已经聚合的 jsonb 数组（使用 json_agg 函数）未嵌套到服务 jsonb 数组。

 SELECT a.filtered_payload, s.*
FROM   segments s
           cross  JOIN   LATERAL (
    SELECT json_agg(j.elem) AS filtered_payload
    FROM   jsonb_array_elements(s.payload) j(elem)
    WHERE  j.elem @> '{"kind":"person"}') a;

Answer 1

我相信 LATERAL JOIN 正在发挥作用。在您的原始查询中，您对整个 table 数据集使用 json_agg（按 '{"kind":"person"}' 过滤）

with a  as 
    (
        select jsonb_array_elements(s.payload) j 
        from segments s
    )
SELECT  json_agg(a.j) AS filtered_payload 
from a
where j @> '{"kind":"person"}';

与此同时，在第二个实例中，您当时正在使用 LATERAL 玩一行。这就是为什么您最终有 3 行具有单个 "kind":"person" 值，而不是具有 3 个值的唯一行。

不确定您要实现的目标，但以下内容可以为您指明正确的方向

SELECT a.filtered_payload,
    a.ct_elem_row, 
    sum(ct_elem_row) OVER () AS ct_elem_total, 
    count(*)         OVER () AS ct_rows
FROM segments s
 JOIN LATERAL (
    SELECT json_agg(j.elem) AS filtered_payload, 
        count(*) AS ct_elem_row
    FROM   segments d, lateral jsonb_array_elements(d.payload) j(elem)
    WHERE  j.elem @> '{"kind":"person"}'
    ) a ON ct_elem_row > 0
WHERE  s.payload @> '[{"kind":"person"}]';

结果

                                            filtered_payload                                            | ct_elem_row | ct_elem_total | ct_rows
--------------------------------------------------------------------------------------------------------+-------------+---------------+---------
 [{"kind": "person", "limit": "1"}, {"kind": "person", "limit": "3"}, {"kind": "person", "limit": "2"}] |           3 |             9 |       3
 [{"kind": "person", "limit": "1"}, {"kind": "person", "limit": "3"}, {"kind": "person", "limit": "2"}] |           3 |             9 |       3
 [{"kind": "person", "limit": "1"}, {"kind": "person", "limit": "3"}, {"kind": "person", "limit": "2"}] |           3 |             9 |       3
(3 rows)

在这种情况下不理解 json_agg

not understand json_agg in this context

postgresql

jsonb