json - Postgres : Nested Aggregate JSON

Question

我正在查询的表如下所示:

namespace | key   | value
---------------------------
foo       | bar   | baz
foo       | alpha | beta
gamma     | delta | epsilon

我想像这样从数据库中提取它:

{
    "foo": {
        "bar": "baz",
        "alpha": "beta"
    },
    "gamma": {
        "delta": "epsilon"
    }
}

尝试使用 json_object_agg 并不能真正让我通过第一级，因为不允许嵌套聚合函数。但据我所知，我需要在 GROUP BY 中使用 GROUP BY，但我不确定这是否可行。也许解决方案与 WINDOW 有关？

Answer 1

with t (namespace, key, value) as (
    values
    ('foo','bar','baz'),('foo','alpha','beta'),('gamma','delta','epsilon')
), s as (
    select namespace, json_object_agg(key, value) as joa
    from t
    group by namespace
)
select json_object_agg(namespace, joa)
from s
;
                                  json_object_agg                                   
------------------------------------------------------------------------------------
 { "foo" : { "bar" : "baz", "alpha" : "beta" }, "gamma" : { "delta" : "epsilon" } }

由于 CTE 是一个优化障碍，这个版本可能会更快:

with t (namespace, key, value) as (
    values
    ('foo','bar','baz'),('foo','alpha','beta'),('gamma','delta','epsilon')
)
select json_object_agg(namespace, joa)
from (
    select namespace, json_object_agg(key, value) as joa
    from t
    group by namespace
) s