Postgres 13

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WITH mytable AS (
    SELECT '[{"key1":10},{"key1":20},{"key1":30}]'::json as mydata
)

SELECT
    *
FROM mytable,
    json_array_elements(mydata) as elements

mydata
value

[{"key1":10},{"key1":20},{"key1":30}]
{"key1":10}

[{"key1":10},{"key1":20},{"key1":30}]
{"key1":20}

[{"key1":10},{"key1":20},{"key1":30}]
{"key1":30}

SELECT 3

WITH mytable AS (
    SELECT '[{"key1":10},{"key1":20},{"key1":30}]'::json as mydata
)

SELECT
    *
FROM mytable,
    json_array_elements(mydata) as elements
WHERE (elements ->> 'key1')::int < 25

mydata
value

[{"key1":10},{"key1":20},{"key1":30}]
{"key1":10}

[{"key1":10},{"key1":20},{"key1":30}]
{"key1":20}

SELECT 2

WITH mytable AS (
    SELECT '[{"key1":10},{"key1":20},{"key1":30}]'::json as mydata
)

SELECT
    json_agg(elements)
FROM mytable,
    json_array_elements(mydata) as elements
WHERE (elements ->> 'key1')::int < 25

json_agg

[{"key1":10}, {"key1":20}]

SELECT 1

mydata	value
[{"key1":10},{"key1":20},{"key1":30}]	{"key1":10}
[{"key1":10},{"key1":20},{"key1":30}]	{"key1":20}
[{"key1":10},{"key1":20},{"key1":30}]	{"key1":30}