Postgres 16

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create table your_table(your_json_column jsonb, malformed jsonb);
insert into your_table select '{"Reason": null, "RequestedByUser": false, "CancellationDate": "2024-01-16T00:40:59.0928615+00:00", "RequestedBySystem": null, "RequestedBySellerNotification": null, "RequestedByPaymentNotification": true}',
                              '["{\"RequestedByUser\":false,\"RequestedBySystem\":null,\"RequestedBySellerNotification\":null,\"RequestedByPaymentNotification\":true,\"Reason\":null,\"CancellationDate\":\"2024-01-16T00:40:59.0928615+00:00\"}"]'
returning jsonb_pretty(your_json_column) as jdata,
          jsonb_pretty(malformed) as malformed;

CREATE TABLE

jdata
malformed

{
    "Reason": null,
    "RequestedByUser": false,
    "CancellationDate": "2024-01-16T00:40:59.0928615+00:00",
    "RequestedBySystem": null,
    "RequestedBySellerNotification": null,
    "RequestedByPaymentNotification": true
}
[
    "{"RequestedByUser":false,"RequestedBySystem":null,"RequestedBySellerNotification":null,"RequestedByPaymentNotification":true,"Reason":null,"CancellationDate":"2024-01-16T00:40:59.0928615+00:00"}"
]

INSERT 0 1

select your_json_column->>'CancellationDate' from your_table;

?column?

2024-01-16T00:40:59.0928615+00:00

SELECT 1

select (malformed->>0) from your_table;

?column?

{"RequestedByUser":false,"RequestedBySystem":null,"RequestedBySellerNotification":null,"RequestedByPaymentNotification":true,"Reason":null,"CancellationDate":"2024-01-16T00:40:59.0928615+00:00"}

SELECT 1

select (malformed->>0)::jsonb from your_table;

jsonb

{"Reason": null, "RequestedByUser": false, "CancellationDate": "2024-01-16T00:40:59.0928615+00:00", "RequestedBySystem": null, "RequestedBySellerNotification": null, "RequestedByPaymentNotification": true}

SELECT 1

select ((malformed->>0)::json)->>'CancellationDate' from your_table;

?column?

2024-01-16T00:40:59.0928615+00:00

SELECT 1