How to diagnose and fix the 22030 duplicate_json_object_key_value error code in Postgres.

The 22030 duplicate_json_object_key_value error in PostgreSQL occurs when there is an attempt to create a JSON object with duplicate keys, which is not allowed in a strictly conforming JSON object. Here’s how to diagnose and fix this error with examples and sample code:

  1. Check for Duplicate Keys: Ensure that each key in a JSON object is unique. If you have duplicate keys, remove or rename them to resolve the error. -- Bad Example: Duplicate keys 'a' SELECT '{"a": 1, "a": 2}'::json; -- Good Example: Unique keys SELECT '{"a": 1, "b": 2}'::json;
  2. Use JSONB Instead of JSON: If your use case can tolerate duplicate keys (for example, when you’re aggregating data and don’t care about key uniqueness), consider using the JSONB data type instead. JSONB does not enforce key uniqueness, and it will keep the value of the last duplicate key. -- Using JSONB to avoid duplicate key error SELECT '{"a": 1, "a": 2}'::jsonb; -- Result: {"a": 2}
  3. Aggregate Data Properly: When constructing JSON objects dynamically using aggregate functions, ensure that you’re not inadvertently creating duplicate keys. Use the json_object_agg function to create a JSON object from a set of key-value pairs, and ensure that the keys are distinct. -- Bad Example: Aggregating with potential duplicate keys SELECT json_object_agg(key, value) FROM (VALUES ('a', 1), ('a', 2)) AS t(key, value); -- Good Example: Ensure distinct keys before aggregation SELECT json_object_agg(key, value) FROM (SELECT DISTINCT key, value FROM your_table) AS t;
  4. Merge JSON Objects Correctly: When merging JSON objects, ensure that you’re handling potential duplicate keys according to your application’s requirements. You can use the jsonb type and its operators to merge objects and resolve duplicates. -- Merging JSON objects with potential duplicates using JSONB SELECT '{"a": 1}'::jsonb || '{"a": 2, "b": 3}'::jsonb; -- Result: {"a": 2, "b": 3}
  5. Validate Input Data: If you’re constructing JSON objects from user input or external data sources, validate the input to ensure that there are no duplicate keys before attempting to create a JSON object. -- Example of a function to check for duplicate keys in input data CREATE OR REPLACE FUNCTION validate_json_keys(input_data json) RETURNS boolean AS $$ DECLARE key text; keys text[] := '{}'; BEGIN FOR key IN SELECT json_object_keys(input_data) LOOP IF key = ANY(keys) THEN RETURN false; -- Duplicate key found ELSE keys := array_append(keys, key); END IF; END LOOP; RETURN true; -- No duplicates END; $$ LANGUAGE plpgsql;
  6. Correcting Data in Application Logic: If possible, address the issue in the application logic before the data is sent to the database. This can prevent the error from occurring by ensuring that the JSON objects constructed in the application are valid.

When encountering the 22030 duplicate_json_object_key_value error, review the JSON data to identify and resolve any duplicate keys. Depending on the situation, you may want to remove duplicates, rename keys, use the JSONB data type, or handle the merging of JSON objects with care to maintain key uniqueness. If you often deal with JSON data, consider implementing validation checks in your application code to prevent this error from occurring.

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