Summary: json_each(json) expands a JSON object into one row per top-level key, returning each key as text and its value as json, making it the right choice for iterating over a JSON object with an unknown or dynamic schema in SQL. Use it instead of jsonb_each when the input column is typed json rather than jsonb, and instead of json_each_text when preserving the JSON type on values matters. For large JSON objects, the row-expansion carries performance overhead; json_object_keys is lighter when only the keys are needed.

Expands JSON into a record per key-value pair

The json_each function in Postgres is used to expand a JSON object into a set of key-value pairs.

It is useful when you need to iterate over a JSON object's keys and values, such as when you're working with dynamic JSON structures where the schema is not fixed. Another important use case is performing data transformations and analytics.

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json_each(json JSON) -> SETOF record(key text, value json)

The function returns a set of rows, each containing a key and the corresponding value for each field in the input JSON object. The key is of type text, while the value is of type json.

Consider a JSON object representing a user's profile information. The JSON data will have multiple attributes and might look like this:

{
  "username": "johndoe",
  "age": 30,
  "email": "johndoe@example.com"
}

We can go over all the fields in the profile JSON object using json_each, and produce a row for each key-value pair.

SELECT key, value
FROM json_each('{"username": "johndoe", "age": 30, "email": "johndoe@example.com"}');

This query returns the following results:

| key      | value                 |
|----------|-----------------------|
| username | "johndoe"             |
| age      | 30                    |
| email    | "johndoe@example.com" |

You can use AS to specify custom column names for the key and value columns.

SELECT attr_name, attr_value
FROM json_each('{"username": "johndoe", "age": 30, "email": "johndoe@example.com"}')
AS user_data(attr_name, attr_value);

This query returns the following results:

| attr_name | attr_value            |
|-----------|-----------------------|
| username  | "johndoe"             |
| age       | 30                    |
| email     | "johndoe@example.com" |

Since json_each returns a set of rows, you can use it as a table source in a FROM clause. This lets us join the expanded JSON data in the output with other tables.

Here, we're joining each row in the user_data table with the output of json_each:

CREATE TABLE user_data (
    id INT,
    profile JSON
);
INSERT INTO user_data (id, profile)
VALUES
    (123, '{"username": "johndoe", "age": 30, "email": "johndoe@example.com"}'),
    (140, '{"username": "mikesmith", "age": 40, "email": "mikesmith@example.com"}');

SELECT id, key, value
FROM user_data, json_each(user_data.profile);

This query returns the following results:

| id  | key      | value                   |
|-----|----------|-------------------------|
| 123 | username | "johndoe"               |
| 123 | age      | 30                      |
| 123 | email    | "johndoe@example.com"   |
| 140 | username | "mikesmith"             |
| 140 | age      | 40                      |
| 140 | email    | "mikesmith@example.com" |

When working with large JSON objects, json_each may lead to performance overhead, as it expands each key-value pair into a separate row.

• json_each_text - Similar functionality to json_each but returns the value as a text type instead of JSON.
• json_object_keys - It returns only the set of keys in the JSON object, without the values.
• jsonb_each - It provides the same functionality as json_each, but accepts JSONB input instead of JSON.

• PostgreSQL documentation: JSON functions

• array_to_json
• json
• json_agg
• json_array_elements
• json_build_object
• json_exists
• json_extract_path
• json_extract_path_text
• json_object
• json_populate_record
• json_query
• json_scalar
• json_serialize
• json_table
• json_to_record
• json_value
• jsonb_array_elements
• jsonb_each
• jsonb_extract_path
• jsonb_extract_path_text
• jsonb_object
• jsonb_populate_record
• jsonb_to_record

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