Summary: json_serialize() is a PostgreSQL 17 function that converts a JSON value into text (text) or binary (bytea) output, with an optional RETURNING clause to control the output type. Use it when you need to serialize JSON for storage, transmission, or export in a specific format, as opposed to json(), which converts text or binary into a JSON value. The function returns text by default and raises SQLSTATE 22P02 on invalid JSON input.

Convert JSON Values to Text or Binary Format

The json_serialize() function introduced in PostgreSQL 17 provides a flexible way to convert JSON values into text or binary format. Use it when you need to control the output format of JSON data or prepare it for transmission or storage in specific formats.

Use json_serialize() when you need to:

• Convert JSON values to specific text formats
• Transform JSON into binary representation
• Ensure consistent JSON string formatting
• Prepare JSON data for external systems or storage

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The json_serialize() function uses the following syntax:

json_serialize(
    expression                              -- Input JSON expression
    [ FORMAT JSON [ ENCODING UTF8 ] ]       -- Optional input format specification
    [ RETURNING data_type                   -- Optional return type specification
      [ FORMAT JSON [ ENCODING UTF8 ] ] ]   -- Optional output format specification
) → text | bytea

Parameters:

• expression: Input JSON value or expression to serialize
• FORMAT JSON: Explicitly specifies JSON format for input (optional)
• ENCODING UTF8: Specifies UTF8 encoding for input/output (optional)
• RETURNING data_type: Specifies the desired output type (optional, defaults to text)

Let's explore various ways to use the json_serialize() function with different inputs and output formats.

-- Serialize a simple JSON object to text
SELECT json_serialize('{"name": "Alice", "age": 30}');

# |       json_serialize
--------------------------------
1 | {"name": "Alice", "age": 30}

-- Serialize a JSON array
SELECT json_serialize('[1, 2, 3, "four", true, null]');

# |      json_serialize
----------------------------------
1 | [1, 2, 3, "four", true, null]

-- Convert JSON to binary format
SELECT json_serialize(
    '{"id": 1, "data": "test"}'
    RETURNING bytea
);

# |                   json_serialize
--------------------------------------------------------
1 | \x7b226964223a20312c202264617461223a202274657374227d

-- Serialize nested JSON structures
SELECT json_serialize('{
    "user": {
        "name": "Bob",
        "settings": {
            "theme": "dark",
            "notifications": true
        },
        "tags": ["admin", "active"]
    }
}');

# |                                  json_serialize
---------------------------------------------------------------------------------------------------------------------
1 | { "user": { "name": "Bob", "settings": { "theme": "dark", "notifications": true }, "tags": ["admin", "active"] } }

While both json_serialize() and json() work with JSON data, they serve different purposes:

• json() converts text or binary data into JSON values
• json_serialize() converts JSON values into text or binary format
• json() focuses on input validation (for example, WITH UNIQUE keys)
• json_serialize() focuses on output format control

Think of them as complementary functions:

-- json() for input conversion
SELECT json('{"name": "Alice"}');  -- Text to JSON

-- json_serialize() for output conversion
SELECT json_serialize('{"name": "Alice"}'::json);  -- JSON to Text

-- Create a table with JSON data
CREATE TABLE events (
    id SERIAL PRIMARY KEY,
    event_data json
);

-- Insert sample data
INSERT INTO events (event_data) VALUES
    ('{"type": "login", "user_id": 123}'),
    ('{"type": "purchase", "amount": 99.99}');

-- Export data in specific format
SELECT id, json_serialize(event_data RETURNING text)
FROM events;

The function handles various error conditions:

-- Invalid JSON input (raises error)
SELECT json_serialize('{"invalid": }');

ERROR: invalid input syntax for type json (SQLSTATE 22P02)

• json() function documentation
• PostgreSQL JSON functions documentation
• PostgreSQL data type formatting functions

• array_to_json
• json
• json_agg
• json_array_elements
• json_build_object
• json_each
• json_exists
• json_extract_path
• json_extract_path_text
• json_object
• json_populate_record
• json_query
• json_scalar
• 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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