Every product has a price and a stock. But a T-shirt has sizes, a mug has a volume, and a plush has a height. A column for each fact would leave most cells NULL.

A JSON column holds a small document in each row. Each row can have its own keys.

JSON

JSON (JavaScript Object Notation) is a text format for data:

{
  "color": "purple",
  "size_cm": 30,
  "washable": true,
  "tags": ["plush", "kraken"]
}

Objects and arrays can hold other objects and arrays.

The jsonb type

ALTER TABLE products ADD COLUMN details jsonb;

UPDATE products
SET details = '{"color": "red", "tags": ["mug"]}'
WHERE id = 10;

• Write a JSON value as text, in single quotes. PostgreSQL checks it and stores it.
• Text that is not valid JSON fails: invalid input syntax for type json.
• jsonb stores the document in a parsed, binary form. It does not keep the order of the keys or the spaces. Queries on it are fast.
• json stores the text exactly as you wrote it. Use jsonb unless you must keep the text.

When to use it

A JSON column fits data that changes shape from row to row, or that you store and read as one piece: settings, extra attributes, a copy of an API response. Data that you join, filter and change often belongs in normal columns, where types and constraints protect it.