Use of INSERT and RETURNING in pgsql

sql-insert-returning.sql

INSERT INTO images (
      post_id
    , extension
    , position
    , created_at)
    (SELECT
          '13'
        , 'jpg'
        , COALESCE(MAX(position), 0) + 1
        , CURRENT_TIMESTAMP
        FROM images
        WHERE post_id = '13')
        RETURNING id, extension, position;