What is the wp_postmeta table
Have a look at this: https://codex.wordpress.org/images/2/2a/WP3.9.4-ERD.png
wp_postmeta (similar to wp_usermeta for users and wp_commentmenta for comments) stores any additional information which is connected to a post somehow especially user-defined.
Use the functions
add_post_meta()update_post_meta()get_post_meta()which are already implementing object and array serialization/deserialization (and a lot of more) in order to store custom information which is not stored within the wp_posts table.