PutItem replaces
PutItem writes a whole item and overwrites any item with the same key.
Why
PutItem writes an entire item. If the key is new, you get a new item. If an item with that key already exists, PutItem replaces it wholesale: it does not merge.
Add a track, then overwrite track 001 with a version that drops the seconds attribute. Look at the row: the old attribute did not survive. That is the footgun and the feature.
PutItem is write-or-replace by key. To change part of an item without losing the rest, use UpdateItem.
Try it. Then change it.
Predict what each request will do. Run it, inspect the response, then change a value in the workbench and try again.
A new key, so a new item appears in the Chill playlist.
Same key as an existing track, but with no 'seconds'. PutItem replaces the whole item, so the old attribute is gone.
A new key, so a new item appears in the Chill playlist.
Choose an access pattern above, or build your own request. See what comes back and what it costs.