You're right, because MVCC works on immutable rows. The structure can be broken up to accommodate large values[1], which saves updating parts that don't change,

You're right, because MVCC works on immutable rows. The structure can be broken up to accommodate large values[1], which saves updating parts that don't change, but json data is a single column and updates need to copy the entire document. [1] http://www.postgresql.org/docs/devel/static/storage-toast.html http://www.postgresql.org/docs/devel/static/storage-toast.ht...