This can happen even with something as robust as postgres. If you have a huge table undergoing lots of read&#x2F;write activity, the locking needed to modify the table schema can take (essentially) infinite time, and&#x2F;or slow your writes to a crawl and&#x2F;or spike memory usage on your DB such that you no longer have faith in process completion.<p>Example from my past: setting a default column value on a huge table undergoing hundreds of writes per second. Oops.