You say "just" like you think that sort of thing is easy. Nothing could be farther from the truth. For one thing in many DBs certain (often nessesary) schema c

You say "just" like you think that sort of thing is easy. Nothing could be farther from the truth. For one thing in many DBs certain (often nessesary) schema changes require a global write lock. It's much much easier to just show down for 15 minutes or less than spend hundreds of hours of engineering effort.