Lengthy migrations doesn't matter. What matters is whether they hold long locks or not. Data migrations might take a while but they won't lock anything. Schema

Lengthy migrations doesn't matter. What matters is whether they hold long locks or not. Data migrations might take a while but they won't lock anything. Schema migrations, on the other hand, can easily do so, like if you add a new column with a default value. The whole table must be rewritten and it's locked for the entire time.