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Re:Managing database schema changes without downtime by theptip on Saturday March 24, 2018 @12:09PM (#16667309)
If you do it right, it's zero downtime (i.e. no DB locks held for long enough to cause timeouts to clients).
Re:Managing database schema changes without downtime by way0utwest on Wednesday March 28, 2018 @11:23AM (#16698370)
In most cases. However, some changes, such as adding the column and needing to manipulate data (populate nulls, transforms, etc.) could still cause locking.

This really comes down to knowing the boundaries of what changes could cause timeouts and working around them as best you can. However, some edge cases could cause downtime.