Altering tables without downtime.
WHAT THE PROBLEM IS
Some alterations lock the table for the duration.
WHAT THAT MEANS ON A LARGE TABLE
An outage lasting as long as the operation.
WHAT MODERN VERSIONS SUPPORT
Many changes performed online, without blocking.
WHAT TO CHECK
Whether your specific change is online, in your specific version.
WHY VERSION MATTERS
Capabilities differ, and the documentation is version-specific.
WHAT TOOLS EXIST FOR THE REST
Utilities creating a copy, synchronising it, and swapping.
WHAT THEY REQUIRE
Space for a second copy Time Triggers or log reading
WHAT THE SAFE PATTERN FOR APPLICATIONS IS
Add the new structure Write to both old and new Migrate existing rows in batches Switch reads Stop writing the old Remove it later
WHY IN STAGES
Each step works with the application version before and after.
WHAT THAT ENABLES
Rolling back without losing data.
WHAT TO NEVER DO
Rename a column in one step while code references it Remove a column in the same release that stops using it
WHAT TO TEST
The change, timed, on a copy of production.
WHAT TO PREPARE
A way to stop it partway.
