[surjection]

Any pgroll operations[0] that require a change to an existing column, such as adding a constraint, will create a new copy of the column and backfill it using 'up' SQL defined in the migration and apply the change to that new column.

There are no operations that will modify the data of an existing column in-place, as this would violate the invariant that the old schema must remain usable alongside the new one.

[0] - https://github.com/xataio/pgroll/tree/main/docs#operations-r...

[darkwater]

Maybe this is explained somewhere in the docs but I'm lazy: how does it cope with possible performance issue in highly trafficked tables? Can you somehow control the backfill speed if it's taking up too much I/O?

[surjection]

There's nothing about this in the docs :)

Backfills are done in fixed size batches to avoid taking long-lived row locks on many rows but there is nothing in place to control the overall rate of backfilling.

This would certainly be a nice feature to add soon though.