If columns are added in a migration in another process, you get a unique-index exception

[ageweke]

Steps to reproduce:

1. Have a long-running process (like Rails) using low-card tables.
2. Run a migration that adds new, nullable columns to the low-card table.

Expected results:

1. The long-running process continues working smoothly, seeing the new columns as expected.

Actual results:

1. You get an exception because there is no longer a unique index on just the old columns; instead, there is one on the old columns plus the new columns, and the long-running process doesn’t know about the new columns.