Fix generated column in unique key error

[meiji163]

A Pull Request should be associated with an Issue.

We wish to have discussions in Issues. A single issue may be targeted by multiple PRs.
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and potentially we'll be able to point development in a particular direction.

Related issue: https://github.com/github/gh-ost/issues/0123456789

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Description

This PR [briefly explain what it does]

In case this PR introduced Go code changes:

• contributed code is using same conventions as original code
• script/cibuild returns with no formatting errors, build errors or unit test errors.