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I don't think executing long-running DDL with concurrent read/writes is that bad, especially if the DDL is additive and doesn't lock the whole table.

Concurrency - isn't that what a DB is supposed to do well?

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If the site isn't taken down during an update, some parts of it will always be broken during a schema migration since our ORM expects each table corresponding to a data model to be in sync after a code update. I guess you could always version your data models and handle the pre-upgrade and post-upgrade cases, but that seems like a lot of work in our situation.

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