When would you want to use disable_ddl_transaction in Rails?

Question

My understanding is that all migrations are wrapped in a transaction, meaning, if a change for 1 row, like adding null: false, in a table fails, every other row will fail. When would you not want this to happen with disable_ddl_transaction?

Sorbose

Answer 1

All migrations are wrapped in transaction by default so that they can be rolled back when they are failed.

You need to use disable_ddl_transaction when you want to do something that can not excute inside a transaction. You can check Transactional Migrations section of Migration document for example.

Malta

One example that comes to mind is when adding a custom/user-defined Enum in PGSQL. They cant be defined in a transaction, so adding them via a Rails migration requires disable_ddl_transaction! called when the class is loaded, before its #up method is evoked. – Spooky

Answer 2

When you want to add an index on a large table using a concurrent (algorithm) approach, which prevents the table from getting locked while the index is being added, you need to disable_ddl_transaction since the concurrent approach can't be executed inside the transaction. Please find more details here.

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