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When `pg-roll` makes a schema change, the contract with the user is that it will leave the old version of schema unchanged. When the operation to add a `UNIQUE` constraint was implemented (https://github.com/xataio/pg-roll/pull/53), this contract was not respected. The operation adds a unique index to the existing column, changing the behaviour for users of the old schema. This PR changes the operation so that it follows a similar pattern to other operations that were implemented later: * On `Start`: * Duplicate the column. * Add a `UNIQUE` index concurrently * Create `up` and `down` triggers to copy values between the old and new columns. * Backfill values from the old column into the new using `up` SQL * On `Complete` * Create a unique constraint on the new column using the unique index. * Drop the old column * Rename the column to its old name. * Remove `up` and `down` triggers. Writing correct `up` SQL for the operation is a little more difficult than for other operations (eg set `NOT NULL`) as it is up to the user to ensure uniqueness of values. The example migration in this PR appends a random suffix to each value. |
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