Make set_unique operations respect the contract for old schema versions
#118
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When setting a column UNIQUE, the `up` sql should ensure that the value written to the new schema is unique.
It is now possible to insert values that violate the uniqueness constaint into the old version of the schema.
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Follow up to #118 to make the `down` SQL optional. When making an existing column unique, the `down` sql is almost always going to be a simple copy from the new column to the old.
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When
pg-rollmakes 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
UNIQUEconstraint was implemented (#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:
Start:UNIQUEindex concurrentlyupanddowntriggers to copy values between the old and new columns.upSQLCompleteupanddowntriggers.Writing correct
upSQL for the operation is a little more difficult than for other operations (eg setNOT 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.