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Adding a column, changing column's nullability and adding/changing constraints is already zero-downtime in PG.

Not sure about default value change, but I would also say its zero-downtime as adding a column with default is zero-downtime. Haven't checked, though.

Dropping a column and changing the data type are not zero-downtime.

Renaming a column is not zero-downtime as you might have multiple readers/writes using both.

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> Adding a column, changing column's nullability and adding/changing constraints is already zero-downtime in PG.

Making a previously nullable column NOT NULL is not zero downtime. Neither are adding constraints -- except in some cases with NOT VALID, which isn't quite the same thing.

> Dropping a column and changing the data type are not zero-downtime.

Dropping a column is zero downtime.

Adding a NULL constraint isn't zero downtime since PG scans the whole table ensuring that the constraint is not violated.

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