If I'm not mistaken: Altering a table can be costly and may cause a performance ...

		devmach on May 8, 2015 \| parent \| context \| favorite \| on: Full text search in milliseconds with PostgreSQL If I'm not mistaken: Altering a table can be costly and may cause a performance issues, specially if you have millions of records. Rows will be most likely locked and get updated and during this process other threads should wait until related row become "free".

That only happens if you add a non-nullable column.