PostgreSQL 11 Administration Cookbook

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Indexes that depend on a dropped column are automatically dropped as well. All other objects that depend on the column(s), such as foreign keys from other tables, will cause the ALTER TABLE statement to be rejected. You can override this and drop everything in sight using the CASCADE option, as follows:

ALTER TABLE x DROP COLUMN  last_update_timestampCASCADE;

Adding a column with a non-null default value can be done with ALTER TABLE … ADD COLUMN … DEFAULT …, as we have just shown, but this holds an AccessExclusive lock for the whole duration of the command, which can take a long time if the DEFAULT is a volatile expression, as 100% of the rows must be rewritten.

The script which introduced in the Using psql variables recipe in ...