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3.5.1 Adding a Column

To add a column, use a command like this:

ALTER TABLE products ADD COLUMN description text;

The new column is initially filled with whatever default value is given (null if you don't specify a DEFAULT clause).

You can also define constraints on the column at the same time, using the usual syntax:

ALTER TABLE products ADD COLUMN description text CHECK
 (description <> ”);

In fact all the options that can be applied to a column description in CREATE TABLE can be used here. Keep in mind however that the default value must satisfy the given constraints, or the ADD will fail. Alternatively, you can add constraints later (see below) after you've filled in the new column correctly.

Tip: Adding a column with a default requires updating each row of the table (to store the new column value). However, if no default is specified, PostgreSQL is able to avoid the physical update. So if you intend to fill the column with mostly nondefault values, it's best to add the column with no default, insert the correct values using UPDATE, and then add any desired default as described below.