Feature
You can now create an identity column, sometimes referred to as an auto-incrementing column, on a table.
You can now create an identity column, sometimes referred to as an auto-incrementing column, on a table. You can use identity columns to create and maintain primary keys on your tables. When you insert a row into a table that has an identity column, BigQuery generates a unique integer value for that column. This feature is in Preview.
Read the original announcement