Add Use Not Valid To Immediately Enforce A Constraint as a postgres til

postgres/use-not-valid-to-immediately-enforce-a-constraint.md

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+# Use Not Valid To Immediately Enforce A Constraint
+
+When adding a constraint to a table, you can optionally include `not valid`.
+This tells Postgres that it doesn't need to enforce the constraint on
+existing records in the table. At least not immediately. This constraint
+will be enforced for any updates and subsequent insertions. Thus, you can
+immediately enforce the constraint while giving yourself time to clean up
+or massage any existing records that conflict with the constraint.
+
+Here is an example of how you would add a constraint this way:
+
+```sql
+alter table boxes
+add constraint check_valid_length
+check (length > 0) not valid;
+```
+
+Eventually, you will want to ensure that all data in the table conforms to the
+constraint. Once you get to that point, you can mark the constraint as valid
+with a `validate constraint` command:
+
+```sql
+alter table boxes
+validate constraint check_valid_length;
+```
+
+As long as all records are valid with respect to this constraint, it will be
+marked as valid.
+
+h/t Chris Erin