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

README.md

@@ -7,7 +7,7 @@ variety of languages and technologies. These are things that don't really
 warrant a full blog post. These are mostly things I learn by pairing with
 smart people at [Hashrocket](http://hashrocket.com/).
 
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 - [Types By Category](postgres/types-by-category.md)
 - [Use A psqlrc File For Common Settings](postgres/use-a-psqlrc-file-for-common-settings.md)
 - [Use Argument Indexes](postgres/use-argument-indexes.md)
+- [Use Not Valid To Immediately Enforce A Constraint](postgres/use-not-valid-to-immediately-enforce-a-constraint.md)
 - [Using Expressions In Indexes](postgres/using-expressions-in-indexes.md)
 - [Using Intervals To Offset Time](postgres/using-intervals-to-offset-time.md)
 - [Who Is The Current User](postgres/who-is-the-current-user.md)

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