Add Put Unique Constraint On Generated Column as a Postgres TIL

postgres/put-unique-constraint-on-generated-column.md

@@ -0,0 +1,46 @@
+# Put Unique Constraint On Generated Column
+
+You cannot apply a _unique constraint_ to an expression over a column, e.g.
+`lower(email)`. You can, however, create a [generated
+column](https://www.postgresql.org/docs/current/ddl-generated-columns.html) for
+that expression and then apply the unique constraint to that generated column.
+
+Here is what that could look like:
+
+```sql
+> create table users (
+    id integer generated always as identity primary key,
+    name text not null,
+    email text not null,
+    email_lower text generated always as (lower(email)) stored,
+    unique ( email_lower )
+);
+
+> \d users
++-------------+---------+-----------------------------------------------------------------+
+| Column      | Type    | Modifiers                                                       |
+|-------------+---------+-----------------------------------------------------------------|
+| id          | integer |  not null generated always as identity                          |
+| name        | text    |  not null                                                       |
+| email       | text    |  not null                                                       |
+| email_lower | text    |  default lower(email) generated always as (lower(email)) stored |
++-------------+---------+-----------------------------------------------------------------+
+Indexes:
+    "users_pkey" PRIMARY KEY, btree (id)
+    "users_email_lower_key" UNIQUE CONSTRAINT, btree (email_lower)
+```
+
+And then an demonstration of violating that constraint:
+
+```sql
+
+> insert into users (name, email) values ('Bob', 'bob@email.com');
+INSERT 0 1
+
+> insert into users (name, email) values ('Bobby', 'BOB@email.com');
+duplicate key value violates unique constraint "users_email_lower_key"
+DETAIL:  Key (email_lower)=(bob@email.com) already exists.
+```
+
+The main tradeoff here is that you are doubling the amount of storage you need
+for that column. Unless it is a massive table, that is likely not an issue.