Add Add Unique Constraint Using Existing Index as a Postgres TIL

postgres/add-unique-constraint-using-existing-index.md

@@ -0,0 +1,25 @@
+# Add Unique Constraint Using Existing Index
+
+Adding a unique constraint to an existing column on a production table can
+block updates. If we need to avoid this kind of locking for the duration of
+index creation, then we can first create the index concurrently and then use
+that existing index to back the unique constraint.
+
+```sql
+create index concurrently users_email_idx on users (email);
+
+-- wait for that to complete
+
+alter table users
+  add constraint unique_users_email unique using index users_email_idx;
+```
+
+First, we concurrently create the index. The time this takes will depend on how
+large the table is. That's the blocking time we are avoiding with this
+approach. Then once that completes we can apply a unique constraint using that
+preexisting index.
+
+Note: if a non-unique value exists in the table for that column, adding the
+constraint will fail. You'll need to deal with that _duplicate_ value first.
+
+[source](https://dba.stackexchange.com/questions/81627/postgresql-9-3-add-unique-constraint-using-an-existing-unique-index)