Add Add Unique Constraint Using Existing Index as a Postgres TIL

Author: jbranchaud

README.md

@@ -10,7 +10,7 @@ pairing with smart people at Hashrocket.
 
 For a steady stream of TILs, [sign up for my newsletter](https://crafty-builder-6996.ck.page/e169c61186).
 
-_1484 TILs and counting..._
+_1485 TILs and counting..._
 
 ---
 
@@ -703,6 +703,7 @@ _1484 TILs and counting..._
 - [A Better Null Display Character](postgres/a-better-null-display-character.md)
 - [Add Foreign Key Constraint Without A Full Lock](postgres/add-foreign-key-constraint-without-a-full-lock.md)
 - [Add ON DELETE CASCADE To Foreign Key Constraint](postgres/add-on-delete-cascade-to-foreign-key-constraint.md)
+- [Add Unique Constraint Using Existing Index](postgres/add-unique-constraint-using-existing-index.md)
 - [Adding Composite Uniqueness Constraints](postgres/adding-composite-uniqueness-constraints.md)
 - [Aggregate A Column Into An Array](postgres/aggregate-a-column-into-an-array.md)
 - [Assumed Radius Of The Earth](postgres/assumed-radius-of-the-earth.md)

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)