Add Foreign Key Constraint Without A Full Lock as a postgres til

README.md

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 ### PostgreSQL
 
 - [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)
 - [Adding Composite Uniqueness Constraints](postgres/adding-composite-uniqueness-constraints.md)
 - [Aggregate A Column Into An Array](postgres/aggregate-a-column-into-an-array.md)

postgres/add-foreign-key-constraint-without-a-full-lock.md

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+# Add Foreign Key Constraint Without A Full Lock
+
+Adding a foreign key constraint to a large production table can cause a full
+table lock resulting in downtime. This is because the entire table needs to be
+scanned to check that the constraint is valid.
+
+The amount of locking, and ultimately the impact on your app, can be reduced by
+spreading this action across two commands. First is to add the constraint
+without checking that all the existing records are valid.
+
+```sql
+alter table books
+  add constraint fk_books_authors
+  foreign key (author_id)
+  references authors(id)
+  not valid;
+```
+
+The constraint will be added immediately and any subsequent inserts or updates
+will be subject to the new foreign key constraint.
+
+The second step is to make this constraint valid for all the existing rows.
+
+```sql
+alter table books validate constraint fk_books_authors;
+```
+
+This "validation acquires only a SHARE UPDATE EXCLUSIVE lock on the table being
+altered." This is lower impact than a full table lock.
+
+[Source](https://www.postgresql.org/docs/current/sql-altertable.html#SQL-ALTERTABLE-NOTES)