# Adding Foreign Key without NOT VALID
**Check Name**: `AddForeignKeyCheck`
**Lock Type**: ShareRowExclusiveLock
## Bad
Adding a foreign key with validation requires a `ShareRowExclusiveLock`, which blocks writes on the table.
On large tables, this can cause outages.
```sql
ALTER TABLE orders ADD CONSTRAINT fk_user_id
FOREIGN KEY (user_id) REFERENCES users(id);
```
### Good
Add the foreign key first without validation using the `NOT VALID` clause. Validate the foreign key later in a separate
migration.
```sql
-- Step 1 (no validation scan; short metadata lock)
ALTER TABLE orders ADD CONSTRAINT fk_user_id
FOREIGN KEY (user_id) REFERENCES users(id) NOT VALID;
-- Step 2 (separate migration, acquires ShareUpdateExclusiveLock only)
ALTER TABLE orders VALIDATE CONSTRAINT fk_user_id;
```