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+---
+title: Use UPSERT for Insert-or-Update Operations
+impact: MEDIUM
+impactDescription: Atomic operation, eliminates race conditions
+tags: upsert, on-conflict, insert, update
+---
+
+## Use UPSERT for Insert-or-Update Operations
+
+Using separate SELECT-then-INSERT/UPDATE creates race conditions. Use INSERT ... ON CONFLICT for atomic upserts.
+
+**Incorrect (check-then-insert race condition):**
+
+```sql
+-- Race condition: two requests check simultaneously
+select * from settings where user_id = 123 and key = 'theme';
+-- Both find nothing
+
+-- Both try to insert
+insert into settings (user_id, key, value) values (123, 'theme', 'dark');
+-- One succeeds, one fails with duplicate key error!
+```
+
+**Correct (atomic UPSERT):**
+
+```sql
+-- Single atomic operation
+insert into settings (user_id, key, value)
+values (123, 'theme', 'dark')
+on conflict (user_id, key)
+do update set value = excluded.value, updated_at = now();
+
+-- Returns the inserted/updated row
+insert into settings (user_id, key, value)
+values (123, 'theme', 'dark')
+on conflict (user_id, key)
+do update set value = excluded.value
+returning *;
+```
+
+Insert-or-ignore pattern:
+
+```sql
+-- Insert only if not exists (no update)
+insert into page_views (page_id, user_id)
+values (1, 123)
+on conflict (page_id, user_id) do nothing;
+```
+
+Reference: [INSERT ON CONFLICT](https://www.postgresql.org/docs/current/sql-insert.html#SQL-ON-CONFLICT)