first commit

.agents/skills/supabase-postgres-best-practices/references/lock-advisory.md

@@ -0,0 +1,56 @@
+---
+title: Use Advisory Locks for Application-Level Locking
+impact: MEDIUM
+impactDescription: Efficient coordination without row-level lock overhead
+tags: advisory-locks, coordination, application-locks
+---
+
+## Use Advisory Locks for Application-Level Locking
+
+Advisory locks provide application-level coordination without requiring database rows to lock.
+
+**Incorrect (creating rows just for locking):**
+
+```sql
+-- Creating dummy rows to lock on
+create table resource_locks (
+  resource_name text primary key
+);
+
+insert into resource_locks values ('report_generator');
+
+-- Lock by selecting the row
+select * from resource_locks where resource_name = 'report_generator' for update;
+```
+
+**Correct (advisory locks):**
+
+```sql
+-- Session-level advisory lock (released on disconnect or unlock)
+select pg_advisory_lock(hashtext('report_generator'));
+-- ... do exclusive work ...
+select pg_advisory_unlock(hashtext('report_generator'));
+
+-- Transaction-level lock (released on commit/rollback)
+begin;
+select pg_advisory_xact_lock(hashtext('daily_report'));
+-- ... do work ...
+commit;  -- Lock automatically released
+```
+
+Try-lock for non-blocking operations:
+
+```sql
+-- Returns immediately with true/false instead of waiting
+select pg_try_advisory_lock(hashtext('resource_name'));
+
+-- Use in application
+if (acquired) {
+  -- Do work
+  select pg_advisory_unlock(hashtext('resource_name'));
+} else {
+  -- Skip or retry later
+}
+```
+
+Reference: [Advisory Locks](https://www.postgresql.org/docs/current/explicit-locking.html#ADVISORY-LOCKS)