pg table rewrite trigger

_posts/2024-09-15-postgres-online-ddl.md

@@ -239,6 +239,40 @@ admincoin=# select c.* from pg_cast c
  10129 |       1043 |         25 |        0 | i           | b
 ```
 
+That seems to be the end of the story until I realized
+[this line](https://github.com/postgres/postgres/blob/3f9b9621766796983c37e192f73c5f8751872c18/src/backend/commands/tablecmds.c#L5871).
+Table rewrite has its own trigger! Following
+[this example](https://www.postgresql.org/docs/current/event-trigger-table-rewrite-example.html),
+we can quickly write a simple one.
+
+```sql
+CREATE OR REPLACE FUNCTION log_rewrite()
+ RETURNS event_trigger
+ LANGUAGE plpgsql AS
+$$
+BEGIN
+	RAISE NOTICE 'Table rewrite event detected for table %', pg_event_trigger_table_rewrite_oid()::regclass;
+END;
+$$;
+
+CREATE EVENT TRIGGER no_rewrite_allowed ON table_rewrite EXECUTE FUNCTION log_rewrite();
+```
+
+Not fancy, just log it. Let's test it.
+
+```
+CREATE TABLE my_table (
+    id SERIAL PRIMARY KEY,
+    num INTEGER
+);
+
+ALTER TABLE my_table ALTER COLUMN num TYPE TEXT;
+NOTICE:  Table rewrite event detected for table my_table
+```
+
+OK. So now we have a systematic way to test it locally whenever we are not sure
+whether a rewrite will be triggered.
+
 `castfunc = 0` means no cast needed. `castmethod = b` means that the types are
 binary-coercible.