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Thanks to the <code class="literal">table_rewrite</code> event, it is possible to implement
a table rewriting policy only allowing the rewrite in maintenance windows.
</p><p>
Here's an example implementing such a policy.
</p><pre class="programlisting">
CREATE OR REPLACE FUNCTION no_rewrite()
RETURNS event_trigger
LANGUAGE plpgsql AS
$$
---
--- Implement local Table Rewriting policy:
--- public.foo is not allowed rewriting, ever
--- other tables are only allowed rewriting between 1am and 6am
--- unless they have more than 100 blocks
---
DECLARE
table_oid oid := pg_event_trigger_table_rewrite_oid();
current_hour integer := extract('hour' from current_time);
pages integer;
max_pages integer := 100;
BEGIN
IF pg_event_trigger_table_rewrite_oid() = 'public.foo'::regclass
THEN
RAISE EXCEPTION 'you''re not allowed to rewrite the table %',
table_oid::regclass;
END IF;
SELECT INTO pages relpages FROM pg_class WHERE oid = table_oid;
IF pages > max_pages
THEN
RAISE EXCEPTION 'rewrites only allowed for table with less than % pages',
max_pages;
END IF;
IF current_hour NOT BETWEEN 1 AND 6
THEN
RAISE EXCEPTION 'rewrites only allowed between 1am and 6am';
END IF;
END;
$$;
CREATE EVENT TRIGGER no_rewrite_allowed
ON table_rewrite
EXECUTE FUNCTION no_rewrite();
</pre><p>
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