blog: update what is new in pg 18

Author: tianzhou

content/blog/what-is-new-in-postgres-18.md

@@ -69,6 +69,12 @@ Even when a `NOT NULL` constraint is added as `NOT VALID`, it still prevents new
 
 **DBA Note:** This is a godsend for those of us managing large production databases where downtime is measured in dollars per second. No more scheduling 3 AM maintenance windows just to add a NOT NULL constraint to that 10TB table. The fact that it still prevents new NULLs while being marked as invalid is the perfect balance – forward data integrity without the immediate validation pain.
 
+<HintBlock>
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+Before Postgres 18, you an use a less intuitive `CHECK (column is NOT NULL) NOT VALID`.
+
+</HintBlock>
+
 ## OLD/NEW values in RETURNING
 
 Previously, RETURNING had limitations: it typically returned new values for INSERT and UPDATE, and old values for DELETE. MERGE operations would return values based on the internal query executed. The new syntax is more flexible, allowing INSERT ON CONFLICT to return old values, UPDATE to return old values (previously only new), and DELETE to potentially return new values if an ON DELETE row assignment would produce them. This is facilitated by changeable relation aliases "old" and "new" to specify which values are desired.