doc: mention unusability of dropped CHECK to verify NOT NULL

Álvaro Herrera · Álvaro Herrera · commit 126665289fa8 · 2025-08-04T13:26:45.000+02:00
It's possible to use a CHECK (col IS NOT NULL) constraint to skip scanning a table for nulls when adding a NOT NULL constraint on the same column. However, if the CHECK constraint is dropped on the same command that the NOT NULL is added, this fails, i.e., makes the NOT NULL addition slow. The best we can do about it at this stage is to document this so that users aren't taken by surprise. (In Postgres 18 you can directly add the NOT NULL constraint as NOT VALID instead, so there's no longer much use for the CHECK constraint, therefore no point in building mechanism to support the case better.) Reported-by: Andrew <psy2000usa@yahoo.com> Reviewed-by: David G. Johnston <david.g.johnston@gmail.com> Discussion: https://postgr.es/m/175385113607.786.16774570234342968908@wrigleys.postgresql.org
diff --git a/doc/src/sgml/ref/alter_table.sgml b/doc/src/sgml/ref/alter_table.sgml
@@ -240,9 +240,10 @@ WITH ( MODULUS <replaceable class="parameter">numeric_literal</replaceable>, REM
       provided none of the records in the table contain a
       <literal>NULL</literal> value for the column.  Ordinarily this is
       checked during the <literal>ALTER TABLE</literal> by scanning the
-      entire table; however, if a valid <literal>CHECK</literal> constraint is
-      found which proves no <literal>NULL</literal> can exist, then the
-      table scan is skipped.
+      entire table, unless <literal>NOT VALID</literal> is specified;
+      however, if a valid <literal>CHECK</literal> constraint exists
+      (and is not dropped in the same command) which proves no
+      <literal>NULL</literal> can exist, then the table scan is skipped.
       If a column has an invalid not-null constraint,
       <literal>SET NOT NULL</literal> validates it.
      </para>
