Doc: Clarify lock levels taken during ATTACH PARTITION

It wasn't all that clear which lock levels, if any, would be held on the
DEFAULT partition during an ATTACH PARTITION operation.

Also, clarify which locks will be taken if the DEFAULT partition or the
table being attached are themselves partitioned tables.

Here I'm only backpatching to v12 as before then we obtained an ACCESS
EXCLUSIVE lock on the partitioned table.  It seems much less relevant to
mention which locks are taken on other tables when the partitioned table
itself is locked with an ACCESS EXCLUSIVE lock.

Author: Matthias van de Meent, David Rowley
Discussion: https://postgr.es/m/CAEze2WiTB6iwrV8W_J=fnrnZ7fowW3qu-8iQ8zCHP3FiQ6+o-A@mail.gmail.com
Backpatch-through: 12

doc/src/sgml/ddl.sgml

@@ -3970,6 +3970,11 @@ ALTER TABLE measurement ATTACH PARTITION measurement_y2008m02
 </programlisting>
     </para>
 
+    <para>
+     The <command>ATTACH PARTITION</command> command requires taking a
+     <literal>SHARE UPDATE EXCLUSIVE</literal> lock on the partitioned table.
+    </para>
+
     <para>
      Before running the <command>ATTACH PARTITION</command> command, it is
      recommended to create a <literal>CHECK</literal> constraint on the table to
@@ -3978,10 +3983,27 @@ ALTER TABLE measurement ATTACH PARTITION measurement_y2008m02
      which is otherwise needed to validate the implicit
      partition constraint. Without the <literal>CHECK</literal> constraint,
      the table will be scanned to validate the partition constraint while
-     holding both an <literal>ACCESS EXCLUSIVE</literal> lock on that partition
-     and a <literal>SHARE UPDATE EXCLUSIVE</literal> lock on the parent table.
+     holding an <literal>ACCESS EXCLUSIVE</literal> lock on that partition.
      It is recommended to drop the now-redundant <literal>CHECK</literal>
-     constraint after <command>ATTACH PARTITION</command> is finished.
+     constraint after the <command>ATTACH PARTITION</command> is complete.  If
+     the table being attached is itself a partitioned table then each of its
+     sub-partitions will be recursively locked and scanned until either a
+     suitable <literal>CHECK</literal> constraint is encountered or the leaf
+     partitions are reached.
+    </para>
+
+    <para>
+     Similarly, if the partitioned table has a <literal>DEFAULT</literal>
+     partition, it is recommended to create a <literal>CHECK</literal>
+     constraint which excludes the to-be-attached partition's constraint.  If
+     this is not done then the <literal>DEFAULT</literal> partition will be
+     scanned to verify that it contains no records which should be located in
+     the partition being attached.  This operation will be performed whilst
+     holding an <literal>ACCESS EXCLUSIVE</literal> lock on the <literal>
+     DEFAULT</literal> partition.  If the <literal>DEFAULT</literal> partition
+     is itself a partitioned table then each of its partitions will be
+     recursively checked in the same way as the table being attached, as
+     mentioned above.
     </para>
 
     <para>

doc/src/sgml/ref/alter_table.sgml

@@ -965,8 +965,17 @@ WITH ( MODULUS <replaceable class="parameter">numeric_literal</replaceable>, REM
      <para>
       Attaching a partition acquires a
       <literal>SHARE UPDATE EXCLUSIVE</literal> lock on the parent table,
-      in addition to <literal>ACCESS EXCLUSIVE</literal> locks on the table
-      to be attached and on the default partition (if any).
+      in addition to the <literal>ACCESS EXCLUSIVE</literal> locks on the table
+      being attached and on the default partition (if any).
+     </para>
+
+     <para>
+      Further locks must also be held on all sub-partitions if the table being
+      attached is itself a partitioned table.  Likewise if the default
+      partition is itself a partitioned table.  The locking of the
+      sub-partitions can be avoided by adding a <literal>CHECK</literal>
+      constraint as described in
+      <xref linkend="ddl-partitioning-declarative-maintenance"/>.
      </para>
     </listitem>
    </varlistentry>