Doc: Improve description around ALTER TABLE ATTACH PARTITION

This clarifies more how to use and how to take advantage of constraints
when attaching a new partition.

Author: Justin Pryzby
Reviewed-by: Amit Langote, Álvaro Herrera, Michael Paquier
Discussion: https://postgr.es/m/20191028001207.GB23808@telsasoft.com
Backpatch-through: 10

doc/src/sgml/ddl.sgml

@@ -3952,14 +3952,14 @@ ALTER TABLE measurement ATTACH PARTITION measurement_y2008m02
     <para>
      Before running the <command>ATTACH PARTITION</command> command, it is
      recommended to create a <literal>CHECK</literal> constraint on the table to
-     be attached describing the desired partition constraint.  That way,
+     be attached matching the desired partition constraint. That way,
      the system will be able to skip the scan to validate the implicit
-     partition constraint. Without such a constraint, the table will be
-     scanned to validate the partition constraint while holding an
-     <literal>ACCESS EXCLUSIVE</literal> lock on that partition
+     partition constraint. Without the <literal>CHECK</literal> constraint,
+     the table will be scanned to validate the partition constraint while
+     holding an <literal>ACCESS EXCLUSIVE</literal> lock on that partition
      and a <literal>SHARE UPDATE EXCLUSIVE</literal> lock on the parent table.
-     One may then drop the constraint after <command>ATTACH PARTITION</command>
-     is finished, because it is no longer necessary.
+     It may be desired to drop the redundant <literal>CHECK</literal> constraint
+     after <command>ATTACH PARTITION</command> is finished.
     </para>
 
     <para>

doc/src/sgml/ref/alter_table.sgml

@@ -841,7 +841,7 @@ WITH ( MODULUS <replaceable class="parameter">numeric_literal</replaceable>, REM
       or as a default partition by using <literal>DEFAULT</literal>.
       For each index in the target table, a corresponding
       one will be created in the attached table; or, if an equivalent
-      index already exists, will be attached to the target table's index,
+      index already exists, it will be attached to the target table's index,
       as if <command>ALTER INDEX ATTACH PARTITION</command> had been executed.
       Note that if the existing table is a foreign table, it is currently not
       allowed to attach the table as a partition of the target table if there
@@ -864,23 +864,24 @@ WITH ( MODULUS <replaceable class="parameter">numeric_literal</replaceable>, REM
       already exist.
       If any of the <literal>CHECK</literal> constraints of the table being
       attached is marked <literal>NO INHERIT</literal>, the command will fail;
-      such a constraint must be recreated without the <literal>NO INHERIT</literal>
-      clause.
+      such constraints must be recreated without the
+      <literal>NO INHERIT</literal> clause.
      </para>
 
      <para>
       If the new partition is a regular table, a full table scan is performed
-      to check that no existing row in the table violates the partition
-      constraint.  It is possible to avoid this scan by adding a valid
-      <literal>CHECK</literal> constraint to the table that would allow only
-      the rows satisfying the desired partition constraint before running this
-      command.  It will be determined using such a constraint that the table
-      need not be scanned to validate the partition constraint.  This does not
-      work, however, if any of the partition keys is an expression and the
-      partition does not accept <literal>NULL</literal> values.  If attaching
-      a list partition that will not accept <literal>NULL</literal> values,
-      also add <literal>NOT NULL</literal> constraint to the partition key
-      column, unless it's an expression.
+      to check that existing rows in the table do not violate the partition
+      constraint. It is possible to avoid this scan by adding a valid
+      <literal>CHECK</literal> constraint to the table that allows only
+      rows satisfying the desired partition constraint before running this
+      command. The <literal>CHECK</literal> constraint will be used to
+      determine that the table need not be scanned to validate the partition
+      constraint. This does not work, however, if any of the partition keys
+      is an expression and the partition does not accept
+      <literal>NULL</literal> values. If attaching a list partition that will
+      not accept <literal>NULL</literal> values, also add
+      <literal>NOT NULL</literal> constraint to the partition key column,
+      unless it's an expression.
      </para>
 
      <para>