Allow omitting one or both boundaries in an array slice specifier.

Omitted boundaries represent the upper or lower limit of the corresponding
array subscript.  This allows simpler specification of many common
use-cases.

(Revised version of commit 9246af6799)

YUriy Zhuravlev

doc/src/sgml/array.sgml

@@ -276,6 +276,29 @@ SELECT schedule[1:2][2] FROM sal_emp WHERE name = 'Bill';
   for all dimensions, e.g., <literal>[1:2][1:1]</>, not <literal>[2][1:1]</>.
  </para>
 
+ <para>
+  It is possible to omit the <replaceable>lower-bound</replaceable> and/or
+  <replaceable>upper-bound</replaceable> of a slice specifier; the missing
+  bound is replaced by the lower or upper limit of the array's subscripts.
+  For example:
+
+<programlisting>
+SELECT schedule[:2][2:] FROM sal_emp WHERE name = 'Bill';
+
+        schedule
+------------------------
+ {{lunch},{presentation}}
+(1 row)
+
+SELECT schedule[:][1:1] FROM sal_emp WHERE name = 'Bill';
+
+        schedule
+------------------------
+ {{meeting},{training}}
+(1 row)
+</programlisting>
+ </para>
+
  <para>
   An array subscript expression will return null if either the array itself or
   any of the subscript expressions are null.  Also, null is returned if a
@@ -391,6 +414,10 @@ UPDATE sal_emp SET pay_by_quarter[1:2] = '{27000,27000}'
     WHERE name = 'Carol';
 </programlisting>
 
+  The slice syntaxes with omitted <replaceable>lower-bound</replaceable> and/or
+  <replaceable>upper-bound</replaceable> can be used too, but only when
+  updating an array value that is not NULL or zero-dimensional (otherwise,
+  there is no existing subscript limit to substitute).
  </para>
 
  <para>