Support arrays as input to array_agg() and ARRAY(SELECT ...).

These cases formerly failed with errors about "could not find array type
for data type".  Now they yield arrays of the same element type and one
higher dimension.

The implementation involves creating functions with API similar to the
existing accumArrayResult() family.  I (tgl) also extended the base family
by adding an initArrayResult() function, which allows callers to avoid
special-casing the zero-inputs case if they just want an empty array as
result.  (Not all do, so the previous calling convention remains valid.)
This allowed simplifying some existing code in xml.c and plperl.c.

Ali Akbar, reviewed by Pavel Stehule, significantly modified by me

doc/src/sgml/func.sgml

@@ -12035,7 +12035,7 @@ NULL baz</literallayout>(3 rows)</entry>
        <function>array_agg(<replaceable class="parameter">expression</replaceable>)</function>
       </entry>
       <entry>
-       any
+       any non-array type
       </entry>
       <entry>
        array of the argument type
@@ -12043,6 +12043,21 @@ NULL baz</literallayout>(3 rows)</entry>
       <entry>input values, including nulls, concatenated into an array</entry>
      </row>
 
+     <row>
+      <entry>
+       <function>array_agg(<replaceable class="parameter">expression</replaceable>)</function>
+      </entry>
+      <entry>
+       any array type
+      </entry>
+      <entry>
+       same as argument data type
+      </entry>
+      <entry>input arrays concatenated into array of one higher dimension
+       (inputs must all have same dimensionality,
+        and cannot be empty or NULL)</entry>
+     </row>
+
      <row>
       <entry>
        <indexterm>

doc/src/sgml/syntax.sgml

@@ -2239,11 +2239,22 @@ SELECT ARRAY(SELECT oid FROM pg_proc WHERE proname LIKE 'bytea%');
 -----------------------------------------------------------------------
  {2011,1954,1948,1952,1951,1244,1950,2005,1949,1953,2006,31,2412,2413}
 (1 row)
+
+SELECT ARRAY(SELECT ARRAY[i, i*2] FROM generate_series(1,5) AS a(i));
+              array
+----------------------------------
+ {{1,2},{2,4},{3,6},{4,8},{5,10}}
+(1 row)
 </programlisting>
-   The subquery must return a single column. The resulting
+   The subquery must return a single column.
+   If the subquery's output column is of a non-array type, the resulting
    one-dimensional array will have an element for each row in the
    subquery result, with an element type matching that of the
    subquery's output column.
+   If the subquery's output column is of an array type, the result will be
+   an array of the same type but one higher dimension; in this case all
+   the subquery rows must yield arrays of identical dimensionality, else
+   the result would not be rectangular.
   </para>
 
   <para>

doc/src/sgml/xaggr.sgml

@@ -359,12 +359,12 @@ SELECT attrelid::regclass, array_accum(atttypid::regtype)
    aggregate <function>array_agg</> is equivalent to
 
 <programlisting>
-CREATE FUNCTION array_agg_transfn(internal, anyelement)
+CREATE FUNCTION array_agg_transfn(internal, anynonarray)
   RETURNS internal ...;
-CREATE FUNCTION array_agg_finalfn(internal, anyelement)
+CREATE FUNCTION array_agg_finalfn(internal, anynonarray)
   RETURNS anyarray ...;
 
-CREATE AGGREGATE array_agg (anyelement)
+CREATE AGGREGATE array_agg (anynonarray)
 (
     sfunc = array_agg_transfn,
     stype = internal,
@@ -376,7 +376,7 @@ CREATE AGGREGATE array_agg (anyelement)
    Here, the <literal>finalfunc_extra</> option specifies that the final
    function receives, in addition to the state value, extra dummy
    argument(s) corresponding to the aggregate's input argument(s).
-   The extra <type>anyelement</> argument allows the declaration
+   The extra <type>anynonarray</> argument allows the declaration
    of <function>array_agg_finalfn</> to be valid.
   </para>