Remove "invalid concatenation of jsonb objects" error case.

The jsonb || jsonb operator arbitrarily rejected certain combinations
of scalar and non-scalar inputs, while being willing to concatenate
other combinations.  This was of course quite undocumented.  Rather
than trying to document it, let's just remove the restriction,
creating a uniform rule that unless we are handling an object-to-object
concatenation, non-array inputs are converted to one-element arrays,
resulting in an array-to-array concatenation.  (This does not change
the behavior for any case that didn't throw an error before.)

Per complaint from Joel Jacobson.  Back-patch to all supported branches.

Discussion: https://postgr.es/m/163099.1608312033@sss.pgh.pa.us

1 files changed, 7 insertions, 4 deletions

diff --git a/doc/src/sgml/func.sgml b/doc/src/sgml/func.sgml
index 903e52814d5..20626b2a13b 100644
--- a/doc/src/sgml/func.sgml
+++ b/doc/src/sgml/func.sgml
@@ -11313,10 +11313,13 @@ table2-mapping
 
   <note>
    <para>
-    The <literal>||</> operator concatenates the elements at the top level of
-    each of its operands. It does not operate recursively. For example, if
-    both operands are objects with a common key field name, the value of the
-    field in the result will just be the value from the right hand operand.
+    The <literal>||</literal> operator concatenates two JSON objects by
+    generating an object containing the union of their keys, taking the
+    second object's value when there are duplicate keys.  All other cases
+    produce a JSON array: first, any non-array input is converted into a
+    single-element array, and then the two arrays are concatenated.
+    It does not operate recursively; only the top-level array or object
+    structure is merged.
    </para>
   </note>