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

2 files changed, 39 insertions, 2 deletions

diff --git a/src/test/regress/expected/jsonb.out b/src/test/regress/expected/jsonb.out
index d32a2dabb8c..821fcb8a627 100644
--- a/src/test/regress/expected/jsonb.out
+++ b/src/test/regress/expected/jsonb.out
@@ -3054,9 +3054,41 @@ select '{"a":"b"}'::jsonb || '[]'::jsonb;
 (1 row)
 
 select '"a"'::jsonb || '{"a":1}';
-ERROR:  invalid concatenation of jsonb objects
+    ?column?     
+-----------------
+ ["a", {"a": 1}]
+(1 row)
+
 select '{"a":1}' || '"a"'::jsonb;
-ERROR:  invalid concatenation of jsonb objects
+    ?column?     
+-----------------
+ [{"a": 1}, "a"]
+(1 row)
+
+select '[3]'::jsonb || '{}'::jsonb;
+ ?column? 
+----------
+ [3, {}]
+(1 row)
+
+select '3'::jsonb || '[]'::jsonb;
+ ?column? 
+----------
+ [3]
+(1 row)
+
+select '3'::jsonb || '4'::jsonb;
+ ?column? 
+----------
+ [3, 4]
+(1 row)
+
+select '3'::jsonb || '{}'::jsonb;
+ ?column? 
+----------
+ [3, {}]
+(1 row)
+
 select '["a", "b"]'::jsonb || '{"c":1}';
        ?column?       
 ----------------------
diff --git a/src/test/regress/sql/jsonb.sql b/src/test/regress/sql/jsonb.sql
index 4c195b5527a..f1539682abd 100644
--- a/src/test/regress/sql/jsonb.sql
+++ b/src/test/regress/sql/jsonb.sql
@@ -772,6 +772,11 @@ select '{"a":"b"}'::jsonb || '[]'::jsonb;
 select '"a"'::jsonb || '{"a":1}';
 select '{"a":1}' || '"a"'::jsonb;
 
+select '[3]'::jsonb || '{}'::jsonb;
+select '3'::jsonb || '[]'::jsonb;
+select '3'::jsonb || '4'::jsonb;
+select '3'::jsonb || '{}'::jsonb;
+
 select '["a", "b"]'::jsonb || '{"c":1}';
 select '{"c": 1}'::jsonb || '["a", "b"]';