Add json_strip_nulls and jsonb_strip_nulls functions.

The functions remove object fields, including in nested objects, that
have null as a value. In certain cases this can lead to considerably
smaller datums, with no loss of semantic information.

Andrew Dunstan, reviewed by Pavel Stehule.

src/test/regress/expected/json.out

@@ -1586,3 +1586,53 @@ select * from json_to_recordset('[{"a":1,"b":{"d":"foo"},"c":true},{"a":2,"c":fa
  2 | {"d":"bar"} | f
 (2 rows)
 
+-- json_strip_nulls
+select json_strip_nulls(null);
+ json_strip_nulls 
+------------------
+ 
+(1 row)
+
+select json_strip_nulls('1');
+ json_strip_nulls 
+------------------
+ 1
+(1 row)
+
+select json_strip_nulls('"a string"');
+ json_strip_nulls 
+------------------
+ "a string"
+(1 row)
+
+select json_strip_nulls('null');
+ json_strip_nulls 
+------------------
+ null
+(1 row)
+
+select json_strip_nulls('[1,2,null,3,4]');
+ json_strip_nulls 
+------------------
+ [1,2,null,3,4]
+(1 row)
+
+select json_strip_nulls('{"a":1,"b":null,"c":[2,null,3],"d":{"e":4,"f":null}}');
+          json_strip_nulls          
+------------------------------------
+ {"a":1,"c":[2,null,3],"d":{"e":4}}
+(1 row)
+
+select json_strip_nulls('[1,{"a":1,"b":null,"c":2},3]');
+  json_strip_nulls   
+---------------------
+ [1,{"a":1,"c":2},3]
+(1 row)
+
+-- an empty object is not null and should not be stripped
+select json_strip_nulls('{"a": {"b": null, "c": null}, "d": {} }');
+ json_strip_nulls 
+------------------
+ {"a":{},"d":{}}
+(1 row)
+

src/test/regress/expected/json_1.out

@@ -1582,3 +1582,53 @@ select * from json_to_recordset('[{"a":1,"b":{"d":"foo"},"c":true},{"a":2,"c":fa
  2 | {"d":"bar"} | f
 (2 rows)
 
+-- json_strip_nulls
+select json_strip_nulls(null);
+ json_strip_nulls 
+------------------
+ 
+(1 row)
+
+select json_strip_nulls('1');
+ json_strip_nulls 
+------------------
+ 1
+(1 row)
+
+select json_strip_nulls('"a string"');
+ json_strip_nulls 
+------------------
+ "a string"
+(1 row)
+
+select json_strip_nulls('null');
+ json_strip_nulls 
+------------------
+ null
+(1 row)
+
+select json_strip_nulls('[1,2,null,3,4]');
+ json_strip_nulls 
+------------------
+ [1,2,null,3,4]
+(1 row)
+
+select json_strip_nulls('{"a":1,"b":null,"c":[2,null,3],"d":{"e":4,"f":null}}');
+          json_strip_nulls          
+------------------------------------
+ {"a":1,"c":[2,null,3],"d":{"e":4}}
+(1 row)
+
+select json_strip_nulls('[1,{"a":1,"b":null,"c":2},3]');
+  json_strip_nulls   
+---------------------
+ [1,{"a":1,"c":2},3]
+(1 row)
+
+-- an empty object is not null and should not be stripped
+select json_strip_nulls('{"a": {"b": null, "c": null}, "d": {} }');
+ json_strip_nulls 
+------------------
+ {"a":{},"d":{}}
+(1 row)
+

src/test/regress/expected/jsonb.out

@@ -2472,3 +2472,53 @@ SELECT '{"n":null,"a":1,"b":[1,2],"c":{"1":2},"d":{"1":[2,3]}}'::jsonb ? 'e';
  f
 (1 row)
 
+-- jsonb_strip_nulls
+select jsonb_strip_nulls(null);
+ jsonb_strip_nulls 
+-------------------
+ 
+(1 row)
+
+select jsonb_strip_nulls('1');
+ jsonb_strip_nulls 
+-------------------
+ 1
+(1 row)
+
+select jsonb_strip_nulls('"a string"');
+ jsonb_strip_nulls 
+-------------------
+ "a string"
+(1 row)
+
+select jsonb_strip_nulls('null');
+ jsonb_strip_nulls 
+-------------------
+ null
+(1 row)
+
+select jsonb_strip_nulls('[1,2,null,3,4]');
+ jsonb_strip_nulls  
+--------------------
+ [1, 2, null, 3, 4]
+(1 row)
+
+select jsonb_strip_nulls('{"a":1,"b":null,"c":[2,null,3],"d":{"e":4,"f":null}}');
+             jsonb_strip_nulls              
+--------------------------------------------
+ {"a": 1, "c": [2, null, 3], "d": {"e": 4}}
+(1 row)
+
+select jsonb_strip_nulls('[1,{"a":1,"b":null,"c":2},3]');
+    jsonb_strip_nulls     
+--------------------------
+ [1, {"a": 1, "c": 2}, 3]
+(1 row)
+
+-- an empty object is not null and should not be stripped
+select jsonb_strip_nulls('{"a": {"b": null, "c": null}, "d": {} }');
+ jsonb_strip_nulls  
+--------------------
+ {"a": {}, "d": {}}
+(1 row)
+

src/test/regress/expected/jsonb_1.out

@@ -2472,3 +2472,53 @@ SELECT '{"n":null,"a":1,"b":[1,2],"c":{"1":2},"d":{"1":[2,3]}}'::jsonb ? 'e';
  f
 (1 row)
 
+-- jsonb_strip_nulls
+select jsonb_strip_nulls(null);
+ jsonb_strip_nulls 
+-------------------
+ 
+(1 row)
+
+select jsonb_strip_nulls('1');
+ jsonb_strip_nulls 
+-------------------
+ 1
+(1 row)
+
+select jsonb_strip_nulls('"a string"');
+ jsonb_strip_nulls 
+-------------------
+ "a string"
+(1 row)
+
+select jsonb_strip_nulls('null');
+ jsonb_strip_nulls 
+-------------------
+ null
+(1 row)
+
+select jsonb_strip_nulls('[1,2,null,3,4]');
+ jsonb_strip_nulls  
+--------------------
+ [1, 2, null, 3, 4]
+(1 row)
+
+select jsonb_strip_nulls('{"a":1,"b":null,"c":[2,null,3],"d":{"e":4,"f":null}}');
+             jsonb_strip_nulls              
+--------------------------------------------
+ {"a": 1, "c": [2, null, 3], "d": {"e": 4}}
+(1 row)
+
+select jsonb_strip_nulls('[1,{"a":1,"b":null,"c":2},3]');
+    jsonb_strip_nulls     
+--------------------------
+ [1, {"a": 1, "c": 2}, 3]
+(1 row)
+
+-- an empty object is not null and should not be stripped
+select jsonb_strip_nulls('{"a": {"b": null, "c": null}, "d": {} }');
+ jsonb_strip_nulls  
+--------------------
+ {"a": {}, "d": {}}
+(1 row)
+

src/test/regress/sql/json.sql

@@ -509,3 +509,23 @@ select * from json_to_recordset('[{"a":1,"b":"foo","d":false},{"a":2,"b":"bar","
 
 select * from json_to_recordset('[{"a":1,"b":{"d":"foo"},"c":true},{"a":2,"c":false,"b":{"d":"bar"}}]')
     as x(a int, b json, c boolean);
+
+
+-- json_strip_nulls
+
+select json_strip_nulls(null);
+
+select json_strip_nulls('1');
+
+select json_strip_nulls('"a string"');
+
+select json_strip_nulls('null');
+
+select json_strip_nulls('[1,2,null,3,4]');
+
+select json_strip_nulls('{"a":1,"b":null,"c":[2,null,3],"d":{"e":4,"f":null}}');
+
+select json_strip_nulls('[1,{"a":1,"b":null,"c":2},3]');
+
+-- an empty object is not null and should not be stripped
+select json_strip_nulls('{"a": {"b": null, "c": null}, "d": {} }');

src/test/regress/sql/jsonb.sql

@@ -542,3 +542,22 @@ SELECT '{"n":null,"a":1,"b":[1,2],"c":{"1":2},"d":{"1":[2,3]}}'::jsonb ? 'b';
 SELECT '{"n":null,"a":1,"b":[1,2],"c":{"1":2},"d":{"1":[2,3]}}'::jsonb ? 'c';
 SELECT '{"n":null,"a":1,"b":[1,2],"c":{"1":2},"d":{"1":[2,3]}}'::jsonb ? 'd';
 SELECT '{"n":null,"a":1,"b":[1,2],"c":{"1":2},"d":{"1":[2,3]}}'::jsonb ? 'e';
+
+-- jsonb_strip_nulls
+
+select jsonb_strip_nulls(null);
+
+select jsonb_strip_nulls('1');
+
+select jsonb_strip_nulls('"a string"');
+
+select jsonb_strip_nulls('null');
+
+select jsonb_strip_nulls('[1,2,null,3,4]');
+
+select jsonb_strip_nulls('{"a":1,"b":null,"c":[2,null,3],"d":{"e":4,"f":null}}');
+
+select jsonb_strip_nulls('[1,{"a":1,"b":null,"c":2},3]');
+
+-- an empty object is not null and should not be stripped
+select jsonb_strip_nulls('{"a": {"b": null, "c": null}, "d": {} }');