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.

Author: adunstan

doc/src/sgml/func.sgml

@@ -10716,6 +10716,19 @@ table2-mapping
 </programlisting>
        </entry>
       </row>
+      <row>
+       <entry><para><literal>json_strip_nulls(from_json json)</literal>
+         </para><para><literal>jsonb_strip_nulls(from_json jsonb)</literal>
+       </para></entry>
+       <entry><para><type>json</type></para><para><type>jsonb</type></para></entry>
+       <entry>
+         Returns <replaceable>from_json</replaceable>
+         with all object fields that have null values omitted. Other null values
+         are untouched.
+       </entry>
+       <entry><literal>json_strip_nulls('[{"f1":1,"f2":null},2,null,3]')</literal></entry>
+       <entry><literal>[{"f1":1},2,null,3]</literal></entry>
+       </row>
      </tbody>
     </tgroup>
    </table>
@@ -10752,6 +10765,16 @@ table2-mapping
     </para>
   </note>
 
+  <note>
+    <para>
+      If the argument to <literal>json_strip_nulls</> contains duplicate
+      field names in any object, the result could be semantically somewhat
+      different, depending on the order in which they occur. This is not an
+      issue for <literal>jsonb_strip_nulls</> since jsonb values never have
+      duplicate object field names.
+    </para>
+  </note>
+
   <para>
     See also <xref linkend="functions-aggregate"> for the aggregate
     function <function>json_agg</function> which aggregates record

src/backend/utils/adt/jsonfuncs.c

@@ -105,6 +105,15 @@ static void populate_recordset_object_end(void *state);
 static void populate_recordset_array_start(void *state);
 static void populate_recordset_array_element_start(void *state, bool isnull);
 
+/* semantic action functions for json_strip_nulls */
+static void sn_object_start(void *state);
+static void sn_object_end(void *state);
+static void sn_array_start(void *state);
+static void sn_array_end(void *state);
+static void sn_object_field_start (void *state, char *fname, bool isnull);
+static void sn_array_element_start (void *state, bool isnull);
+static void sn_scalar(void *state, char *token, JsonTokenType tokentype);
+
 /* worker function for populate_recordset and to_recordset */
 static Datum populate_recordset_worker(FunctionCallInfo fcinfo, const char *funcname,
 						  bool have_record_arg);
@@ -225,6 +234,13 @@ typedef struct PopulateRecordsetState
 	MemoryContext fn_mcxt;		/* used to stash IO funcs */
 } PopulateRecordsetState;
 
+/* state for json_strip_nulls */
+typedef struct StripnullState{
+	JsonLexContext *lex;
+	StringInfo  strval;
+	bool skip_next_null;
+} StripnullState;
+
 /* Turn a jsonb object into a record */
 static void make_row_from_rec_and_jsonb(Jsonb *element,
 							PopulateRecordsetState *state);
@@ -2996,3 +3012,184 @@ findJsonbValueFromContainerLen(JsonbContainer *container, uint32 flags,
 
 	return findJsonbValueFromContainer(container, flags, &k);
 }
+
+/*
+ * Semantic actions for json_strip_nulls.
+ *
+ * Simply repeat the input on the output unless we encounter
+ * a null object field. State for this is set when the field
+ * is started and reset when the scalar action (which must be next)
+ * is called.
+ */
+
+static void
+sn_object_start(void *state)
+{
+	StripnullState *_state = (StripnullState *) state;
+	appendStringInfoCharMacro(_state->strval, '{');
+}
+
+static void
+sn_object_end(void *state)
+{
+	StripnullState *_state = (StripnullState *) state;
+	appendStringInfoCharMacro(_state->strval, '}');
+}
+
+static void
+sn_array_start(void *state)
+{
+	StripnullState *_state = (StripnullState *) state;
+	appendStringInfoCharMacro(_state->strval, '[');
+}
+
+static void
+sn_array_end(void *state)
+{
+	StripnullState *_state = (StripnullState *) state;
+	appendStringInfoCharMacro(_state->strval, ']');
+}
+
+static void
+sn_object_field_start (void *state, char *fname, bool isnull)
+{
+	StripnullState *_state = (StripnullState *) state;
+
+	if (isnull)
+	{
+		/*
+		 * The next thing must be a scalar or isnull couldn't be true,
+		 * so there is no danger of this state being carried down
+		 * into a nested  object or array. The flag will be reset in the
+		 * scalar action.
+		 */
+		_state->skip_next_null = true;
+		return;
+	}
+
+	if (_state->strval->data[_state->strval->len - 1] != '{')
+		appendStringInfoCharMacro(_state->strval, ',');
+
+	/*
+	 * Unfortunately we don't have the quoted and escaped string any more,
+	 * so we have to re-escape it.
+	 */
+	escape_json(_state->strval,fname);
+
+	appendStringInfoCharMacro(_state->strval, ':');
+}
+
+static void
+sn_array_element_start (void *state, bool isnull)
+{
+	StripnullState *_state = (StripnullState *) state;
+
+	if (_state->strval->data[_state->strval->len - 1] != '[')
+		appendStringInfoCharMacro(_state->strval, ',');
+}
+
+static void
+sn_scalar(void *state, char *token, JsonTokenType tokentype)
+{
+	StripnullState *_state = (StripnullState *) state;
+
+	if (_state->skip_next_null)
+	{
+		Assert (tokentype == JSON_TOKEN_NULL);
+		_state->skip_next_null = false;
+		return;
+	}
+
+	if (tokentype == JSON_TOKEN_STRING)
+		escape_json(_state->strval, token);
+	else
+		appendStringInfoString(_state->strval, token);
+}
+
+/*
+ * SQL function json_strip_nulls(json) -> json
+ */
+Datum
+json_strip_nulls(PG_FUNCTION_ARGS)
+{
+	text	   *json = PG_GETARG_TEXT_P(0);
+	StripnullState  *state;
+	JsonLexContext *lex;
+	JsonSemAction *sem;
+
+	lex = makeJsonLexContext(json, true);
+	state = palloc0(sizeof(StripnullState));
+	sem = palloc0(sizeof(JsonSemAction));
+
+	state->strval = makeStringInfo();
+	state->skip_next_null = false;
+	state->lex = lex;
+
+	sem->semstate = (void *) state;
+	sem->object_start = sn_object_start;
+	sem->object_end = sn_object_end;
+	sem->array_start = sn_array_start;
+	sem->array_end = sn_array_end;
+	sem->scalar = sn_scalar;
+	sem->array_element_start = sn_array_element_start;
+	sem->object_field_start = sn_object_field_start;
+
+	pg_parse_json(lex, sem);
+
+	PG_RETURN_TEXT_P(cstring_to_text_with_len(state->strval->data,
+											  state->strval->len));
+
+}
+
+/*
+ * SQL function jsonb_strip_nulls(jsonb) -> jsonb
+ */
+Datum
+jsonb_strip_nulls(PG_FUNCTION_ARGS)
+{
+	Jsonb * jb = PG_GETARG_JSONB(0);
+	JsonbIterator *it;
+	JsonbParseState *parseState = NULL;
+	JsonbValue *res = NULL;
+	int type;
+	JsonbValue v,k;
+	bool last_was_key = false;
+
+	if (JB_ROOT_IS_SCALAR(jb))
+		PG_RETURN_POINTER(jb);
+
+	it = JsonbIteratorInit(&jb->root);
+
+	while ((type = JsonbIteratorNext(&it, &v, false)) != WJB_DONE)
+	{
+		Assert( ! (type == WJB_KEY && last_was_key));
+
+		if (type == WJB_KEY)
+		{
+			/* stash the key until we know if it has a null value */
+			k = v;
+			last_was_key = true;
+			continue;
+		}
+
+		if (last_was_key)
+		{
+			/* if the last element was a key this one can't be */
+			last_was_key = false;
+
+			/* skip this field if value is null */
+			if (type == WJB_VALUE && v.type == jbvNull)
+				continue;
+
+			/* otherwise, do a delayed push of the key */
+			res = pushJsonbValue(&parseState, WJB_KEY, &k);
+		}
+
+		if (type == WJB_VALUE || type == WJB_ELEM)
+			res = pushJsonbValue(&parseState, type, &v);
+		else
+			res = pushJsonbValue(&parseState, type, NULL);
+	}
+
+	PG_RETURN_POINTER(JsonbValueToJsonb(res));
+}

src/include/catalog/catversion.h

@@ -53,6 +53,6 @@
  */
 
 /*							yyyymmddN */
-#define CATALOG_VERSION_NO	201412022
+#define CATALOG_VERSION_NO	201412121
 
 #endif

src/include/catalog/pg_proc.h

@@ -4292,6 +4292,8 @@ DATA(insert OID = 3203 (  json_object	 PGNSP PGUID 12 1 0 0 0 f f f f t f i 2 0
 DESCR("map text arrays of keys and values to json object");
 DATA(insert OID = 3176 (  to_json	   PGNSP PGUID 12 1 0 0 0 f f f f t f s 1 0 114 "2283" _null_ _null_ _null_ _null_ to_json _null_ _null_ _null_ ));
 DESCR("map input to json");
+DATA(insert OID = 3261 (  json_strip_nulls	   PGNSP PGUID 12 1 0 0 0 f f f f t f i 1 0 114 "114" _null_ _null_ _null_ _null_ json_strip_nulls _null_ _null_ _null_ ));
+DESCR("remove object fields with null values from json");
 
 DATA(insert OID = 3947 (  json_object_field			PGNSP PGUID 12 1 0 0 0 f f f f t f i 2 0 114 "114 25" _null_ _null_ "{from_json, field_name}" _null_ json_object_field _null_ _null_ _null_ ));
 DATA(insert OID = 3948 (  json_object_field_text	PGNSP PGUID 12 1 0 0 0 f f f f t f i 2 0 25  "114 25" _null_ _null_ "{from_json, field_name}" _null_ json_object_field_text _null_ _null_ _null_ ));
@@ -4654,6 +4656,9 @@ DESCR("I/O");
 DATA(insert OID =  3803 (  jsonb_send		PGNSP PGUID 12 1 0 0 0 f f f f t f i 1 0 17 "3802" _null_ _null_ _null_ _null_	jsonb_send _null_ _null_ _null_ ));
 DESCR("I/O");
 
+DATA(insert OID = 3262 (  jsonb_strip_nulls	   PGNSP PGUID 12 1 0 0 0 f f f f t f i 1 0 3802 "3802" _null_ _null_ _null_ _null_ jsonb_strip_nulls _null_ _null_ _null_ ));
+DESCR("remove object fields with null values from jsonb");
+
 DATA(insert OID = 3478 (  jsonb_object_field			PGNSP PGUID 12 1 0 0 0 f f f f t f i 2 0 3802 "3802 25" _null_ _null_ "{from_json, field_name}" _null_ jsonb_object_field _null_ _null_ _null_ ));
 DATA(insert OID = 3214 (  jsonb_object_field_text	PGNSP PGUID 12 1 0 0 0 f f f f t f i 2 0 25  "3802 25" _null_ _null_ "{from_json, field_name}" _null_ jsonb_object_field_text _null_ _null_ _null_ ));
 DATA(insert OID = 3215 (  jsonb_array_element		PGNSP PGUID 12 1 0 0 0 f f f f t f i 2 0 3802 "3802 23" _null_ _null_ "{from_json, element_index}" _null_ jsonb_array_element _null_ _null_ _null_ ));

src/include/utils/json.h

@@ -63,6 +63,7 @@ extern Datum json_populate_record(PG_FUNCTION_ARGS);
 extern Datum json_populate_recordset(PG_FUNCTION_ARGS);
 extern Datum json_to_record(PG_FUNCTION_ARGS);
 extern Datum json_to_recordset(PG_FUNCTION_ARGS);
+extern Datum json_strip_nulls(PG_FUNCTION_ARGS);
 
 extern Datum jsonb_object_field(PG_FUNCTION_ARGS);
 extern Datum jsonb_object_field_text(PG_FUNCTION_ARGS);
@@ -80,5 +81,6 @@ extern Datum jsonb_populate_record(PG_FUNCTION_ARGS);
 extern Datum jsonb_populate_recordset(PG_FUNCTION_ARGS);
 extern Datum jsonb_to_record(PG_FUNCTION_ARGS);
 extern Datum jsonb_to_recordset(PG_FUNCTION_ARGS);
+extern Datum jsonb_strip_nulls(PG_FUNCTION_ARGS);
 
 #endif   /* JSON_H */

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)
+