1. support '' as default value of set type

2. '' as set type value: compatible with MySQL

src/common/backend/nodes/outfuncs.cpp

@@ -1134,7 +1134,7 @@ static void _outCommonRemoteQueryPart(StringInfo str, T* node)
         appendStringInfo(str, " :rq_param_types");
     }
     for (i = 0; i < node->rq_num_params; i++) {
-        appendStringInfo(str, " %d", node->rq_param_types[i]);    
+        appendStringInfo(str, " %d", node->rq_param_types[i]);
     }
     
     WRITE_BOOL_FIELD(rq_params_internal);
@@ -2139,18 +2139,25 @@ static void _outConst(StringInfo str, Const* node)
     } else {
         Oid typoutput;
         bool typIsVarlena = false;
-
-        /*
-         * For user-define type
-         *
-         * decode datum to string with Output functions identified by
-         * datatype since data type oid may be inconsistent on different
-         * nodes. the string will be translated to datum with inputfunction
-         * when we read the node string
+        /* For user-define set type: if the default value is ''(means empty
+         * set, not null), It should not be output as :udftypevalue <>.
+         * It will be taken as null value.
          */
-        appendStringInfo(str, " :udftypevalue ");
-        getTypeOutputInfo(node->consttype, &typoutput, &typIsVarlena);
-        _outToken(str, OidOutputFunctionCall(typoutput, node->constvalue));
+        if (type_is_set(node->consttype)) {
+            _outDatum(str, node->constvalue, node->constlen, node->constbyval);
+        } else {
+            /*
+             * For user-define type
+             *
+             * decode datum to string with Output functions identified by
+             * datatype since data type oid may be inconsistent on different
+             * nodes. the string will be translated to datum with inputfunction
+             * when we read the node string
+             */
+            appendStringInfo(str, " :udftypevalue ");
+            getTypeOutputInfo(node->consttype, &typoutput, &typIsVarlena);
+            _outToken(str, OidOutputFunctionCall(typoutput, node->constvalue));
+        }
     }
 
     WRITE_CFGINFO_FIELD(consttype, constvalue);

src/common/backend/utils/adt/set.cpp

@@ -1118,7 +1118,7 @@ static void preprocess_set_value(char *value, Oid settypoid, VarBit **result)
     bool hasEmpty = false;
     
     if (*value == '\0') {
-        hasEmpty = true;
+        return;
     } else {
         int len = strlen(value);
         if (*value == ',' || *(value + len - 1) == ',') {
@@ -1140,7 +1140,7 @@ static VarBit* get_set_in_result(Oid settypoid, char *setlabels)
     VarBit *result = NULL;
     char* next_token = NULL;
     char* labels = pstrdup(setlabels);
-    
+
     preprocess_set_value(setlabels, settypoid, &result);
 
     char* token = strtok_s(labels, SETLABELDELIMIT, &next_token);

src/test/regress/expected/hw_datatype_set.out

@@ -4105,3 +4105,48 @@ select pg_catalog.pg_get_tabledef('t2');
  WITH (orientation=row, compression=no);
 (1 row)
 
+-- test '' as set member
+-- 1. define set type not include ''
+drop table if exists t1;
+create table t1 (c1 set('a', 'b') not null default '');
+NOTICE:  CREATE TABLE will create implicit set "t1_c1_set" for column "t1.c1"
+insert into t1 values ('');
+insert into t1 values (default);
+select c1, c1+0 from t1; -- expect value 0
+ c1 | ?column? 
+----+----------
+    |        0
+    |        0
+(2 rows)
+
+insert into t1 values ('a,,b'); -- expect error
+ERROR:  invalid input value for set t1_c1_set: ''
+LINE 1: insert into t1 values ('a,,b');
+                               ^
+CONTEXT:  referenced column: c1
+-- 2. define set type include ''
+drop table if exists t2;
+create table t2 (c1 set('a', '', 'b') default '');
+NOTICE:  CREATE TABLE will create implicit set "t2_c1_set" for column "t2.c1"
+insert into t2 values (''); -- expect value 0, not 2
+insert into t2 values (default);
+select c1, c1+0 from t2;
+ c1 | ?column? 
+----+----------
+    |        0
+    |        0
+(2 rows)
+
+insert into t2 values ('a,,b'); -- expect 2, not 0
+insert into t2 values (',a,b'); -- expect 2, not 0
+select c1, c1+0 from t2 order by 2;
+  c1  | ?column? 
+------+----------
+      |        0
+      |        0
+ a,,b |        7
+ a,,b |        7
+(4 rows)
+
+drop table t1;
+drop table t2;

src/test/regress/sql/hw_datatype_set.sql

@@ -711,4 +711,25 @@ create table t2 (
   c2 set('1', '', '3'),
   c3 set('1','2','3','4','5','6','7','8','9','10','11','12','13','14','15','16','17','18','19','20','21','22','23','24','25','26','27','28','29','30','31','32','33','34','35','36','37','38','39','40','41','42','43','44','45','46','47','48','49','50','51','52','53','54','55','56','57','58','59','60','61','62','63','64')
 );
-select pg_catalog.pg_get_tabledef('t2');
+select pg_catalog.pg_get_tabledef('t2');
+
+-- test '' as set member
+-- 1. define set type not include ''
+drop table if exists t1;
+create table t1 (c1 set('a', 'b') not null default '');
+insert into t1 values ('');
+insert into t1 values (default);
+select c1, c1+0 from t1; -- expect value 0
+insert into t1 values ('a,,b'); -- expect error
+
+-- 2. define set type include ''
+drop table if exists t2;
+create table t2 (c1 set('a', '', 'b') default '');
+insert into t2 values (''); -- expect value 0, not 2
+insert into t2 values (default);
+select c1, c1+0 from t2;
+insert into t2 values ('a,,b'); -- expect 2, not 0
+insert into t2 values (',a,b'); -- expect 2, not 0
+select c1, c1+0 from t2 order by 2;
+drop table t1;
+drop table t2;