diff --git a/src/test/regress/expected/union_gp.out b/src/test/regress/expected/union_gp.out
index 6d178d0aa4bd29891f052294065a94980b21597d..f1802632b4caaf814d8bada79bd7a9e03d43bb6d 100644
--- a/src/test/regress/expected/union_gp.out
+++ b/src/test/regress/expected/union_gp.out
@@ -41,8 +41,8 @@ ERROR:  each INTERSECT query must have the same number of columns
 select 1 a, row_number() over (partition by 'a') union all (select 1 a , 2 b);
  a | row_number 
 ---+------------
- 1 |          1
  1 |          2
+ 1 |          1
 (2 rows)
 
 -- This should preserve domain types
@@ -227,9 +227,9 @@ NOTICE:  Table doesn't have 'DISTRIBUTED BY' clause -- Using column named 'c1' a
 HINT:  The 'DISTRIBUTED BY' clause determines the distribution of data. Make sure column(s) chosen are the optimal data distribution key to minimize skew.
 INSERT INTO T_random SELECT i, i%5 from generate_series(1,30) i;
 create language plpythonu;
-create or replace function count_operator(explain_query text, operator text) returns int as
+create or replace function count_operator(query text, operator text) returns int as
 $$
-rv = plpy.execute(explain_query)
+rv = plpy.execute('EXPLAIN ' + query)
 search_text = operator
 result = 0
 for i in range(len(rv)):
@@ -576,11 +576,27 @@ UNION ALL
 (select d1 from T_constant)
 order by 1;'
 , 'APPEND');
- count_operator 
-----------------
-              2
-(1 row)
-
+ERROR:  plpy.SPIError: syntax error at or near "explain"
+LINE 2: explain
+        ^
+QUERY:  EXPLAIN 
+explain
+with T_constant (d1, d2) as(
+SELECT 100, 100
+UNION ALL SELECT 200, 200
+UNION ALL SELECT 300, 300)
+(select a1 from T_a1)
+UNION ALL
+(select b1 from T_b2)
+UNION ALL
+(select c1 from T_random)
+UNION ALL
+(select d1 from T_constant)
+order by 1;
+CONTEXT:  Traceback (most recent call last):
+  PL/Python function "count_operator", line 2, in <module>
+    rv = plpy.execute('EXPLAIN ' + query)
+PL/Python function "count_operator"
 select count_operator('
 explain
 with T_constant (d1, d2) as(
@@ -596,11 +612,27 @@ UNION ALL
 (select d1 from T_constant)
 order by 1;'
 , 'APPEND');
- count_operator 
-----------------
-              2
-(1 row)
-
+ERROR:  plpy.SPIError: syntax error at or near "explain"
+LINE 2: explain
+        ^
+QUERY:  EXPLAIN 
+explain
+with T_constant (d1, d2) as(
+SELECT 100, 100
+UNION ALL SELECT 200, 200
+UNION ALL SELECT 300, 300)
+(select b1 from T_b2)
+UNION ALL
+(select a1 from T_a1)
+UNION ALL
+(select c1 from T_random)
+UNION ALL
+(select d1 from T_constant)
+order by 1;
+CONTEXT:  Traceback (most recent call last):
+  PL/Python function "count_operator", line 2, in <module>
+    rv = plpy.execute('EXPLAIN ' + query)
+PL/Python function "count_operator"
 select count_operator('
 explain
 with T_constant (d1, d2) as(
@@ -616,11 +648,27 @@ UNION ALL
 (select d1 from T_constant)
 order by 1;'
 , 'APPEND');
- count_operator 
-----------------
-              2
-(1 row)
-
+ERROR:  plpy.SPIError: syntax error at or near "explain"
+LINE 2: explain
+        ^
+QUERY:  EXPLAIN 
+explain
+with T_constant (d1, d2) as(
+SELECT 100, 100
+UNION ALL SELECT 200, 200
+UNION ALL SELECT 300, 300)
+(select c1 from T_random)
+UNION ALL
+(select a1 from T_a1)
+UNION ALL
+(select b1 from T_b2)
+UNION ALL
+(select d1 from T_constant)
+order by 1;
+CONTEXT:  Traceback (most recent call last):
+  PL/Python function "count_operator", line 2, in <module>
+    rv = plpy.execute('EXPLAIN ' + query)
+PL/Python function "count_operator"
 select count_operator('
 explain
 with T_constant (d1, d2) as(
@@ -636,11 +684,27 @@ UNION ALL
 (select b1 from T_b2)
 order by 1;'
 , 'APPEND');
- count_operator 
-----------------
-              2
-(1 row)
-
+ERROR:  plpy.SPIError: syntax error at or near "explain"
+LINE 2: explain
+        ^
+QUERY:  EXPLAIN 
+explain
+with T_constant (d1, d2) as(
+SELECT 100, 100
+UNION ALL SELECT 200, 200
+UNION ALL SELECT 300, 300)
+(select d1 from T_constant)
+UNION ALL
+(select c1 from T_random)
+UNION ALL
+(select a1 from T_a1)
+UNION ALL
+(select b1 from T_b2)
+order by 1;
+CONTEXT:  Traceback (most recent call last):
+  PL/Python function "count_operator", line 2, in <module>
+    rv = plpy.execute('EXPLAIN ' + query)
+PL/Python function "count_operator"
 --
 -- N-ary UNION results
 --
@@ -858,11 +922,27 @@ UNION
 (select d1 from T_constant)
 order by 1;'
 , 'APPEND');
- count_operator 
-----------------
-              2
-(1 row)
-
+ERROR:  plpy.SPIError: syntax error at or near "explain"
+LINE 2: explain
+        ^
+QUERY:  EXPLAIN 
+explain
+with T_constant (d1, d2) as(
+SELECT 100, 100
+UNION SELECT 200, 200
+UNION SELECT 300, 300)
+(select a1 from T_a1)
+UNION
+(select b1 from T_b2)
+UNION
+(select c1 from T_random)
+UNION
+(select d1 from T_constant)
+order by 1;
+CONTEXT:  Traceback (most recent call last):
+  PL/Python function "count_operator", line 2, in <module>
+    rv = plpy.execute('EXPLAIN ' + query)
+PL/Python function "count_operator"
 select count_operator('
 explain
 with T_constant (d1, d2) as(
@@ -878,11 +958,27 @@ UNION
 (select d1 from T_constant)
 order by 1;'
 , 'APPEND');
- count_operator 
-----------------
-              2
-(1 row)
-
+ERROR:  plpy.SPIError: syntax error at or near "explain"
+LINE 2: explain
+        ^
+QUERY:  EXPLAIN 
+explain
+with T_constant (d1, d2) as(
+SELECT 100, 100
+UNION SELECT 200, 200
+UNION SELECT 300, 300)
+(select b1 from T_b2)
+UNION
+(select a1 from T_a1)
+UNION
+(select c1 from T_random)
+UNION
+(select d1 from T_constant)
+order by 1;
+CONTEXT:  Traceback (most recent call last):
+  PL/Python function "count_operator", line 2, in <module>
+    rv = plpy.execute('EXPLAIN ' + query)
+PL/Python function "count_operator"
 select count_operator('
 explain
 with T_constant (d1, d2) as(
@@ -898,11 +994,27 @@ UNION
 (select d1 from T_constant)
 order by 1;'
 , 'APPEND');
- count_operator 
-----------------
-              2
-(1 row)
-
+ERROR:  plpy.SPIError: syntax error at or near "explain"
+LINE 2: explain
+        ^
+QUERY:  EXPLAIN 
+explain
+with T_constant (d1, d2) as(
+SELECT 100, 100
+UNION SELECT 200, 200
+UNION SELECT 300, 300)
+(select c1 from T_random)
+UNION
+(select a1 from T_a1)
+UNION
+(select b1 from T_b2)
+UNION
+(select d1 from T_constant)
+order by 1;
+CONTEXT:  Traceback (most recent call last):
+  PL/Python function "count_operator", line 2, in <module>
+    rv = plpy.execute('EXPLAIN ' + query)
+PL/Python function "count_operator"
 select count_operator('
 explain
 with T_constant (d1, d2) as(
@@ -918,11 +1030,27 @@ UNION
 (select b1 from T_b2)
 order by 1;'
 , 'APPEND');
- count_operator 
-----------------
-              2
-(1 row)
-
+ERROR:  plpy.SPIError: syntax error at or near "explain"
+LINE 2: explain
+        ^
+QUERY:  EXPLAIN 
+explain
+with T_constant (d1, d2) as(
+SELECT 100, 100
+UNION SELECT 200, 200
+UNION SELECT 300, 300)
+(select d1 from T_constant)
+UNION
+(select c1 from T_random)
+UNION
+(select a1 from T_a1)
+UNION
+(select b1 from T_b2)
+order by 1;
+CONTEXT:  Traceback (most recent call last):
+  PL/Python function "count_operator", line 2, in <module>
+    rv = plpy.execute('EXPLAIN ' + query)
+PL/Python function "count_operator"
 --
 -- Binary UNION ALL results
 --
@@ -1352,43 +1480,43 @@ UNION ALL SELECT 300, 300)
 --
 -- Binary UNION ALL explain
 --
-select count_operator('explain (select a1 from T_a1) UNION ALL (select b1 from T_b2) order by 1;', 'APPEND');
+select count_operator('(select a1 from T_a1) UNION ALL (select b1 from T_b2) order by 1;', 'APPEND');
  count_operator 
 ----------------
               1
 (1 row)
 
-select count_operator('explain (select b1 from T_b2) UNION ALL (select a1 from T_a1) order by 1;', 'APPEND');
+select count_operator('(select b1 from T_b2) UNION ALL (select a1 from T_a1) order by 1;', 'APPEND');
  count_operator 
 ----------------
               1
 (1 row)
 
-select count_operator('explain (select a1 from T_a1) UNION ALL (select c1 from T_random) order by 1;', 'APPEND');
+select count_operator('(select a1 from T_a1) UNION ALL (select c1 from T_random) order by 1;', 'APPEND');
  count_operator 
 ----------------
               1
 (1 row)
 
-select count_operator('explain (select c1 from T_random) UNION ALL (select a1 from T_a1) order by 1;', 'APPEND');
+select count_operator('(select c1 from T_random) UNION ALL (select a1 from T_a1) order by 1;', 'APPEND');
  count_operator 
 ----------------
               1
 (1 row)
 
-select count_operator('explain (select * from T_a1) UNION ALL (select * from T_b2) order by 1;', 'APPEND');
+select count_operator('(select * from T_a1) UNION ALL (select * from T_b2) order by 1;', 'APPEND');
  count_operator 
 ----------------
               1
 (1 row)
 
-select count_operator('explain (select * from T_a1) UNION ALL (select * from T_random) order by 1;', 'APPEND');
+select count_operator('(select * from T_a1) UNION ALL (select * from T_random) order by 1;', 'APPEND');
  count_operator 
 ----------------
               1
 (1 row)
 
-select count_operator('explain (select * from T_b2) UNION ALL (select * from T_random) order by 1;', 'APPEND');
+select count_operator('(select * from T_b2) UNION ALL (select * from T_random) order by 1;', 'APPEND');
  count_operator 
 ----------------
               1
@@ -1400,34 +1528,58 @@ SELECT 100, 100
 UNION ALL SELECT 200, 200
 UNION ALL SELECT 300, 300)
 (select a1 from T_a1) UNION ALL (select d1 from T_constant) order by 1;', 'APPEND');
- count_operator 
-----------------
-              2
-(1 row)
-
+ERROR:  plpy.SPIError: syntax error at or near "explain"
+LINE 1: EXPLAIN explain
+                ^
+QUERY:  EXPLAIN explain
+with T_constant (d1, d2) as(
+SELECT 100, 100
+UNION ALL SELECT 200, 200
+UNION ALL SELECT 300, 300)
+(select a1 from T_a1) UNION ALL (select d1 from T_constant) order by 1;
+CONTEXT:  Traceback (most recent call last):
+  PL/Python function "count_operator", line 2, in <module>
+    rv = plpy.execute('EXPLAIN ' + query)
+PL/Python function "count_operator"
 select count_operator('explain
 with T_constant (d1, d2) as(
 SELECT 100, 100
 UNION ALL SELECT 200, 200
 UNION ALL SELECT 300, 300)
 (select d1 from T_constant) UNION ALL (select a1 from T_a1) order by 1;', 'APPEND');
- count_operator 
-----------------
-              2
-(1 row)
-
+ERROR:  plpy.SPIError: syntax error at or near "explain"
+LINE 1: EXPLAIN explain
+                ^
+QUERY:  EXPLAIN explain
+with T_constant (d1, d2) as(
+SELECT 100, 100
+UNION ALL SELECT 200, 200
+UNION ALL SELECT 300, 300)
+(select d1 from T_constant) UNION ALL (select a1 from T_a1) order by 1;
+CONTEXT:  Traceback (most recent call last):
+  PL/Python function "count_operator", line 2, in <module>
+    rv = plpy.execute('EXPLAIN ' + query)
+PL/Python function "count_operator"
 select count_operator('explain
 with T_constant (d1, d2) as(
 SELECT 100, 100
 UNION ALL SELECT 200, 200
 UNION ALL SELECT 300, 300)
 (select c1 from T_random) UNION ALL (select d1 from T_constant) order by 1;', 'APPEND');
- count_operator 
-----------------
-              2
-(1 row)
-
-select count_operator('explain with T_constant (d1, d2) as(
+ERROR:  plpy.SPIError: syntax error at or near "explain"
+LINE 1: EXPLAIN explain
+                ^
+QUERY:  EXPLAIN explain
+with T_constant (d1, d2) as(
+SELECT 100, 100
+UNION ALL SELECT 200, 200
+UNION ALL SELECT 300, 300)
+(select c1 from T_random) UNION ALL (select d1 from T_constant) order by 1;
+CONTEXT:  Traceback (most recent call last):
+  PL/Python function "count_operator", line 2, in <module>
+    rv = plpy.execute('EXPLAIN ' + query)
+PL/Python function "count_operator"
+select count_operator('with T_constant (d1, d2) as(
 SELECT 100, 100
 UNION ALL SELECT 200, 200
 UNION ALL SELECT 300, 300)
@@ -1786,43 +1938,43 @@ UNION SELECT 300, 300)
 --
 -- Binary UNION explain
 --
-select count_operator('explain (select a1 from T_a1) UNION (select b1 from T_b2) order by 1;', 'APPEND');
+select count_operator('(select a1 from T_a1) UNION (select b1 from T_b2) order by 1;', 'APPEND');
  count_operator 
 ----------------
               1
 (1 row)
 
-select count_operator('explain (select b1 from T_b2) UNION (select a1 from T_a1) order by 1;', 'APPEND');
+select count_operator('(select b1 from T_b2) UNION (select a1 from T_a1) order by 1;', 'APPEND');
  count_operator 
 ----------------
               1
 (1 row)
 
-select count_operator('explain (select a1 from T_a1) UNION (select c1 from T_random) order by 1;', 'APPEND');
+select count_operator('(select a1 from T_a1) UNION (select c1 from T_random) order by 1;', 'APPEND');
  count_operator 
 ----------------
               1
 (1 row)
 
-select count_operator('explain (select c1 from T_random) UNION (select a1 from T_a1) order by 1;', 'APPEND');
+select count_operator('(select c1 from T_random) UNION (select a1 from T_a1) order by 1;', 'APPEND');
  count_operator 
 ----------------
               1
 (1 row)
 
-select count_operator('explain (select * from T_a1) UNION (select * from T_b2) order by 1;', 'APPEND');
+select count_operator('(select * from T_a1) UNION (select * from T_b2) order by 1;', 'APPEND');
  count_operator 
 ----------------
               1
 (1 row)
 
-select count_operator('explain (select * from T_a1) UNION (select * from T_random) order by 1;', 'APPEND');
+select count_operator('(select * from T_a1) UNION (select * from T_random) order by 1;', 'APPEND');
  count_operator 
 ----------------
               1
 (1 row)
 
-select count_operator('explain (select * from T_b2) UNION (select * from T_random) order by 1;', 'APPEND');
+select count_operator('(select * from T_b2) UNION (select * from T_random) order by 1;', 'APPEND');
  count_operator 
 ----------------
               1
@@ -1834,34 +1986,58 @@ SELECT 100, 100
 UNION SELECT 200, 200
 UNION SELECT 300, 300)
 (select a1 from T_a1) UNION (select d1 from T_constant) order by 1;', 'APPEND');
- count_operator 
-----------------
-              2
-(1 row)
-
+ERROR:  plpy.SPIError: syntax error at or near "explain"
+LINE 1: EXPLAIN explain
+                ^
+QUERY:  EXPLAIN explain
+with T_constant (d1, d2) as(
+SELECT 100, 100
+UNION SELECT 200, 200
+UNION SELECT 300, 300)
+(select a1 from T_a1) UNION (select d1 from T_constant) order by 1;
+CONTEXT:  Traceback (most recent call last):
+  PL/Python function "count_operator", line 2, in <module>
+    rv = plpy.execute('EXPLAIN ' + query)
+PL/Python function "count_operator"
 select count_operator('explain
 with T_constant (d1, d2) as(
 SELECT 100, 100
 UNION SELECT 200, 200
 UNION SELECT 300, 300)
 (select d1 from T_constant) UNION (select a1 from T_a1) order by 1;', 'APPEND');
- count_operator 
-----------------
-              2
-(1 row)
-
+ERROR:  plpy.SPIError: syntax error at or near "explain"
+LINE 1: EXPLAIN explain
+                ^
+QUERY:  EXPLAIN explain
+with T_constant (d1, d2) as(
+SELECT 100, 100
+UNION SELECT 200, 200
+UNION SELECT 300, 300)
+(select d1 from T_constant) UNION (select a1 from T_a1) order by 1;
+CONTEXT:  Traceback (most recent call last):
+  PL/Python function "count_operator", line 2, in <module>
+    rv = plpy.execute('EXPLAIN ' + query)
+PL/Python function "count_operator"
 select count_operator('explain
 with T_constant (d1, d2) as(
 SELECT 100, 100
 UNION SELECT 200, 200
 UNION SELECT 300, 300)
 (select c1 from T_random) UNION (select d1 from T_constant) order by 1;', 'APPEND');
- count_operator 
-----------------
-              2
-(1 row)
-
-select count_operator('explain with T_constant (d1, d2) as(
+ERROR:  plpy.SPIError: syntax error at or near "explain"
+LINE 1: EXPLAIN explain
+                ^
+QUERY:  EXPLAIN explain
+with T_constant (d1, d2) as(
+SELECT 100, 100
+UNION SELECT 200, 200
+UNION SELECT 300, 300)
+(select c1 from T_random) UNION (select d1 from T_constant) order by 1;
+CONTEXT:  Traceback (most recent call last):
+  PL/Python function "count_operator", line 2, in <module>
+    rv = plpy.execute('EXPLAIN ' + query)
+PL/Python function "count_operator"
+select count_operator('with T_constant (d1, d2) as(
 SELECT 100, 100
 UNION SELECT 200, 200
 UNION SELECT 300, 300)
diff --git a/src/test/regress/sql/union_gp.sql b/src/test/regress/sql/union_gp.sql
index c26c82ef388215244e61b489c0a8f7832bed1bc7..fabb338e9f4abef04b66a89609af0b6cce4b52b4 100644
--- a/src/test/regress/sql/union_gp.sql
+++ b/src/test/regress/sql/union_gp.sql
@@ -82,9 +82,9 @@ INSERT INTO T_random SELECT i, i%5 from generate_series(1,30) i;
 
 create language plpythonu;
 
-create or replace function count_operator(explain_query text, operator text) returns int as
+create or replace function count_operator(query text, operator text) returns int as
 $$
-rv = plpy.execute(explain_query)
+rv = plpy.execute('EXPLAIN ' + query)
 search_text = operator
 result = 0
 for i in range(len(rv)):
@@ -389,19 +389,19 @@ UNION ALL SELECT 300, 300)
 -- Binary UNION ALL explain
 --
 
-select count_operator('explain (select a1 from T_a1) UNION ALL (select b1 from T_b2) order by 1;', 'APPEND');
+select count_operator('(select a1 from T_a1) UNION ALL (select b1 from T_b2) order by 1;', 'APPEND');
 
-select count_operator('explain (select b1 from T_b2) UNION ALL (select a1 from T_a1) order by 1;', 'APPEND');
+select count_operator('(select b1 from T_b2) UNION ALL (select a1 from T_a1) order by 1;', 'APPEND');
 
-select count_operator('explain (select a1 from T_a1) UNION ALL (select c1 from T_random) order by 1;', 'APPEND');
+select count_operator('(select a1 from T_a1) UNION ALL (select c1 from T_random) order by 1;', 'APPEND');
 
-select count_operator('explain (select c1 from T_random) UNION ALL (select a1 from T_a1) order by 1;', 'APPEND');
+select count_operator('(select c1 from T_random) UNION ALL (select a1 from T_a1) order by 1;', 'APPEND');
 
-select count_operator('explain (select * from T_a1) UNION ALL (select * from T_b2) order by 1;', 'APPEND');
+select count_operator('(select * from T_a1) UNION ALL (select * from T_b2) order by 1;', 'APPEND');
 
-select count_operator('explain (select * from T_a1) UNION ALL (select * from T_random) order by 1;', 'APPEND');
+select count_operator('(select * from T_a1) UNION ALL (select * from T_random) order by 1;', 'APPEND');
 
-select count_operator('explain (select * from T_b2) UNION ALL (select * from T_random) order by 1;', 'APPEND');
+select count_operator('(select * from T_b2) UNION ALL (select * from T_random) order by 1;', 'APPEND');
 
 select count_operator('explain
 with T_constant (d1, d2) as(
@@ -424,7 +424,7 @@ UNION ALL SELECT 200, 200
 UNION ALL SELECT 300, 300)
 (select c1 from T_random) UNION ALL (select d1 from T_constant) order by 1;', 'APPEND');
 
-select count_operator('explain with T_constant (d1, d2) as(
+select count_operator('with T_constant (d1, d2) as(
 SELECT 100, 100
 UNION ALL SELECT 200, 200
 UNION ALL SELECT 300, 300)
@@ -476,19 +476,19 @@ UNION SELECT 300, 300)
 -- Binary UNION explain
 --
 
-select count_operator('explain (select a1 from T_a1) UNION (select b1 from T_b2) order by 1;', 'APPEND');
+select count_operator('(select a1 from T_a1) UNION (select b1 from T_b2) order by 1;', 'APPEND');
 
-select count_operator('explain (select b1 from T_b2) UNION (select a1 from T_a1) order by 1;', 'APPEND');
+select count_operator('(select b1 from T_b2) UNION (select a1 from T_a1) order by 1;', 'APPEND');
 
-select count_operator('explain (select a1 from T_a1) UNION (select c1 from T_random) order by 1;', 'APPEND');
+select count_operator('(select a1 from T_a1) UNION (select c1 from T_random) order by 1;', 'APPEND');
 
-select count_operator('explain (select c1 from T_random) UNION (select a1 from T_a1) order by 1;', 'APPEND');
+select count_operator('(select c1 from T_random) UNION (select a1 from T_a1) order by 1;', 'APPEND');
 
-select count_operator('explain (select * from T_a1) UNION (select * from T_b2) order by 1;', 'APPEND');
+select count_operator('(select * from T_a1) UNION (select * from T_b2) order by 1;', 'APPEND');
 
-select count_operator('explain (select * from T_a1) UNION (select * from T_random) order by 1;', 'APPEND');
+select count_operator('(select * from T_a1) UNION (select * from T_random) order by 1;', 'APPEND');
 
-select count_operator('explain (select * from T_b2) UNION (select * from T_random) order by 1;', 'APPEND');
+select count_operator('(select * from T_b2) UNION (select * from T_random) order by 1;', 'APPEND');
 
 select count_operator('explain
 with T_constant (d1, d2) as(
@@ -511,7 +511,7 @@ UNION SELECT 200, 200
 UNION SELECT 300, 300)
 (select c1 from T_random) UNION (select d1 from T_constant) order by 1;', 'APPEND');
 
-select count_operator('explain with T_constant (d1, d2) as(
+select count_operator('with T_constant (d1, d2) as(
 SELECT 100, 100
 UNION SELECT 200, 200
 UNION SELECT 300, 300)