Add test for using transaction ID in parallel worker

src/test/regress/expected/multi_distributed_transaction_id.out

@@ -125,3 +125,41 @@ SELECT initiator_node_identifier, transaction_number, transaction_stamp, (proces
 ROLLBACK PREPARED 'dist_xact_id_test';
 -- set back to the original zone
 SET TIME ZONE DEFAULT;
+-- parallel safe wrapper for getting the current transaction number
+CREATE OR REPLACE FUNCTION parallel_worker_transaction_id_test() 
+RETURNS bigint STRICT VOLATILE PARALLEL SAFE AS $$
+	SELECT transaction_number FROM get_current_transaction_id();
+$$ LANGUAGE sql;
+-- force the transaction ID to be used in a parallel plan
+BEGIN;
+SELECT assign_distributed_transaction_id(50, 1234567, '2016-01-01 00:00:00+0');
+ assign_distributed_transaction_id 
+-----------------------------------
+ 
+(1 row)
+
+-- create >8MB table
+CREATE UNLOGGED TABLE parallel_id_test AS
+SELECT s AS a, s AS b, s AS c, s AS d, s AS e, s AS f, s AS g, s AS h FROM generate_series(1,150000) s;
+INSERT INTO parallel_id_test VALUES (1234567), (1234567), (1234568), (1234568);
+ANALYSE parallel_id_test;
+SET LOCAL max_parallel_workers_per_gather TO 8;
+SET LOCAL cpu_tuple_cost TO 1000000;
+EXPLAIN (COSTS OFF)
+SELECT a FROM parallel_id_test WHERE a = parallel_worker_transaction_id_test();
+                         QUERY PLAN                          
+-------------------------------------------------------------
+ Gather
+   Workers Planned: 1
+   ->  Parallel Seq Scan on parallel_id_test
+         Filter: (a = parallel_worker_transaction_id_test())
+(4 rows)
+
+SELECT a FROM parallel_id_test WHERE a = parallel_worker_transaction_id_test();
+    a    
+---------
+ 1234567
+ 1234567
+(2 rows)
+
+ROLLBACK;

src/test/regress/sql/multi_distributed_transaction_id.sql

@@ -79,3 +79,27 @@ ROLLBACK PREPARED 'dist_xact_id_test';
 
 -- set back to the original zone
 SET TIME ZONE DEFAULT;
+
+-- parallel safe wrapper for getting the current transaction number
+CREATE OR REPLACE FUNCTION parallel_worker_transaction_id_test() 
+RETURNS bigint STRICT VOLATILE PARALLEL SAFE AS $$
+	SELECT transaction_number FROM get_current_transaction_id();
+$$ LANGUAGE sql;
+
+-- force the transaction ID to be used in a parallel plan
+BEGIN;
+SELECT assign_distributed_transaction_id(50, 1234567, '2016-01-01 00:00:00+0');
+
+-- create >8MB table
+CREATE UNLOGGED TABLE parallel_id_test AS
+SELECT s AS a, s AS b, s AS c, s AS d, s AS e, s AS f, s AS g, s AS h FROM generate_series(1,150000) s;
+INSERT INTO parallel_id_test VALUES (1234567), (1234567), (1234568), (1234568);
+ANALYSE parallel_id_test;
+
+SET LOCAL max_parallel_workers_per_gather TO 8;
+SET LOCAL cpu_tuple_cost TO 1000000;
+
+EXPLAIN (COSTS OFF)
+SELECT a FROM parallel_id_test WHERE a = parallel_worker_transaction_id_test();
+SELECT a FROM parallel_id_test WHERE a = parallel_worker_transaction_id_test();
+ROLLBACK;