Randomize output segment for non-master gather motion

Via https://github.com/greenplum-db/gporca/pull/400, ORCA will optimize
DML queries by enforcing a gather on segment instead of master, whenever
possible.

Previous to this commit, ORCA always picked the first segment to gather
on while translating the DXL-GatherMotion node to GPDB motion node.

This commit uses GPDB's hash function to select the segment to gather
on, in a round-robin fashion starting with a random segment index. This
will ensure that concurrent DML queries issued via a same session, will
be gathered on different segments to distribute the workload.
Signed-off-by: NDhanashree Kashid <dkashid@pivotal.io>

src/backend/gpopt/gpdbwrappers.cpp

@@ -2824,6 +2824,27 @@ gpdb::CdbHashConst
 	return 0;
 }
 
+// pick a segment randomly from a pool of segments using GPDB's hash function
+int32
+gpdb::CdbHashRandom
+	(
+	int num_segments
+	)
+{
+	GP_WRAP_START;
+	{
+		CdbHash    *pcdbhash = makeCdbHash(num_segments);
+
+		cdbhashinit(pcdbhash);
+
+		cdbhashnokey(pcdbhash);
+
+		return cdbhashreduce(pcdbhash);
+	}
+	GP_WRAP_END;
+	return 0;
+}
+
 // hash a list of const values with GPDB's hash function
 int32 
 gpdb::CdbHashConstList

src/backend/gpopt/translate/CTranslatorDXLToPlStmt.cpp

@@ -1960,6 +1960,21 @@ CTranslatorDXLToPlStmt::TranslateDXLMotion
 			motion->motionType = MOTIONTYPE_FIXED;
 			// get segment id
 			INT segid = CDXLPhysicalGatherMotion::Cast(motion_dxlop)->IOutputSegIdx();
+
+			// if it's a gather on a segment, pick a segment from
+			// available segments using GPDB's hash function.
+			// This function outputs a segment index in a round
+			// robin fashion using a random segment index as the
+			// starting point.
+			// This ensures that concurrent DML queries issued via
+			// a same session, use a different output segment each
+			// time a gather on segment is needed.
+			if (segid >= 0)
+			{
+				segid = gpdb::CdbHashRandom(m_num_of_segments);
+				GPOS_ASSERT(segid >= 0);
+			}
+
 			motion->numOutputSegs = 1;
 			motion->outputSegIdx = (INT *) gpdb::GPDBAlloc(sizeof(INT));
 			*(motion->outputSegIdx) = segid;

src/include/gpopt/gpdbwrappers.h

@@ -595,7 +595,10 @@ namespace gpdb {
 	
 	// hash a const value with GPDB's hash function
 	int32 CdbHashConst(Const *constant, int num_segments);
-	
+
+	// pick a random segment from a pool of segments using GPDB's hash function
+	int32 CdbHashRandom(int num_segments);
+
 	// hash a list of const values with GPDB's hash function
 	int32 CdbHashConstList(List *constants, int num_segments);