Function create_motion_path_for_insert

src/backend/cdb/cdbpath.c:2509–2647

* Add a suitable Motion Path so that the input tuples from 'subpath' are * distributed correctly for insertion into target table. */

 * distributed correctly for insertion into target table.
 */
Path *
create_motion_path_for_insert(PlannerInfo *root, GpPolicy *policy,
							  Path *subpath)
{
	GpPolicyType	policyType = policy->ptype;
	CdbPathLocus	targetLocus;

	if (policyType == POLICYTYPE_PARTITIONED)
	{
		/*
		 * A query to reach here: INSERT INTO t1 VALUES(1).
		 * There is no need to add a motion from General, we could
		 * simply put General on the same segments with target table.
		 */
		/* FIXME: also do this for other targetPolicyType? */
		/* FIXME: also do this for all the subplans */
		if (CdbPathLocus_IsGeneral(subpath->locus))
		{
			subpath->locus.numsegments = policy->numsegments;
		}

		targetLocus = cdbpathlocus_for_insert(root, policy, subpath->pathtarget);

		if (policy->nattrs == 0 && CdbPathLocus_IsPartitioned(subpath->locus))
		{
			/*
			 * If the target table is DISTRIBUTED RANDOMLY, we can insert the
			 * rows anywhere. So if the input path is already partitioned, let
			 * the insertions happen where they are. Unless the GUC gp_force_random_redistribution
			 * tells us to force the redistribution.
			 *
			 * If you `explain` the query insert into tab_random select * from tab_partition
			 * there is not Motion node in plan. However, it is not means that the query only
			 * execute in entry db. It is dispatched to QE and do everything well as we expect.
			 *
			 * But, we need to grant a Motion node if target locus' segnumber is different with
			 * subpath.
			 */
			if (gp_force_random_redistribution || targetLocus.numsegments != subpath->locus.numsegments)
			{
				CdbPathLocus_MakeStrewn(&targetLocus, policy->numsegments, 0);
				subpath = cdbpath_create_motion_path(root, subpath, NIL, false, targetLocus);
			}
		}
		else if (CdbPathLocus_IsNull(targetLocus))
		{
			/* could not create DistributionKeys to represent the distribution keys. */
			CdbPathLocus_MakeStrewn(&targetLocus, policy->numsegments, 0);

			subpath = (Path *) make_motion_path(root, subpath, targetLocus, false, policy);
		}
		else
		{
			subpath = cdbpath_create_motion_path(root, subpath, NIL, false, targetLocus);
		}
	}
	else if (policyType == POLICYTYPE_ENTRY)
	{