Function create_split_update_path

src/backend/cdb/cdbpath.c:2745–2781

* In Postgres planner, we add a SplitUpdate node at top so that updating on * distribution columns could be handled. The SplitUpdate will split each * update into delete + insert. * * There are several important points should be highlighted: * * First, in order to split each update operation into two operations, * delete + insert, we need several junk columns in the subplan's targetlist, *

 * 'rti' is the UPDATE target relation.
 */
Path *
create_split_update_path(PlannerInfo *root, Index rti, GpPolicy *policy, Path *subpath)
{
	GpPolicyType	policyType = policy->ptype;
	CdbPathLocus	targetLocus;

	if (policyType == POLICYTYPE_PARTITIONED)
	{
		/*
		 * If any of the distribution key columns are being changed,
		 * the UPDATE might move tuples from one segment to another.
		 * Create a Split Update node to deal with that.
		 *
		 * If the input is a dummy plan that cannot return any rows,
		 * e.g. because the input was eliminated by constraint
		 * exclusion, we can skip it.
		 */
		targetLocus = cdbpathlocus_for_insert(root, policy, subpath->pathtarget);

		subpath = (Path *) make_splitupdate_path(root, subpath, rti);
		subpath = cdbpath_create_explicit_motion_path(root,
													  subpath,
													  targetLocus);
	}
	else if (policyType == POLICYTYPE_ENTRY)
	{
		/* Master-only table */
		CdbPathLocus_MakeEntry(&targetLocus);
		subpath = cdbpath_create_motion_path(root, subpath, NIL, false, targetLocus);
	}
	else if (policyType == POLICYTYPE_REPLICATED)
	{
	}
	else
		elog(ERROR, "unrecognized policy type %u", policyType);
	return subpath;
}

/*
 * turn_volatile_seggen_to_singleqe