Function create_mergejoin_path

src/backend/optimizer/util/pathnode.c:4107–4284

* create_mergejoin_path * Creates a pathnode corresponding to a mergejoin join between * two relations * * 'joinrel' is the join relation * 'jointype' is the type of join required * 'workspace' is the result from initial_cost_mergejoin * 'extra' contains various information about the join * 'outer_path' is the outer path * 'inner_path' is the inner path * 'restrict_clauses' are the R

 *      or NIL to use existing ordering
 */
Path *
create_mergejoin_path(PlannerInfo *root,
					  RelOptInfo *joinrel,
					  JoinType jointype,
					  JoinType orig_jointype,		/* CDB */
					  JoinCostWorkspace *workspace,
					  JoinPathExtraData *extra,
					  Path *outer_path,
					  Path *inner_path,
					  List *restrict_clauses,
					  List *pathkeys,
					  Relids required_outer,
					  List *mergeclauses,
					  List *redistribution_clauses,	/* CDB */
					  List *outersortkeys,
					  List *innersortkeys)
{
	MergePath  *pathnode = makeNode(MergePath);
	CdbPathLocus join_locus;
	List	   *outermotionkeys;
	List	   *innermotionkeys;
	bool		preserve_outer_ordering;
	bool		preserve_inner_ordering;
	int			rowidexpr_id;

	/*
	 * GPDB_92_MERGE_FIXME: Should we keep the pathkeys_contained_in calls?
	 */
	/*
	 * Do subpaths have useful ordering?
	 */
	if (outersortkeys == NIL)           /* must preserve existing ordering */
		outermotionkeys = outer_path->pathkeys;
	else if (pathkeys_contained_in(outersortkeys, outer_path->pathkeys))
		outermotionkeys = outersortkeys;/* lucky coincidence, already ordered */
	else                                /* existing order useless; must sort */
		outermotionkeys = NIL;

	if (innersortkeys == NIL)
		innermotionkeys = inner_path->pathkeys;
	else if (pathkeys_contained_in(innersortkeys, inner_path->pathkeys))
		innermotionkeys = innersortkeys;
	else
		innermotionkeys = NIL;

	/*
	 * Add motion nodes above subpaths and decide where to join.
	 *
	 * If we're explicitly sorting one or both sides of the join, don't choose
	 * a Motion that would break that ordering again. But as a special case,
	 * if there are no merge clauses, then there is no join order that would need
	 * preserving. That case can occur with a query like "a FULL JOIN b ON true"
	 */
	if (mergeclauses)
	{
		preserve_outer_ordering = (outersortkeys == NIL);
		preserve_inner_ordering = (innersortkeys == NIL);
	}