Function set_function_pathlist

src/backend/optimizer/path/allpaths.c:2707–2770

* set_function_pathlist * Build the (single) access path for a function RTE */

 *		Build the (single) access path for a function RTE
 */
static void
set_function_pathlist(PlannerInfo *root, RelOptInfo *rel, RangeTblEntry *rte)
{
	Relids		required_outer;
	List	   *pathkeys = NIL;

	/*
	 * We don't support pushing join clauses into the quals of a function
	 * scan, but it could still have required parameterization due to LATERAL
	 * refs in the function expression.
	 */
	required_outer = rel->lateral_relids;

	/*
	 * The result is considered unordered unless ORDINALITY was used, in which
	 * case it is ordered by the ordinal column (the last one).  See if we
	 * care, by checking for uses of that Var in equivalence classes.
	 */
	if (rte->funcordinality)
	{
		AttrNumber	ordattno = rel->max_attr;
		Var		   *var = NULL;
		ListCell   *lc;

		/*
		 * Is there a Var for it in rel's targetlist?  If not, the query did
		 * not reference the ordinality column, or at least not in any way
		 * that would be interesting for sorting.
		 */
		foreach(lc, rel->reltarget->exprs)
		{
			Var		   *node = (Var *) lfirst(lc);

			/* checking varno/varlevelsup is just paranoia */
			if (IsA(node, Var) &&
				node->varattno == ordattno &&
				node->varno == rel->relid &&
				node->varlevelsup == 0)
			{
				var = node;
				break;
			}
		}

		/*
		 * Try to build pathkeys for this Var with int8 sorting.  We tell
		 * build_expression_pathkey not to build any new equivalence class; if
		 * the Var isn't already mentioned in some EC, it means that nothing
		 * cares about the ordering.
		 */
		if (var)
			pathkeys = build_expression_pathkey(root,
												(Expr *) var,
												NULL,	/* below outer joins */
												Int8LessOperator,
												rel->relids,
												false);
	}