Function build_joinrel_tlist

src/backend/optimizer/util/relnode.c:1410–1468

* build_joinrel_tlist * Builds a join relation's target list from an input relation. * (This is invoked twice to handle the two input relations.) * * The join's targetlist includes all Vars of its member relations that * will still be needed above the join. This subroutine adds all such * Vars from the specified input rel's tlist to the join rel's tlist. * * We also compute the expect

 * of data that was cached at the baserel level by set_rel_width().
 */
static void
build_joinrel_tlist(PlannerInfo *root, RelOptInfo *joinrel,
					RelOptInfo *input_rel)
{
	Relids		relids = joinrel->relids;
	ListCell   *vars;

	foreach(vars, input_rel->reltarget->exprs)
	{
		Var		   *var = (Var *) lfirst(vars);

		/*
		 * Ignore PlaceHolderVars in the input tlists; we'll make our own
		 * decisions about whether to copy them.
		 */
		if (IsA(var, PlaceHolderVar))
			continue;

		/*
		 * Otherwise, anything in a baserel or joinrel targetlist ought to be
		 * a Var.  (More general cases can only appear in appendrel child
		 * rels, which will never be seen here.)
		 */
		if (!IsA(var, Var))
			elog(ERROR, "unexpected node type in rel targetlist: %d",
				 (int) nodeTag(var));

		if (var->varno == ROWID_VAR)
		{
			/* UPDATE/DELETE row identity vars are always needed */
			RowIdentityVarInfo *ridinfo = (RowIdentityVarInfo *)
			list_nth(root->row_identity_vars, var->varattno - 1);

			joinrel->reltarget->exprs = lappend(joinrel->reltarget->exprs,
												var);
			/* Vars have cost zero, so no need to adjust reltarget->cost */
			joinrel->reltarget->width += ridinfo->rowidwidth;
		}
		else
		{
			RelOptInfo *baserel;
			int			ndx;

			/* Get the Var's original base rel */
			baserel = find_base_rel(root, var->varno);

			/* Is it still needed above this joinrel? */
			ndx = var->varattno - baserel->min_attr;
			if (bms_nonempty_difference(baserel->attr_needed[ndx], relids))
			{
				/* Yup, add it to the output */
				joinrel->reltarget->exprs = lappend(joinrel->reltarget->exprs,
													var);
				/* Vars have cost zero, so no need to adjust reltarget->cost */
				joinrel->reltarget->width += baserel->attr_widths[ndx];
			}
		}
	}