Function set_append_rel_size

src/backend/optimizer/path/allpaths.c:1257–1519

* set_append_rel_size * Set size estimates for a simple "append relation" * * The passed-in rel and RTE represent the entire append relation. The * relation's contents are computed by appending together the output of the * individual member relations. Note that in the non-partitioned inheritance * case, the first member relation is actually the same table as is mentioned * in the parent

 * a good thing because their outputs are not the same size.
 */
static void
set_append_rel_size(PlannerInfo *root, RelOptInfo *rel,
					Index rti, RangeTblEntry *rte)
{
	int			parentRTindex = rti;
	bool		has_live_children;
	double		parent_rows;
	double		parent_size;
	double	   *parent_attrsizes;
	int			nattrs;
	ListCell   *l;

	/* Guard against stack overflow due to overly deep inheritance tree. */
	check_stack_depth();

	Assert(IS_SIMPLE_REL(rel));

	/*
	 * If this is a partitioned baserel, set the consider_partitionwise_join
	 * flag; currently, we only consider partitionwise joins with the baserel
	 * if its targetlist doesn't contain a whole-row Var.
	 */
	if (enable_partitionwise_join &&
		rel->reloptkind == RELOPT_BASEREL &&
		rte->relkind == RELKIND_PARTITIONED_TABLE &&
		rel->attr_needed[InvalidAttrNumber - rel->min_attr] == NULL)
		rel->consider_partitionwise_join = true;

	/*
	 * Initialize to compute size estimates for whole append relation.
	 *
	 * We handle width estimates by weighting the widths of different child
	 * rels proportionally to their number of rows.  This is sensible because
	 * the use of width estimates is mainly to compute the total relation
	 * "footprint" if we have to sort or hash it.  To do this, we sum the
	 * total equivalent size (in "double" arithmetic) and then divide by the
	 * total rowcount estimate.  This is done separately for the total rel
	 * width and each attribute.
	 *
	 * Note: if you consider changing this logic, beware that child rels could
	 * have zero rows and/or width, if they were excluded by constraints.
	 */
	has_live_children = false;
	parent_rows = 0;
	parent_size = 0;
	nattrs = rel->max_attr - rel->min_attr + 1;
	parent_attrsizes = (double *) palloc0(nattrs * sizeof(double));

	foreach(l, root->append_rel_list)
	{
		AppendRelInfo *appinfo = (AppendRelInfo *) lfirst(l);
		int			childRTindex;
		RangeTblEntry *childRTE;
		RelOptInfo *childrel;
		ListCell   *parentvars;
		ListCell   *childvars;

		/* append_rel_list contains all append rels; ignore others */