planner.c

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			else
			{
				/* caller absolute, limit fractional; use caller's value */
			}
		}
		else if (tuple_fraction > 0.0)
		{
			if (limit_fraction >= 1.0)
			{
				/* caller fractional, limit absolute; use limit */
				tuple_fraction = limit_fraction;
			}
			else
			{
				/* both fractional */
				tuple_fraction = Min(tuple_fraction, limit_fraction);
			}
		}
		else
		{
			/* no info from caller, just use limit */
			tuple_fraction = limit_fraction;
		}
	}
	else if (*offset_est != 0 && tuple_fraction > 0.0)
	{
		/*
		 * We have an OFFSET but no LIMIT.	This acts entirely differently
		 * from the LIMIT case: here, we need to increase rather than decrease
		 * the caller's tuple_fraction, because the OFFSET acts to cause more
		 * tuples to be fetched instead of fewer.  This only matters if we got
		 * a tuple_fraction > 0, however.
		 *
		 * As above, use 10% if OFFSET is present but unestimatable.
		 */
		if (*offset_est < 0)
			limit_fraction = 0.10;
		else
			limit_fraction = (double) *offset_est;

		/*
		 * If we have absolute counts from both caller and OFFSET, add them
		 * together; likewise if they are both fractional.	If one is
		 * fractional and the other absolute, we want to take the larger, and
		 * we heuristically assume that's the fractional one.
		 */
		if (tuple_fraction >= 1.0)
		{
			if (limit_fraction >= 1.0)
			{
				/* both absolute, so add them together */
				tuple_fraction += limit_fraction;
			}
			else
			{
				/* caller absolute, limit fractional; use limit */
				tuple_fraction = limit_fraction;
			}
		}
		else
		{
			if (limit_fraction >= 1.0)
			{
				/* caller fractional, limit absolute; use caller's value */
			}
			else
			{
				/* both fractional, so add them together */
				tuple_fraction += limit_fraction;
				if (tuple_fraction >= 1.0)
					tuple_fraction = 0.0;		/* assume fetch all */
			}
		}
	}

	return tuple_fraction;
}

/*
 * extract_grouping_ops - make an array of the equality operator OIDs
 *		for the GROUP BY clause
 */
static Oid *
extract_grouping_ops(List *groupClause)
{
	int			numCols = list_length(groupClause);
	int			colno = 0;
	Oid		   *groupOperators;
	ListCell   *glitem;

	groupOperators = (Oid *) palloc(sizeof(Oid) * numCols);

	foreach(glitem, groupClause)
	{
		GroupClause *groupcl = (GroupClause *) lfirst(glitem);

		groupOperators[colno] = get_equality_op_for_ordering_op(groupcl->sortop);
		if (!OidIsValid(groupOperators[colno])) /* shouldn't happen */
			elog(ERROR, "could not find equality operator for ordering operator %u",
				 groupcl->sortop);
		colno++;
	}

	return groupOperators;
}

/*
 * choose_hashed_grouping - should we use hashed grouping?
 */
static bool
choose_hashed_grouping(PlannerInfo *root,
					   double tuple_fraction, double limit_tuples,
					   Path *cheapest_path, Path *sorted_path,
					   Oid *groupOperators, double dNumGroups,
					   AggClauseCounts *agg_counts)
{
	int			numGroupCols = list_length(root->parse->groupClause);
	double		cheapest_path_rows;
	int			cheapest_path_width;
	Size		hashentrysize;
	List	   *current_pathkeys;
	Path		hashed_p;
	Path		sorted_p;
	int			i;

	/*
	 * Check can't-do-it conditions, including whether the grouping operators
	 * are hashjoinable.  (We assume hashing is OK if they are marked
	 * oprcanhash.	If there isn't actually a supporting hash function, the
	 * executor will complain at runtime.)
	 *
	 * Executor doesn't support hashed aggregation with DISTINCT aggregates.
	 * (Doing so would imply storing *all* the input values in the hash table,
	 * which seems like a certain loser.)
	 */
	if (!enable_hashagg)
		return false;
	if (agg_counts->numDistinctAggs != 0)
		return false;
	for (i = 0; i < numGroupCols; i++)
	{
		if (!op_hashjoinable(groupOperators[i]))
			return false;
	}

	/*
	 * Don't do it if it doesn't look like the hashtable will fit into
	 * work_mem.
	 *
	 * Beware here of the possibility that cheapest_path->parent is NULL. This
	 * could happen if user does something silly like SELECT 'foo' GROUP BY 1;
	 */
	if (cheapest_path->parent)
	{
		cheapest_path_rows = cheapest_path->parent->rows;
		cheapest_path_width = cheapest_path->parent->width;
	}
	else
	{
		cheapest_path_rows = 1; /* assume non-set result */
		cheapest_path_width = 100;		/* arbitrary */
	}

	/* Estimate per-hash-entry space at tuple width... */
	hashentrysize = MAXALIGN(cheapest_path_width) + MAXALIGN(sizeof(MinimalTupleData));
	/* plus space for pass-by-ref transition values... */
	hashentrysize += agg_counts->transitionSpace;
	/* plus the per-hash-entry overhead */
	hashentrysize += hash_agg_entry_size(agg_counts->numAggs);

	if (hashentrysize * dNumGroups > work_mem * 1024L)
		return false;

	/*
	 * See if the estimated cost is no more than doing it the other way. While
	 * avoiding the need for sorted input is usually a win, the fact that the
	 * output won't be sorted may be a loss; so we need to do an actual cost
	 * comparison.
	 *
	 * We need to consider cheapest_path + hashagg [+ final sort] versus
	 * either cheapest_path [+ sort] + group or agg [+ final sort] or
	 * presorted_path + group or agg [+ final sort] where brackets indicate a
	 * step that may not be needed. We assume query_planner() will have
	 * returned a presorted path only if it's a winner compared to
	 * cheapest_path for this purpose.
	 *
	 * These path variables are dummies that just hold cost fields; we don't
	 * make actual Paths for these steps.
	 */
	cost_agg(&hashed_p, root, AGG_HASHED, agg_counts->numAggs,
			 numGroupCols, dNumGroups,
			 cheapest_path->startup_cost, cheapest_path->total_cost,
			 cheapest_path_rows);
	/* Result of hashed agg is always unsorted */
	if (root->sort_pathkeys)
		cost_sort(&hashed_p, root, root->sort_pathkeys, hashed_p.total_cost,
				  dNumGroups, cheapest_path_width, limit_tuples);

	if (sorted_path)
	{
		sorted_p.startup_cost = sorted_path->startup_cost;
		sorted_p.total_cost = sorted_path->total_cost;
		current_pathkeys = sorted_path->pathkeys;
	}
	else
	{
		sorted_p.startup_cost = cheapest_path->startup_cost;
		sorted_p.total_cost = cheapest_path->total_cost;
		current_pathkeys = cheapest_path->pathkeys;
	}
	if (!pathkeys_contained_in(root->group_pathkeys, current_pathkeys))
	{
		cost_sort(&sorted_p, root, root->group_pathkeys, sorted_p.total_cost,
				  cheapest_path_rows, cheapest_path_width, -1.0);
		current_pathkeys = root->group_pathkeys;
	}

	if (root->parse->hasAggs)
		cost_agg(&sorted_p, root, AGG_SORTED, agg_counts->numAggs,
				 numGroupCols, dNumGroups,
				 sorted_p.startup_cost, sorted_p.total_cost,
				 cheapest_path_rows);
	else
		cost_group(&sorted_p, root, numGroupCols, dNumGroups,
				   sorted_p.startup_cost, sorted_p.total_cost,
				   cheapest_path_rows);
	/* The Agg or Group node will preserve ordering */
	if (root->sort_pathkeys &&
		!pathkeys_contained_in(root->sort_pathkeys, current_pathkeys))
		cost_sort(&sorted_p, root, root->sort_pathkeys, sorted_p.total_cost,
				  dNumGroups, cheapest_path_width, limit_tuples);

	/*
	 * Now make the decision using the top-level tuple fraction.  First we
	 * have to convert an absolute count (LIMIT) into fractional form.
	 */
	if (tuple_fraction >= 1.0)
		tuple_fraction /= dNumGroups;

	if (compare_fractional_path_costs(&hashed_p, &sorted_p,
									  tuple_fraction) < 0)
	{
		/* Hashed is cheaper, so use it */
		return true;
	}
	return false;
}

/*---------------
 * make_subplanTargetList
 *	  Generate appropriate target list when grouping is required.
 *
 * When grouping_planner inserts Aggregate, Group, or Result plan nodes
 * above the result of query_planner, we typically want to pass a different
 * target list to query_planner than the outer plan nodes should have.
 * This routine generates the correct target list for the subplan.
 *
 * The initial target list passed from the parser already contains entries
 * for all ORDER BY and GROUP BY expressions, but it will not have entries
 * for variables used only in HAVING clauses; so we need to add those
 * variables to the subplan target list.  Also, we flatten all expressions
 * except GROUP BY items into their component variables; the other expressions
 * will be computed by the inserted nodes rather than by the subplan.
 * For example, given a query like
 *		SELECT a+b,SUM(c+d) FROM table GROUP BY a+b;
 * we want to pass this targetlist to the subplan:
 *		a,b,c,d,a+b
 * where the a+b target will be used by the Sort/Group steps, and the
 * other targets will be used for computing the final results.	(In the
 * above example we could theoretically suppress the a and b targets and
 * pass down only c,d,a+b, but it's not really worth the trouble to
 * eliminate simple var references from the subplan.  We will avoid doing
 * the extra computation to recompute a+b at the outer level; see
 * fix_upper_expr() in setrefs.c.)
 *
 * If we are grouping or aggregating, *and* there are no non-Var grouping
 * expressions, then the returned tlist is effectively dummy; we do not
 * need to force it to be evaluated, because all the Vars it contains
 * should be present in the output of query_planner anyway.
 *
 * 'tlist' is the query's target list.
 * 'groupColIdx' receives an array of column numbers for the GROUP BY
 *			expressions (if there are any) in the subplan's target list.
 * 'need_tlist_eval' is set true if we really need to evaluate the
 *			result tlist.
 *
 * The result is the targetlist to be passed to the subplan.
 *---------------
 */
static List *
make_subplanTargetList(PlannerInfo *root,
					   List *tlist,
					   AttrNumber **groupColIdx,
					   bool *need_tlist_eval)
{
	Query	   *parse = root->parse;
	List	   *sub_tlist;
	List	   *extravars;
	int			numCols;

	*groupColIdx = NULL;

	/*
	 * If we're not grouping or aggregating, there's nothing to do here;
	 * query_planner should receive the unmodified target list.
	 */
	if (!parse->hasAggs && !parse->groupClause && !root->hasHavingQual)
	{
		*need_tlist_eval = true;
		return tlist;
	}

	/*
	 * Otherwise, start with a "flattened" tlist (having just the vars
	 * mentioned in the targetlist and HAVING qual --- but not upper- level
	 * Vars; they will be replaced by Params later on).
	 */
	sub_tlist = flatten_tlist(tlist);
	extravars = pull_var_clause(parse->havingQual, false);
	sub_tlist = add_to_flat_tlist(sub_tlist, extravars);
	list_free(extravars);
	*need_tlist_eval = false;	/* only eval if not flat tlist */

	/*
	 * If grouping, create sub_tlist entries for all GROUP BY expressions
	 * (GROUP BY items that are simple Vars should be in the list already),
	 * and make an array showing where the group columns are in the sub_tlist.
	 */
	numCols = list_length(parse->groupClause);
	if (numCols > 0)
	{
		int			keyno = 0;
		AttrNumber *grpColIdx;
		ListCell   *gl;

		grpColIdx = (AttrNumber *) palloc(sizeof(AttrNumber) * numCols);
		*groupColIdx = grpColIdx;

		foreach(gl, parse->groupClause)
		{
			GroupClause *grpcl = (GroupClause *) lfirst(gl);
			Node	   *groupexpr = get_sortgroupclause_expr(grpcl, tlist);
			TargetEntry *te = NULL;
			ListCell   *sl;

			/* Find or make a matching sub_tlist entry */
			foreach(sl, sub_tlist)
			{
				te = (TargetEntry *) lfirst(sl);
				if (equal(groupexpr, te->expr))
					break;
			}
			if (!sl)
			{
				te = makeTargetEntry((Expr *) groupexpr,
									 list_length(sub_tlist) + 1,
									 NULL,
									 false);
				sub_tlist = lappend(sub_tlist, te);
				*need_tlist_eval = true;		/* it's not flat anymore */
			}

			/* and save its resno */
			grpColIdx[keyno++] = te->resno;
		}
	}

	return sub_tlist;
}

/*
 * locate_grouping_columns
 *		Locate grouping columns in the tlist chosen by query_planner.
 *
 * This is only needed if we don't use the sub_tlist chosen by
 * make_subplanTargetList.	We have to forget the column indexes found
 * by that routine and re-locate the grouping vars in the real sub_tlist.
 */
static void
locate_grouping_columns(PlannerInfo *root,
						List *tlist,
						List *sub_tlist,
						AttrNumber *groupColIdx)
{
	int			keyno = 0;
	ListCell   *gl;

	/*
	 * No work unless grouping.
	 */
	if (!root->parse->groupClause)
	{
		Assert(groupColIdx == NULL);
		return;
	}
	Assert(groupColIdx != NULL);

	foreach(gl, root->parse->groupClause)
	{
		GroupClause *grpcl = (GroupClause *) lfirst(gl);
		Node	   *groupexpr = get_sortgroupclause_expr(grpcl, tlist);
		TargetEntry *te = NULL;
		ListCell   *sl;

		foreach(sl, sub_tlist)
		{
			te = (TargetEntry *) lfirst(sl);
			if (equal(groupexpr, te->expr))
				break;
		}
		if (!sl)
			elog(ERROR, "failed to locate grouping columns");

		groupColIdx[keyno++] = te->resno;
	}
}

/*
 * postprocess_setop_tlist
 *	  Fix up targetlist returned by plan_set_operations().
 *
 * We need to transpose sort key info from the orig_tlist into new_tlist.
 * NOTE: this would not be good enough if we supported resjunk sort keys
 * for results of set operations --- then, we'd need to project a whole
 * new tlist to evaluate the resjunk columns.  For now, just ereport if we
 * find any resjunk columns in orig_tlist.
 */
static List *
postprocess_setop_tlist(List *new_tlist, List *orig_tlist)
{
	ListCell   *l;
	ListCell   *orig_tlist_item = list_head(orig_tlist);

	foreach(l, new_tlist)
	{
		TargetEntry *new_tle = (TargetEntry *) lfirst(l);
		TargetEntry *orig_tle;

		/* ignore resjunk columns in setop result */
		if (new_tle->resjunk)
			continue;

		Assert(orig_tlist_item != NULL);
		orig_tle = (TargetEntry *) lfirst(orig_tlist_item);
		orig_tlist_item = lnext(orig_tlist_item);
		if (orig_tle->resjunk)	/* should not happen */
			elog(ERROR, "resjunk output columns are not implemented");
		Assert(new_tle->resno == orig_tle->resno);
		new_tle->ressortgroupref = orig_tle->ressortgroupref;
	}
	if (orig_tlist_item != NULL)
		elog(ERROR, "resjunk output columns are not implemented");
	return new_tlist;
}