postgres/src/backend/optimizer/plan/planner.c

/*-------------------------------------------------------------------------
 *
 * planner.c
 *	  The query optimizer external interface.
 *
 * Portions Copyright (c) 1996-2008, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *	  $PostgreSQL: pgsql/src/backend/optimizer/plan/planner.c,v 1.229 2008/03/28 02:00:11 tgl Exp $
 *
 *-------------------------------------------------------------------------
 */

#include "postgres.h"

#include <limits.h>

#include "catalog/pg_operator.h"
#include "executor/executor.h"
#include "executor/nodeAgg.h"
#include "miscadmin.h"
#include "nodes/makefuncs.h"
#include "optimizer/clauses.h"
#include "optimizer/cost.h"
#include "optimizer/pathnode.h"
#include "optimizer/paths.h"
#include "optimizer/planmain.h"
#include "optimizer/planner.h"
#include "optimizer/prep.h"
#include "optimizer/subselect.h"
#include "optimizer/tlist.h"
#include "optimizer/var.h"
#ifdef OPTIMIZER_DEBUG
#include "nodes/print.h"
#endif
#include "parser/parse_expr.h"
#include "parser/parse_oper.h"
#include "parser/parsetree.h"
#include "utils/lsyscache.h"
#include "utils/syscache.h"


/* Hook for plugins to get control in planner() */
planner_hook_type planner_hook = NULL;


/* Expression kind codes for preprocess_expression */
#define EXPRKIND_QUAL		0
#define EXPRKIND_TARGET		1
#define EXPRKIND_RTFUNC		2
#define EXPRKIND_VALUES		3
#define EXPRKIND_LIMIT		4
#define EXPRKIND_ININFO		5
#define EXPRKIND_APPINFO	6


static Node *preprocess_expression(PlannerInfo *root, Node *expr, int kind);
static void preprocess_qual_conditions(PlannerInfo *root, Node *jtnode);
static Plan *inheritance_planner(PlannerInfo *root);
static Plan *grouping_planner(PlannerInfo *root, double tuple_fraction);
static bool is_dummy_plan(Plan *plan);
static double preprocess_limit(PlannerInfo *root,
				 double tuple_fraction,
				 int64 *offset_est, int64 *count_est);
static Oid *extract_grouping_ops(List *groupClause);
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);
static List *make_subplanTargetList(PlannerInfo *root, List *tlist,
					   AttrNumber **groupColIdx, bool *need_tlist_eval);
static void locate_grouping_columns(PlannerInfo *root,
						List *tlist,
						List *sub_tlist,
						AttrNumber *groupColIdx);
static List *postprocess_setop_tlist(List *new_tlist, List *orig_tlist);


/*****************************************************************************
 *
 *	   Query optimizer entry point
 *
 * To support loadable plugins that monitor or modify planner behavior,
 * we provide a hook variable that lets a plugin get control before and
 * after the standard planning process.  The plugin would normally call
 * standard_planner().
 *
 * Note to plugin authors: standard_planner() scribbles on its Query input,
 * so you'd better copy that data structure if you want to plan more than once.
 *
 *****************************************************************************/
PlannedStmt *
planner(Query *parse, int cursorOptions, ParamListInfo boundParams)
{
	PlannedStmt *result;

	if (planner_hook)
		result = (*planner_hook) (parse, cursorOptions, boundParams);
	else
		result = standard_planner(parse, cursorOptions, boundParams);
	return result;
}

PlannedStmt *
standard_planner(Query *parse, int cursorOptions, ParamListInfo boundParams)
{
	PlannedStmt *result;
	PlannerGlobal *glob;
	double		tuple_fraction;
	PlannerInfo *root;
	Plan	   *top_plan;
	ListCell   *lp,
			   *lr;

	/* Cursor options may come from caller or from DECLARE CURSOR stmt */
	if (parse->utilityStmt &&
		IsA(parse->utilityStmt, DeclareCursorStmt))
		cursorOptions |= ((DeclareCursorStmt *) parse->utilityStmt)->options;

	/*
	 * Set up global state for this planner invocation.  This data is needed
	 * across all levels of sub-Query that might exist in the given command,
	 * so we keep it in a separate struct that's linked to by each per-Query
	 * PlannerInfo.
	 */
	glob = makeNode(PlannerGlobal);

	glob->boundParams = boundParams;
	glob->paramlist = NIL;
	glob->subplans = NIL;
	glob->subrtables = NIL;
	glob->rewindPlanIDs = NULL;
	glob->finalrtable = NIL;
	glob->relationOids = NIL;
	glob->transientPlan = false;

	/* Determine what fraction of the plan is likely to be scanned */
	if (cursorOptions & CURSOR_OPT_FAST_PLAN)
	{
		/*
		 * We have no real idea how many tuples the user will ultimately FETCH
		 * from a cursor, but it seems a good bet that he doesn't want 'em
		 * all.  Optimize for 10% retrieval (you gotta better number?  Should
		 * this be a SETtable parameter?)
		 */
		tuple_fraction = 0.10;
	}
	else
	{
		/* Default assumption is we need all the tuples */
		tuple_fraction = 0.0;
	}

	/* primary planning entry point (may recurse for subqueries) */
	top_plan = subquery_planner(glob, parse, 1, tuple_fraction, &root);

	/*
	 * If creating a plan for a scrollable cursor, make sure it can run
	 * backwards on demand.  Add a Material node at the top at need.
	 */
	if (cursorOptions & CURSOR_OPT_SCROLL)
	{
		if (!ExecSupportsBackwardScan(top_plan))
			top_plan = materialize_finished_plan(top_plan);
	}

	/* final cleanup of the plan */
	Assert(glob->finalrtable == NIL);
	top_plan = set_plan_references(glob, top_plan, root->parse->rtable);
	/* ... and the subplans (both regular subplans and initplans) */
	Assert(list_length(glob->subplans) == list_length(glob->subrtables));
	forboth(lp, glob->subplans, lr, glob->subrtables)
	{
		Plan	   *subplan = (Plan *) lfirst(lp);
		List	   *subrtable = (List *) lfirst(lr);

		lfirst(lp) = set_plan_references(glob, subplan, subrtable);
	}

	/* build the PlannedStmt result */
	result = makeNode(PlannedStmt);

	result->commandType = parse->commandType;
	result->canSetTag = parse->canSetTag;
	result->transientPlan = glob->transientPlan;
	result->planTree = top_plan;
	result->rtable = glob->finalrtable;
	result->resultRelations = root->resultRelations;
	result->utilityStmt = parse->utilityStmt;
	result->intoClause = parse->intoClause;
	result->subplans = glob->subplans;
	result->rewindPlanIDs = glob->rewindPlanIDs;
	result->returningLists = root->returningLists;
	result->rowMarks = parse->rowMarks;
	result->relationOids = glob->relationOids;
	result->nParamExec = list_length(glob->paramlist);

	return result;
}


/*--------------------
 * subquery_planner
 *	  Invokes the planner on a subquery.  We recurse to here for each
 *	  sub-SELECT found in the query tree.
 *
 * glob is the global state for the current planner run.
 * parse is the querytree produced by the parser & rewriter.
 * level is the current recursion depth (1 at the top-level Query).
 * tuple_fraction is the fraction of tuples we expect will be retrieved.
 * tuple_fraction is interpreted as explained for grouping_planner, below.
 *
 * If subroot isn't NULL, we pass back the query's final PlannerInfo struct;
 * among other things this tells the output sort ordering of the plan.
 *
 * Basically, this routine does the stuff that should only be done once
 * per Query object.  It then calls grouping_planner.  At one time,
 * grouping_planner could be invoked recursively on the same Query object;
 * that's not currently true, but we keep the separation between the two
 * routines anyway, in case we need it again someday.
 *
 * subquery_planner will be called recursively to handle sub-Query nodes
 * found within the query's expressions and rangetable.
 *
 * Returns a query plan.
 *--------------------
 */
Plan *
subquery_planner(PlannerGlobal *glob, Query *parse,
				 Index level, double tuple_fraction,
				 PlannerInfo **subroot)
{
	int			num_old_subplans = list_length(glob->subplans);
	PlannerInfo *root;
	Plan	   *plan;
	List	   *newHaving;
	ListCell   *l;

	/* Create a PlannerInfo data structure for this subquery */
	root = makeNode(PlannerInfo);
	root->parse = parse;
	root->glob = glob;
	root->query_level = level;
	root->planner_cxt = CurrentMemoryContext;
	root->init_plans = NIL;
	root->eq_classes = NIL;
	root->in_info_list = NIL;
	root->append_rel_list = NIL;

	/*
	 * Look for IN clauses at the top level of WHERE, and transform them into
	 * joins.  Note that this step only handles IN clauses originally at top
	 * level of WHERE; if we pull up any subqueries below, their INs are
	 * processed just before pulling them up.
	 */
	if (parse->hasSubLinks)
		parse->jointree->quals = pull_up_IN_clauses(root,
													parse->jointree->quals);

	/*
	 * Scan the rangetable for set-returning functions, and inline them
	 * if possible (producing subqueries that might get pulled up next).
	 * Recursion issues here are handled in the same way as for IN clauses.
	 */
	inline_set_returning_functions(root);

	/*
	 * Check to see if any subqueries in the rangetable can be merged into
	 * this query.
	 */
	parse->jointree = (FromExpr *)
		pull_up_subqueries(root, (Node *) parse->jointree, false, false);

	/*
	 * Detect whether any rangetable entries are RTE_JOIN kind; if not, we can
	 * avoid the expense of doing flatten_join_alias_vars().  Also check for
	 * outer joins --- if none, we can skip reduce_outer_joins() and some
	 * other processing.  This must be done after we have done
	 * pull_up_subqueries, of course.
	 *
	 * Note: if reduce_outer_joins manages to eliminate all outer joins,
	 * root->hasOuterJoins is not reset currently.	This is OK since its
	 * purpose is merely to suppress unnecessary processing in simple cases.
	 */
	root->hasJoinRTEs = false;
	root->hasOuterJoins = false;
	foreach(l, parse->rtable)
	{
		RangeTblEntry *rte = (RangeTblEntry *) lfirst(l);

		if (rte->rtekind == RTE_JOIN)
		{
			root->hasJoinRTEs = true;
			if (IS_OUTER_JOIN(rte->jointype))
			{
				root->hasOuterJoins = true;
				/* Can quit scanning once we find an outer join */
				break;
			}
		}
	}

	/*
	 * Expand any rangetable entries that are inheritance sets into "append
	 * relations".  This can add entries to the rangetable, but they must be
	 * plain base relations not joins, so it's OK (and marginally more
	 * efficient) to do it after checking for join RTEs.  We must do it after
	 * pulling up subqueries, else we'd fail to handle inherited tables in
	 * subqueries.
	 */
	expand_inherited_tables(root);

	/*
	 * Set hasHavingQual to remember if HAVING clause is present.  Needed
	 * because preprocess_expression will reduce a constant-true condition to
	 * an empty qual list ... but "HAVING TRUE" is not a semantic no-op.
	 */
	root->hasHavingQual = (parse->havingQual != NULL);

	/* Clear this flag; might get set in distribute_qual_to_rels */
	root->hasPseudoConstantQuals = false;

	/*
	 * Do expression preprocessing on targetlist and quals.
	 */
	parse->targetList = (List *)
		preprocess_expression(root, (Node *) parse->targetList,
							  EXPRKIND_TARGET);

	parse->returningList = (List *)
		preprocess_expression(root, (Node *) parse->returningList,
							  EXPRKIND_TARGET);

	preprocess_qual_conditions(root, (Node *) parse->jointree);

	parse->havingQual = preprocess_expression(root, parse->havingQual,
											  EXPRKIND_QUAL);

	parse->limitOffset = preprocess_expression(root, parse->limitOffset,
											   EXPRKIND_LIMIT);
	parse->limitCount = preprocess_expression(root, parse->limitCount,
											  EXPRKIND_LIMIT);

	root->in_info_list = (List *)
		preprocess_expression(root, (Node *) root->in_info_list,
							  EXPRKIND_ININFO);
	root->append_rel_list = (List *)
		preprocess_expression(root, (Node *) root->append_rel_list,
							  EXPRKIND_APPINFO);

	/* Also need to preprocess expressions for function and values RTEs */
	foreach(l, parse->rtable)
	{
		RangeTblEntry *rte = (RangeTblEntry *) lfirst(l);

		if (rte->rtekind == RTE_FUNCTION)
			rte->funcexpr = preprocess_expression(root, rte->funcexpr,
												  EXPRKIND_RTFUNC);
		else if (rte->rtekind == RTE_VALUES)
			rte->values_lists = (List *)
				preprocess_expression(root, (Node *) rte->values_lists,
									  EXPRKIND_VALUES);
	}

	/*
	 * In some cases we may want to transfer a HAVING clause into WHERE. We
	 * cannot do so if the HAVING clause contains aggregates (obviously) or
	 * volatile functions (since a HAVING clause is supposed to be executed
	 * only once per group).  Also, it may be that the clause is so expensive
	 * to execute that we're better off doing it only once per group, despite
	 * the loss of selectivity.  This is hard to estimate short of doing the
	 * entire planning process twice, so we use a heuristic: clauses
	 * containing subplans are left in HAVING.	Otherwise, we move or copy the
	 * HAVING clause into WHERE, in hopes of eliminating tuples before
	 * aggregation instead of after.
	 *
	 * If the query has explicit grouping then we can simply move such a
	 * clause into WHERE; any group that fails the clause will not be in the
	 * output because none of its tuples will reach the grouping or
	 * aggregation stage.  Otherwise we must have a degenerate (variable-free)
	 * HAVING clause, which we put in WHERE so that query_planner() can use it
	 * in a gating Result node, but also keep in HAVING to ensure that we
	 * don't emit a bogus aggregated row. (This could be done better, but it
	 * seems not worth optimizing.)
	 *
	 * Note that both havingQual and parse->jointree->quals are in
	 * implicitly-ANDed-list form at this point, even though they are declared
	 * as Node *.
	 */
	newHaving = NIL;
	foreach(l, (List *) parse->havingQual)
	{
		Node	   *havingclause = (Node *) lfirst(l);

		if (contain_agg_clause(havingclause) ||
			contain_volatile_functions(havingclause) ||
			contain_subplans(havingclause))
		{
			/* keep it in HAVING */
			newHaving = lappend(newHaving, havingclause);
		}
		else if (parse->groupClause)
		{
			/* move it to WHERE */
			parse->jointree->quals = (Node *)
				lappend((List *) parse->jointree->quals, havingclause);
		}
		else
		{
			/* put a copy in WHERE, keep it in HAVING */
			parse->jointree->quals = (Node *)
				lappend((List *) parse->jointree->quals,
						copyObject(havingclause));
			newHaving = lappend(newHaving, havingclause);
		}
	}
	parse->havingQual = (Node *) newHaving;

	/*
	 * If we have any outer joins, try to reduce them to plain inner joins.
	 * This step is most easily done after we've done expression
	 * preprocessing.
	 */
	if (root->hasOuterJoins)
		reduce_outer_joins(root);

	/*
	 * Do the main planning.  If we have an inherited target relation, that
	 * needs special processing, else go straight to grouping_planner.
	 */
	if (parse->resultRelation &&
		rt_fetch(parse->resultRelation, parse->rtable)->inh)
		plan = inheritance_planner(root);
	else
		plan = grouping_planner(root, tuple_fraction);

	/*
	 * If any subplans were generated, or if we're inside a subplan, build
	 * initPlan list and extParam/allParam sets for plan nodes, and attach the
	 * initPlans to the top plan node.
	 */
	if (list_length(glob->subplans) != num_old_subplans ||
		root->query_level > 1)
		SS_finalize_plan(root, plan);

	/* Return internal info if caller wants it */
	if (subroot)
		*subroot = root;

	return plan;
}

/*
 * preprocess_expression
 *		Do subquery_planner's preprocessing work for an expression,
 *		which can be a targetlist, a WHERE clause (including JOIN/ON
 *		conditions), or a HAVING clause.
 */
static Node *
preprocess_expression(PlannerInfo *root, Node *expr, int kind)
{
	/*
	 * Fall out quickly if expression is empty.  This occurs often enough to
	 * be worth checking.  Note that null->null is the correct conversion for
	 * implicit-AND result format, too.
	 */
	if (expr == NULL)
		return NULL;

	/*
	 * If the query has any join RTEs, replace join alias variables with
	 * base-relation variables. We must do this before sublink processing,
	 * else sublinks expanded out from join aliases wouldn't get processed. We
	 * can skip it in VALUES lists, however, since they can't contain any Vars
	 * at all.
	 */
	if (root->hasJoinRTEs && kind != EXPRKIND_VALUES)
		expr = flatten_join_alias_vars(root, expr);

	/*
	 * Simplify constant expressions.
	 *
	 * Note: this also flattens nested AND and OR expressions into N-argument
	 * form.  All processing of a qual expression after this point must be
	 * careful to maintain AND/OR flatness --- that is, do not generate a tree
	 * with AND directly under AND, nor OR directly under OR.
	 *
	 * Because this is a relatively expensive process, we skip it when the
	 * query is trivial, such as "SELECT 2+2;" or "INSERT ... VALUES()". The
	 * expression will only be evaluated once anyway, so no point in
	 * pre-simplifying; we can't execute it any faster than the executor can,
	 * and we will waste cycles copying the tree.  Notice however that we
	 * still must do it for quals (to get AND/OR flatness); and if we are in a
	 * subquery we should not assume it will be done only once.
	 *
	 * For VALUES lists we never do this at all, again on the grounds that we
	 * should optimize for one-time evaluation.
	 */
	if (kind != EXPRKIND_VALUES &&
		(root->parse->jointree->fromlist != NIL ||
		 kind == EXPRKIND_QUAL ||
		 root->query_level > 1))
		expr = eval_const_expressions(expr);

	/*
	 * If it's a qual or havingQual, canonicalize it.
	 */
	if (kind == EXPRKIND_QUAL)
	{
		expr = (Node *) canonicalize_qual((Expr *) expr);

#ifdef OPTIMIZER_DEBUG
		printf("After canonicalize_qual()\n");
		pprint(expr);
#endif
	}

	/* Expand SubLinks to SubPlans */
	if (root->parse->hasSubLinks)
		expr = SS_process_sublinks(root, expr, (kind == EXPRKIND_QUAL));

	/*
	 * XXX do not insert anything here unless you have grokked the comments in
	 * SS_replace_correlation_vars ...
	 */

	/* Replace uplevel vars with Param nodes (this IS possible in VALUES) */
	if (root->query_level > 1)
		expr = SS_replace_correlation_vars(root, expr);

	/*
	 * If it's a qual or havingQual, convert it to implicit-AND format. (We
	 * don't want to do this before eval_const_expressions, since the latter
	 * would be unable to simplify a top-level AND correctly. Also,
	 * SS_process_sublinks expects explicit-AND format.)
	 */
	if (kind == EXPRKIND_QUAL)
		expr = (Node *) make_ands_implicit((Expr *) expr);

	return expr;
}

/*
 * preprocess_qual_conditions
 *		Recursively scan the query's jointree and do subquery_planner's
 *		preprocessing work on each qual condition found therein.
 */
static void
preprocess_qual_conditions(PlannerInfo *root, Node *jtnode)
{
	if (jtnode == NULL)
		return;
	if (IsA(jtnode, RangeTblRef))
	{
		/* nothing to do here */
	}
	else if (IsA(jtnode, FromExpr))
	{
		FromExpr   *f = (FromExpr *) jtnode;
		ListCell   *l;

		foreach(l, f->fromlist)
			preprocess_qual_conditions(root, lfirst(l));

		f->quals = preprocess_expression(root, f->quals, EXPRKIND_QUAL);
	}
	else if (IsA(jtnode, JoinExpr))
	{
		JoinExpr   *j = (JoinExpr *) jtnode;

		preprocess_qual_conditions(root, j->larg);
		preprocess_qual_conditions(root, j->rarg);

		j->quals = preprocess_expression(root, j->quals, EXPRKIND_QUAL);
	}
	else
		elog(ERROR, "unrecognized node type: %d",
			 (int) nodeTag(jtnode));
}

/*
 * inheritance_planner
 *	  Generate a plan in the case where the result relation is an
 *	  inheritance set.
 *
 * We have to handle this case differently from cases where a source relation
 * is an inheritance set. Source inheritance is expanded at the bottom of the
 * plan tree (see allpaths.c), but target inheritance has to be expanded at
 * the top.  The reason is that for UPDATE, each target relation needs a
 * different targetlist matching its own column set.  Also, for both UPDATE
 * and DELETE, the executor needs the Append plan node at the top, else it
 * can't keep track of which table is the current target table.  Fortunately,
 * the UPDATE/DELETE target can never be the nullable side of an outer join,
 * so it's OK to generate the plan this way.
 *
 * Returns a query plan.
 */
static Plan *
inheritance_planner(PlannerInfo *root)
{
	Query	   *parse = root->parse;
	int			parentRTindex = parse->resultRelation;
	List	   *subplans = NIL;
	List	   *resultRelations = NIL;
	List	   *returningLists = NIL;
	List	   *rtable = NIL;
	List	   *tlist = NIL;
	PlannerInfo subroot;
	ListCell   *l;

	foreach(l, root->append_rel_list)
	{
		AppendRelInfo *appinfo = (AppendRelInfo *) lfirst(l);
		Plan	   *subplan;

		/* append_rel_list contains all append rels; ignore others */
		if (appinfo->parent_relid != parentRTindex)
			continue;

		/*
		 * Generate modified query with this rel as target.  We have to be
		 * prepared to translate varnos in in_info_list as well as in the
		 * Query proper.
		 */
		memcpy(&subroot, root, sizeof(PlannerInfo));
		subroot.parse = (Query *)
			adjust_appendrel_attrs((Node *) parse,
								   appinfo);
		subroot.in_info_list = (List *)
			adjust_appendrel_attrs((Node *) root->in_info_list,
								   appinfo);
		subroot.init_plans = NIL;
		/* There shouldn't be any OJ info to translate, as yet */
		Assert(subroot.oj_info_list == NIL);

		/* Generate plan */
		subplan = grouping_planner(&subroot, 0.0 /* retrieve all tuples */ );

		/*
		 * If this child rel was excluded by constraint exclusion, exclude it
		 * from the plan.
		 */
		if (is_dummy_plan(subplan))
			continue;

		/* Save rtable and tlist from first rel for use below */
		if (subplans == NIL)
		{
			rtable = subroot.parse->rtable;
			tlist = subplan->targetlist;
		}

		subplans = lappend(subplans, subplan);

		/* Make sure any initplans from this rel get into the outer list */
		root->init_plans = list_concat(root->init_plans, subroot.init_plans);

		/* Build target-relations list for the executor */
		resultRelations = lappend_int(resultRelations, appinfo->child_relid);

		/* Build list of per-relation RETURNING targetlists */
		if (parse->returningList)
		{
			Assert(list_length(subroot.returningLists) == 1);
			returningLists = list_concat(returningLists,
										 subroot.returningLists);
		}
	}

	root->resultRelations = resultRelations;
	root->returningLists = returningLists;

	/* Mark result as unordered (probably unnecessary) */
	root->query_pathkeys = NIL;

	/*
	 * If we managed to exclude every child rel, return a dummy plan
	 */
	if (subplans == NIL)
	{
		root->resultRelations = list_make1_int(parentRTindex);
		/* although dummy, it must have a valid tlist for executor */
		tlist = preprocess_targetlist(root, parse->targetList);
		return (Plan *) make_result(root,
									tlist,
									(Node *) list_make1(makeBoolConst(false,
																	  false)),
									NULL);
	}

	/*
	 * Planning might have modified the rangetable, due to changes of the
	 * Query structures inside subquery RTEs.  We have to ensure that this
	 * gets propagated back to the master copy.  But can't do this until we
	 * are done planning, because all the calls to grouping_planner need
	 * virgin sub-Queries to work from.  (We are effectively assuming that
	 * sub-Queries will get planned identically each time, or at least that
	 * the impacts on their rangetables will be the same each time.)
	 *
	 * XXX should clean this up someday
	 */
	parse->rtable = rtable;

	/* Suppress Append if there's only one surviving child rel */
	if (list_length(subplans) == 1)
		return (Plan *) linitial(subplans);

	return (Plan *) make_append(subplans, true, tlist);
}

/*--------------------
 * grouping_planner
 *	  Perform planning steps related to grouping, aggregation, etc.
 *	  This primarily means adding top-level processing to the basic
 *	  query plan produced by query_planner.
 *
 * tuple_fraction is the fraction of tuples we expect will be retrieved
 *
 * tuple_fraction is interpreted as follows:
 *	  0: expect all tuples to be retrieved (normal case)
 *	  0 < tuple_fraction < 1: expect the given fraction of tuples available
 *		from the plan to be retrieved
 *	  tuple_fraction >= 1: tuple_fraction is the absolute number of tuples
 *		expected to be retrieved (ie, a LIMIT specification)
 *
 * Returns a query plan.  Also, root->query_pathkeys is returned as the
 * actual output ordering of the plan (in pathkey format).
 *--------------------
 */
static Plan *
grouping_planner(PlannerInfo *root, double tuple_fraction)
{
	Query	   *parse = root->parse;
	List	   *tlist = parse->targetList;
	int64		offset_est = 0;
	int64		count_est = 0;
	double		limit_tuples = -1.0;
	Plan	   *result_plan;
	List	   *current_pathkeys;
	List	   *sort_pathkeys;
	double		dNumGroups = 0;

	/* Tweak caller-supplied tuple_fraction if have LIMIT/OFFSET */
	if (parse->limitCount || parse->limitOffset)
	{
		tuple_fraction = preprocess_limit(root, tuple_fraction,
										  &offset_est, &count_est);

		/*
		 * If we have a known LIMIT, and don't have an unknown OFFSET, we can
		 * estimate the effects of using a bounded sort.
		 */
		if (count_est > 0 && offset_est >= 0)
			limit_tuples = (double) count_est + (double) offset_est;
	}

	if (parse->setOperations)
	{
		List	   *set_sortclauses;

		/*
		 * If there's a top-level ORDER BY, assume we have to fetch all the
		 * tuples.	This might seem too simplistic given all the hackery below
		 * to possibly avoid the sort ... but a nonzero tuple_fraction is only
		 * of use to plan_set_operations() when the setop is UNION ALL, and
		 * the result of UNION ALL is always unsorted.
		 */
		if (parse->sortClause)
			tuple_fraction = 0.0;

		/*
		 * Construct the plan for set operations.  The result will not need
		 * any work except perhaps a top-level sort and/or LIMIT.
		 */
		result_plan = plan_set_operations(root, tuple_fraction,
										  &set_sortclauses);

		/*
		 * Calculate pathkeys representing the sort order (if any) of the set
		 * operation's result.  We have to do this before overwriting the sort
		 * key information...
		 */
		current_pathkeys = make_pathkeys_for_sortclauses(root,
														 set_sortclauses,
													 result_plan->targetlist,
														 true);

		/*
		 * We should not need to call preprocess_targetlist, since we must be
		 * in a SELECT query node.	Instead, use the targetlist returned by
		 * plan_set_operations (since this tells whether it returned any
		 * resjunk columns!), and transfer any sort key information from the
		 * original tlist.
		 */
		Assert(parse->commandType == CMD_SELECT);

		tlist = postprocess_setop_tlist(result_plan->targetlist, tlist);

		/*
		 * Can't handle FOR UPDATE/SHARE here (parser should have checked
		 * already, but let's make sure).
		 */
		if (parse->rowMarks)
			ereport(ERROR,
					(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
					 errmsg("SELECT FOR UPDATE/SHARE is not allowed with UNION/INTERSECT/EXCEPT")));

		/*
		 * Calculate pathkeys that represent result ordering requirements
		 */
		sort_pathkeys = make_pathkeys_for_sortclauses(root,
													  parse->sortClause,
													  tlist,
													  true);
	}
	else
	{
		/* No set operations, do regular planning */
		List	   *sub_tlist;
		List	   *group_pathkeys;
		AttrNumber *groupColIdx = NULL;
		Oid		   *groupOperators = NULL;
		bool		need_tlist_eval = true;
		QualCost	tlist_cost;
		Path	   *cheapest_path;
		Path	   *sorted_path;
		Path	   *best_path;
		long		numGroups = 0;
		AggClauseCounts agg_counts;
		int			numGroupCols = list_length(parse->groupClause);
		bool		use_hashed_grouping = false;

		MemSet(&agg_counts, 0, sizeof(AggClauseCounts));

		/*
		 * If the query involves ungrouped aggregation, then it can produce
		 * at most one row, so we can ignore any ORDER BY or DISTINCT
		 * request.  This isn't all that exciting as an optimization, but it
		 * prevents a corner case when optimize_minmax_aggregates succeeds:
		 * if ORDER BY or DISTINCT were present we'd try, and fail, to match
		 * the EquivalenceClasses we're about to build with the modified
		 * targetlist entries it will create.
		 */
		if (parse->hasAggs && parse->groupClause == NIL)
		{
			parse->sortClause = NIL;
			parse->distinctClause = NIL;
		}

		/* Preprocess targetlist */
		tlist = preprocess_targetlist(root, tlist);

		/*
		 * Generate appropriate target list for subplan; may be different from
		 * tlist if grouping or aggregation is needed.
		 */
		sub_tlist = make_subplanTargetList(root, tlist,
										   &groupColIdx, &need_tlist_eval);

		/*
		 * Calculate pathkeys that represent grouping/ordering requirements.
		 * Stash them in PlannerInfo so that query_planner can canonicalize
		 * them after EquivalenceClasses have been formed.
		 */
		root->group_pathkeys =
			make_pathkeys_for_sortclauses(root,
										  parse->groupClause,
										  tlist,
										  false);
		root->sort_pathkeys =
			make_pathkeys_for_sortclauses(root,
										  parse->sortClause,
										  tlist,
										  false);

		/*
		 * Will need actual number of aggregates for estimating costs.
		 *
		 * Note: we do not attempt to detect duplicate aggregates here; a
		 * somewhat-overestimated count is okay for our present purposes.
		 *
		 * Note: think not that we can turn off hasAggs if we find no aggs. It
		 * is possible for constant-expression simplification to remove all
		 * explicit references to aggs, but we still have to follow the
		 * aggregate semantics (eg, producing only one output row).
		 */
		if (parse->hasAggs)
		{
			count_agg_clauses((Node *) tlist, &agg_counts);
			count_agg_clauses(parse->havingQual, &agg_counts);
		}

		/*
		 * Figure out whether we need a sorted result from query_planner.
		 *
		 * If we have a GROUP BY clause, then we want a result sorted properly
		 * for grouping.  Otherwise, if there is an ORDER BY clause, we want
		 * to sort by the ORDER BY clause.	(Note: if we have both, and ORDER
		 * BY is a superset of GROUP BY, it would be tempting to request sort
		 * by ORDER BY --- but that might just leave us failing to exploit an
		 * available sort order at all. Needs more thought...)
		 */
		if (parse->groupClause)
			root->query_pathkeys = root->group_pathkeys;
		else if (parse->sortClause)
			root->query_pathkeys = root->sort_pathkeys;
		else
			root->query_pathkeys = NIL;

		/*
		 * Generate the best unsorted and presorted paths for this Query (but
		 * note there may not be any presorted path).  query_planner will also
		 * estimate the number of groups in the query, and canonicalize all
		 * the pathkeys.
		 */
		query_planner(root, sub_tlist, tuple_fraction, limit_tuples,
					  &cheapest_path, &sorted_path, &dNumGroups);

		group_pathkeys = root->group_pathkeys;
		sort_pathkeys = root->sort_pathkeys;

		/*
		 * If grouping, extract the grouping operators and decide whether we
		 * want to use hashed grouping.
		 */
		if (parse->groupClause)
		{
			groupOperators = extract_grouping_ops(parse->groupClause);
			use_hashed_grouping =
				choose_hashed_grouping(root, tuple_fraction, limit_tuples,
									   cheapest_path, sorted_path,
									   groupOperators, dNumGroups,
									   &agg_counts);

			/* Also convert # groups to long int --- but 'ware overflow! */
			numGroups = (long) Min(dNumGroups, (double) LONG_MAX);
		}

		/*
		 * Select the best path.  If we are doing hashed grouping, we will
		 * always read all the input tuples, so use the cheapest-total path.
		 * Otherwise, trust query_planner's decision about which to use.
		 */
		if (use_hashed_grouping || !sorted_path)
			best_path = cheapest_path;
		else
			best_path = sorted_path;

		/*
		 * Check to see if it's possible to optimize MIN/MAX aggregates. If
		 * so, we will forget all the work we did so far to choose a "regular"
		 * path ... but we had to do it anyway to be able to tell which way is
		 * cheaper.
		 */
		result_plan = optimize_minmax_aggregates(root,
												 tlist,
												 best_path);
		if (result_plan != NULL)
		{
			/*
			 * optimize_minmax_aggregates generated the full plan, with the
			 * right tlist, and it has no sort order.
			 */
			current_pathkeys = NIL;
		}
		else
		{
			/*
			 * Normal case --- create a plan according to query_planner's
			 * results.
			 */
			result_plan = create_plan(root, best_path);
			current_pathkeys = best_path->pathkeys;

			/*
			 * create_plan() returns a plan with just a "flat" tlist of
			 * required Vars.  Usually we need to insert the sub_tlist as the
			 * tlist of the top plan node.	However, we can skip that if we
			 * determined that whatever query_planner chose to return will be
			 * good enough.
			 */
			if (need_tlist_eval)
			{
				/*
				 * If the top-level plan node is one that cannot do expression
				 * evaluation, we must insert a Result node to project the
				 * desired tlist.
				 */
				if (!is_projection_capable_plan(result_plan))
				{
					result_plan = (Plan *) make_result(root,
													   sub_tlist,
													   NULL,
													   result_plan);
				}
				else
				{
					/*
					 * Otherwise, just replace the subplan's flat tlist with
					 * the desired tlist.
					 */
					result_plan->targetlist = sub_tlist;
				}

				/*
				 * Also, account for the cost of evaluation of the sub_tlist.
				 *
				 * Up to now, we have only been dealing with "flat" tlists,
				 * containing just Vars.  So their evaluation cost is zero
				 * according to the model used by cost_qual_eval() (or if you
				 * prefer, the cost is factored into cpu_tuple_cost).  Thus we
				 * can avoid accounting for tlist cost throughout
				 * query_planner() and subroutines.  But now we've inserted a
				 * tlist that might contain actual operators, sub-selects, etc
				 * --- so we'd better account for its cost.
				 *
				 * Below this point, any tlist eval cost for added-on nodes
				 * should be accounted for as we create those nodes.
				 * Presently, of the node types we can add on, only Agg and
				 * Group project new tlists (the rest just copy their input
				 * tuples) --- so make_agg() and make_group() are responsible
				 * for computing the added cost.
				 */
				cost_qual_eval(&tlist_cost, sub_tlist, root);
				result_plan->startup_cost += tlist_cost.startup;
				result_plan->total_cost += tlist_cost.startup +
					tlist_cost.per_tuple * result_plan->plan_rows;
			}
			else
			{
				/*
				 * Since we're using query_planner's tlist and not the one
				 * make_subplanTargetList calculated, we have to refigure any
				 * grouping-column indexes make_subplanTargetList computed.
				 */
				locate_grouping_columns(root, tlist, result_plan->targetlist,
										groupColIdx);
			}

			/*
			 * Insert AGG or GROUP node if needed, plus an explicit sort step
			 * if necessary.
			 *
			 * HAVING clause, if any, becomes qual of the Agg or Group node.
			 */
			if (use_hashed_grouping)
			{
				/* Hashed aggregate plan --- no sort needed */
				result_plan = (Plan *) make_agg(root,
												tlist,
												(List *) parse->havingQual,
												AGG_HASHED,
												numGroupCols,
												groupColIdx,
												groupOperators,
												numGroups,
												agg_counts.numAggs,
												result_plan);
				/* Hashed aggregation produces randomly-ordered results */
				current_pathkeys = NIL;
			}
			else if (parse->hasAggs)
			{
				/* Plain aggregate plan --- sort if needed */
				AggStrategy aggstrategy;

				if (parse->groupClause)
				{
					if (!pathkeys_contained_in(group_pathkeys,
											   current_pathkeys))
					{
						result_plan = (Plan *)
							make_sort_from_groupcols(root,
													 parse->groupClause,
													 groupColIdx,
													 result_plan);
						current_pathkeys = group_pathkeys;
					}
					aggstrategy = AGG_SORTED;

					/*
					 * The AGG node will not change the sort ordering of its
					 * groups, so current_pathkeys describes the result too.
					 */
				}
				else
				{
					aggstrategy = AGG_PLAIN;
					/* Result will be only one row anyway; no sort order */
					current_pathkeys = NIL;
				}

				result_plan = (Plan *) make_agg(root,
												tlist,
												(List *) parse->havingQual,
												aggstrategy,
												numGroupCols,
												groupColIdx,
												groupOperators,
												numGroups,
												agg_counts.numAggs,
												result_plan);
			}
			else if (parse->groupClause)
			{
				/*
				 * GROUP BY without aggregation, so insert a group node (plus
				 * the appropriate sort node, if necessary).
				 *
				 * Add an explicit sort if we couldn't make the path come out
				 * the way the GROUP node needs it.
				 */
				if (!pathkeys_contained_in(group_pathkeys, current_pathkeys))
				{
					result_plan = (Plan *)
						make_sort_from_groupcols(root,
												 parse->groupClause,
												 groupColIdx,
												 result_plan);
					current_pathkeys = group_pathkeys;
				}

				result_plan = (Plan *) make_group(root,
												  tlist,
												  (List *) parse->havingQual,
												  numGroupCols,
												  groupColIdx,
												  groupOperators,
												  dNumGroups,
												  result_plan);
				/* The Group node won't change sort ordering */
			}
			else if (root->hasHavingQual)
			{
				/*
				 * No aggregates, and no GROUP BY, but we have a HAVING qual.
				 * This is a degenerate case in which we are supposed to emit
				 * either 0 or 1 row depending on whether HAVING succeeds.
				 * Furthermore, there cannot be any variables in either HAVING
				 * or the targetlist, so we actually do not need the FROM
				 * table at all!  We can just throw away the plan-so-far and
				 * generate a Result node.	This is a sufficiently unusual
				 * corner case that it's not worth contorting the structure of
				 * this routine to avoid having to generate the plan in the
				 * first place.
				 */
				result_plan = (Plan *) make_result(root,
												   tlist,
												   parse->havingQual,
												   NULL);
			}
		}						/* end of non-minmax-aggregate case */
	}							/* end of if (setOperations) */

	/*
	 * If we were not able to make the plan come out in the right order, add
	 * an explicit sort step.
	 */
	if (parse->sortClause)
	{
		if (!pathkeys_contained_in(sort_pathkeys, current_pathkeys))
		{
			result_plan = (Plan *) make_sort_from_pathkeys(root,
														   result_plan,
														   sort_pathkeys,
														   limit_tuples);
			current_pathkeys = sort_pathkeys;
		}
	}

	/*
	 * If there is a DISTINCT clause, add the UNIQUE node.
	 */
	if (parse->distinctClause)
	{
		result_plan = (Plan *) make_unique(result_plan, parse->distinctClause);

		/*
		 * If there was grouping or aggregation, leave plan_rows as-is (ie,
		 * assume the result was already mostly unique).  If not, use the
		 * number of distinct-groups calculated by query_planner.
		 */
		if (!parse->groupClause && !root->hasHavingQual && !parse->hasAggs)
			result_plan->plan_rows = dNumGroups;
	}

	/*
	 * Finally, if there is a LIMIT/OFFSET clause, add the LIMIT node.
	 */
	if (parse->limitCount || parse->limitOffset)
	{
		result_plan = (Plan *) make_limit(result_plan,
										  parse->limitOffset,
										  parse->limitCount,
										  offset_est,
										  count_est);
	}

	/*
	 * Deal with the RETURNING clause if any.  It's convenient to pass the
	 * returningList through setrefs.c now rather than at top level (if we
	 * waited, handling inherited UPDATE/DELETE would be much harder).
	 */
	if (parse->returningList)
	{
		List	   *rlist;

		Assert(parse->resultRelation);
		rlist = set_returning_clause_references(root->glob,
												parse->returningList,
												result_plan,
												parse->resultRelation);
		root->returningLists = list_make1(rlist);
	}
	else
		root->returningLists = NIL;

	/* Compute result-relations list if needed */
	if (parse->resultRelation)
		root->resultRelations = list_make1_int(parse->resultRelation);
	else
		root->resultRelations = NIL;

	/*
	 * Return the actual output ordering in query_pathkeys for possible use by
	 * an outer query level.
	 */
	root->query_pathkeys = current_pathkeys;

	return result_plan;
}

/*
 * Detect whether a plan node is a "dummy" plan created when a relation
 * is deemed not to need scanning due to constraint exclusion.
 *
 * Currently, such dummy plans are Result nodes with constant FALSE
 * filter quals.
 */
static bool
is_dummy_plan(Plan *plan)
{
	if (IsA(plan, Result))
	{
		List	   *rcqual = (List *) ((Result *) plan)->resconstantqual;

		if (list_length(rcqual) == 1)
		{
			Const	   *constqual = (Const *) linitial(rcqual);

			if (constqual && IsA(constqual, Const))
			{
				if (!constqual->constisnull &&
					!DatumGetBool(constqual->constvalue))
					return true;
			}
		}
	}
	return false;
}

/*
 * preprocess_limit - do pre-estimation for LIMIT and/or OFFSET clauses
 *
 * We try to estimate the values of the LIMIT/OFFSET clauses, and pass the
 * results back in *count_est and *offset_est.	These variables are set to
 * 0 if the corresponding clause is not present, and -1 if it's present
 * but we couldn't estimate the value for it.  (The "0" convention is OK
 * for OFFSET but a little bit bogus for LIMIT: effectively we estimate
 * LIMIT 0 as though it were LIMIT 1.  But this is in line with the planner's
 * usual practice of never estimating less than one row.)  These values will
 * be passed to make_limit, which see if you change this code.
 *
 * The return value is the suitably adjusted tuple_fraction to use for
 * planning the query.	This adjustment is not overridable, since it reflects
 * plan actions that grouping_planner() will certainly take, not assumptions
 * about context.
 */
static double
preprocess_limit(PlannerInfo *root, double tuple_fraction,
				 int64 *offset_est, int64 *count_est)
{
	Query	   *parse = root->parse;
	Node	   *est;
	double		limit_fraction;

	/* Should not be called unless LIMIT or OFFSET */
	Assert(parse->limitCount || parse->limitOffset);

	/*
	 * Try to obtain the clause values.  We use estimate_expression_value
	 * primarily because it can sometimes do something useful with Params.
	 */
	if (parse->limitCount)
	{
		est = estimate_expression_value(root, parse->limitCount);
		if (est && IsA(est, Const))
		{
			if (((Const *) est)->constisnull)
			{
				/* NULL indicates LIMIT ALL, ie, no limit */
				*count_est = 0; /* treat as not present */
			}
			else
			{
				*count_est = DatumGetInt64(((Const *) est)->constvalue);
				if (*count_est <= 0)
					*count_est = 1;		/* force to at least 1 */
			}
		}
		else
			*count_est = -1;	/* can't estimate */
	}
	else
		*count_est = 0;			/* not present */

	if (parse->limitOffset)
	{
		est = estimate_expression_value(root, parse->limitOffset);
		if (est && IsA(est, Const))
		{
			if (((Const *) est)->constisnull)
			{
				/* Treat NULL as no offset; the executor will too */
				*offset_est = 0;	/* treat as not present */
			}
			else
			{
				*offset_est = DatumGetInt64(((Const *) est)->constvalue);
				if (*offset_est < 0)
					*offset_est = 0;	/* less than 0 is same as 0 */
			}
		}
		else
			*offset_est = -1;	/* can't estimate */
	}
	else
		*offset_est = 0;		/* not present */

	if (*count_est != 0)
	{
		/*
		 * A LIMIT clause limits the absolute number of tuples returned.
		 * However, if it's not a constant LIMIT then we have to guess; for
		 * lack of a better idea, assume 10% of the plan's result is wanted.
		 */
		if (*count_est < 0 || *offset_est < 0)
		{
			/* LIMIT or OFFSET is an expression ... punt ... */
			limit_fraction = 0.10;
		}
		else
		{
			/* LIMIT (plus OFFSET, if any) is max number of tuples needed */
			limit_fraction = (double) *count_est + (double) *offset_est;
		}

		/*
		 * If we have absolute limits from both caller and LIMIT, use the
		 * smaller value; likewise if they are both fractional.  If one is
		 * fractional and the other absolute, we can't easily determine which
		 * is smaller, but we use the heuristic that the absolute will usually
		 * be smaller.
		 */
		if (tuple_fraction >= 1.0)
		{
			if (limit_fraction >= 1.0)
			{
				/* both absolute */
				tuple_fraction = Min(tuple_fraction, limit_fraction);
			}
			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;
}