postgres/src/backend/optimizer/util/pathnode.c

/*-------------------------------------------------------------------------
 *
 * pathnode.c
 *	  Routines to manipulate pathlists and create path nodes
 *
 * 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/util/pathnode.c,v 1.146 2008/08/14 18:47:59 tgl Exp $
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"

#include <math.h>

#include "catalog/pg_operator.h"
#include "executor/executor.h"
#include "miscadmin.h"
#include "optimizer/clauses.h"
#include "optimizer/cost.h"
#include "optimizer/pathnode.h"
#include "optimizer/paths.h"
#include "optimizer/tlist.h"
#include "optimizer/var.h"
#include "parser/parse_expr.h"
#include "parser/parsetree.h"
#include "utils/selfuncs.h"
#include "utils/lsyscache.h"
#include "utils/syscache.h"


static List *translate_sub_tlist(List *tlist, int relid);
static bool query_is_distinct_for(Query *query, List *colnos, List *opids);
static Oid	distinct_col_search(int colno, List *colnos, List *opids);


/*****************************************************************************
 *		MISC. PATH UTILITIES
 *****************************************************************************/

/*
 * compare_path_costs
 *	  Return -1, 0, or +1 according as path1 is cheaper, the same cost,
 *	  or more expensive than path2 for the specified criterion.
 */
int
compare_path_costs(Path *path1, Path *path2, CostSelector criterion)
{
	if (criterion == STARTUP_COST)
	{
		if (path1->startup_cost < path2->startup_cost)
			return -1;
		if (path1->startup_cost > path2->startup_cost)
			return +1;

		/*
		 * If paths have the same startup cost (not at all unlikely), order
		 * them by total cost.
		 */
		if (path1->total_cost < path2->total_cost)
			return -1;
		if (path1->total_cost > path2->total_cost)
			return +1;
	}
	else
	{
		if (path1->total_cost < path2->total_cost)
			return -1;
		if (path1->total_cost > path2->total_cost)
			return +1;

		/*
		 * If paths have the same total cost, order them by startup cost.
		 */
		if (path1->startup_cost < path2->startup_cost)
			return -1;
		if (path1->startup_cost > path2->startup_cost)
			return +1;
	}
	return 0;
}

/*
 * compare_fuzzy_path_costs
 *	  Return -1, 0, or +1 according as path1 is cheaper, the same cost,
 *	  or more expensive than path2 for the specified criterion.
 *
 * This differs from compare_path_costs in that we consider the costs the
 * same if they agree to within a "fuzz factor".  This is used by add_path
 * to avoid keeping both of a pair of paths that really have insignificantly
 * different cost.
 */
static int
compare_fuzzy_path_costs(Path *path1, Path *path2, CostSelector criterion)
{
	/*
	 * We use a fuzz factor of 1% of the smaller cost.
	 *
	 * XXX does this percentage need to be user-configurable?
	 */
	if (criterion == STARTUP_COST)
	{
		if (path1->startup_cost > path2->startup_cost * 1.01)
			return +1;
		if (path2->startup_cost > path1->startup_cost * 1.01)
			return -1;

		/*
		 * If paths have the same startup cost (not at all unlikely), order
		 * them by total cost.
		 */
		if (path1->total_cost > path2->total_cost * 1.01)
			return +1;
		if (path2->total_cost > path1->total_cost * 1.01)
			return -1;
	}
	else
	{
		if (path1->total_cost > path2->total_cost * 1.01)
			return +1;
		if (path2->total_cost > path1->total_cost * 1.01)
			return -1;

		/*
		 * If paths have the same total cost, order them by startup cost.
		 */
		if (path1->startup_cost > path2->startup_cost * 1.01)
			return +1;
		if (path2->startup_cost > path1->startup_cost * 1.01)
			return -1;
	}
	return 0;
}

/*
 * compare_path_fractional_costs
 *	  Return -1, 0, or +1 according as path1 is cheaper, the same cost,
 *	  or more expensive than path2 for fetching the specified fraction
 *	  of the total tuples.
 *
 * If fraction is <= 0 or > 1, we interpret it as 1, ie, we select the
 * path with the cheaper total_cost.
 */
int
compare_fractional_path_costs(Path *path1, Path *path2,
							  double fraction)
{
	Cost		cost1,
				cost2;

	if (fraction <= 0.0 || fraction >= 1.0)
		return compare_path_costs(path1, path2, TOTAL_COST);
	cost1 = path1->startup_cost +
		fraction * (path1->total_cost - path1->startup_cost);
	cost2 = path2->startup_cost +
		fraction * (path2->total_cost - path2->startup_cost);
	if (cost1 < cost2)
		return -1;
	if (cost1 > cost2)
		return +1;
	return 0;
}

/*
 * set_cheapest
 *	  Find the minimum-cost paths from among a relation's paths,
 *	  and save them in the rel's cheapest-path fields.
 *
 * This is normally called only after we've finished constructing the path
 * list for the rel node.
 *
 * If we find two paths of identical costs, try to keep the better-sorted one.
 * The paths might have unrelated sort orderings, in which case we can only
 * guess which might be better to keep, but if one is superior then we
 * definitely should keep it.
 */
void
set_cheapest(RelOptInfo *parent_rel)
{
	List	   *pathlist = parent_rel->pathlist;
	ListCell   *p;
	Path	   *cheapest_startup_path;
	Path	   *cheapest_total_path;

	Assert(IsA(parent_rel, RelOptInfo));

	if (pathlist == NIL)
		elog(ERROR, "could not devise a query plan for the given query");

	cheapest_startup_path = cheapest_total_path = (Path *) linitial(pathlist);

	for_each_cell(p, lnext(list_head(pathlist)))
	{
		Path	   *path = (Path *) lfirst(p);
		int			cmp;

		cmp = compare_path_costs(cheapest_startup_path, path, STARTUP_COST);
		if (cmp > 0 ||
			(cmp == 0 &&
			 compare_pathkeys(cheapest_startup_path->pathkeys,
							  path->pathkeys) == PATHKEYS_BETTER2))
			cheapest_startup_path = path;

		cmp = compare_path_costs(cheapest_total_path, path, TOTAL_COST);
		if (cmp > 0 ||
			(cmp == 0 &&
			 compare_pathkeys(cheapest_total_path->pathkeys,
							  path->pathkeys) == PATHKEYS_BETTER2))
			cheapest_total_path = path;
	}

	parent_rel->cheapest_startup_path = cheapest_startup_path;
	parent_rel->cheapest_total_path = cheapest_total_path;
	parent_rel->cheapest_unique_path = NULL;	/* computed only if needed */
}

/*
 * add_path
 *	  Consider a potential implementation path for the specified parent rel,
 *	  and add it to the rel's pathlist if it is worthy of consideration.
 *	  A path is worthy if it has either a better sort order (better pathkeys)
 *	  or cheaper cost (on either dimension) than any of the existing old paths.
 *
 *	  We also remove from the rel's pathlist any old paths that are dominated
 *	  by new_path --- that is, new_path is both cheaper and at least as well
 *	  ordered.
 *
 *	  The pathlist is kept sorted by TOTAL_COST metric, with cheaper paths
 *	  at the front.  No code depends on that for correctness; it's simply
 *	  a speed hack within this routine.  Doing it that way makes it more
 *	  likely that we will reject an inferior path after a few comparisons,
 *	  rather than many comparisons.
 *
 *	  NOTE: discarded Path objects are immediately pfree'd to reduce planner
 *	  memory consumption.  We dare not try to free the substructure of a Path,
 *	  since much of it may be shared with other Paths or the query tree itself;
 *	  but just recycling discarded Path nodes is a very useful savings in
 *	  a large join tree.  We can recycle the List nodes of pathlist, too.
 *
 *	  BUT: we do not pfree IndexPath objects, since they may be referenced as
 *	  children of BitmapHeapPaths as well as being paths in their own right.
 *
 * 'parent_rel' is the relation entry to which the path corresponds.
 * 'new_path' is a potential path for parent_rel.
 *
 * Returns nothing, but modifies parent_rel->pathlist.
 */
void
add_path(RelOptInfo *parent_rel, Path *new_path)
{
	bool		accept_new = true;		/* unless we find a superior old path */
	ListCell   *insert_after = NULL;	/* where to insert new item */
	ListCell   *p1_prev = NULL;
	ListCell   *p1;

	/*
	 * This is a convenient place to check for query cancel --- no part of the
	 * planner goes very long without calling add_path().
	 */
	CHECK_FOR_INTERRUPTS();

	/*
	 * Loop to check proposed new path against old paths.  Note it is possible
	 * for more than one old path to be tossed out because new_path dominates
	 * it.
	 */
	p1 = list_head(parent_rel->pathlist);		/* cannot use foreach here */
	while (p1 != NULL)
	{
		Path	   *old_path = (Path *) lfirst(p1);
		bool		remove_old = false; /* unless new proves superior */
		int			costcmp;

		/*
		 * As of Postgres 8.0, we use fuzzy cost comparison to avoid wasting
		 * cycles keeping paths that are really not significantly different in
		 * cost.
		 */
		costcmp = compare_fuzzy_path_costs(new_path, old_path, TOTAL_COST);

		/*
		 * If the two paths compare differently for startup and total cost,
		 * then we want to keep both, and we can skip the (much slower)
		 * comparison of pathkeys.	If they compare the same, proceed with the
		 * pathkeys comparison.  Note: this test relies on the fact that
		 * compare_fuzzy_path_costs will only return 0 if both costs are
		 * effectively equal (and, therefore, there's no need to call it twice
		 * in that case).
		 */
		if (costcmp == 0 ||
			costcmp == compare_fuzzy_path_costs(new_path, old_path,
												STARTUP_COST))
		{
			switch (compare_pathkeys(new_path->pathkeys, old_path->pathkeys))
			{
				case PATHKEYS_EQUAL:
					if (costcmp < 0)
						remove_old = true;		/* new dominates old */
					else if (costcmp > 0)
						accept_new = false;		/* old dominates new */
					else
					{
						/*
						 * Same pathkeys, and fuzzily the same cost, so keep
						 * just one --- but we'll do an exact cost comparison
						 * to decide which.
						 */
						if (compare_path_costs(new_path, old_path,
											   TOTAL_COST) < 0)
							remove_old = true;	/* new dominates old */
						else
							accept_new = false; /* old equals or dominates new */
					}
					break;
				case PATHKEYS_BETTER1:
					if (costcmp <= 0)
						remove_old = true;		/* new dominates old */
					break;
				case PATHKEYS_BETTER2:
					if (costcmp >= 0)
						accept_new = false;		/* old dominates new */
					break;
				case PATHKEYS_DIFFERENT:
					/* keep both paths, since they have different ordering */
					break;
			}
		}

		/*
		 * Remove current element from pathlist if dominated by new.
		 */
		if (remove_old)
		{
			parent_rel->pathlist = list_delete_cell(parent_rel->pathlist,
													p1, p1_prev);

			/*
			 * Delete the data pointed-to by the deleted cell, if possible
			 */
			if (!IsA(old_path, IndexPath))
				pfree(old_path);
			/* Advance list pointer */
			if (p1_prev)
				p1 = lnext(p1_prev);
			else
				p1 = list_head(parent_rel->pathlist);
		}
		else
		{
			/* new belongs after this old path if it has cost >= old's */
			if (costcmp >= 0)
				insert_after = p1;
			/* Advance list pointers */
			p1_prev = p1;
			p1 = lnext(p1);
		}

		/*
		 * If we found an old path that dominates new_path, we can quit
		 * scanning the pathlist; we will not add new_path, and we assume
		 * new_path cannot dominate any other elements of the pathlist.
		 */
		if (!accept_new)
			break;
	}

	if (accept_new)
	{
		/* Accept the new path: insert it at proper place in pathlist */
		if (insert_after)
			lappend_cell(parent_rel->pathlist, insert_after, new_path);
		else
			parent_rel->pathlist = lcons(new_path, parent_rel->pathlist);
	}
	else
	{
		/* Reject and recycle the new path */
		if (!IsA(new_path, IndexPath))
			pfree(new_path);
	}
}


/*****************************************************************************
 *		PATH NODE CREATION ROUTINES
 *****************************************************************************/

/*
 * create_seqscan_path
 *	  Creates a path corresponding to a sequential scan, returning the
 *	  pathnode.
 */
Path *
create_seqscan_path(PlannerInfo *root, RelOptInfo *rel)
{
	Path	   *pathnode = makeNode(Path);

	pathnode->pathtype = T_SeqScan;
	pathnode->parent = rel;
	pathnode->pathkeys = NIL;	/* seqscan has unordered result */

	cost_seqscan(pathnode, root, rel);

	return pathnode;
}

/*
 * create_index_path
 *	  Creates a path node for an index scan.
 *
 * 'index' is a usable index.
 * 'clause_groups' is a list of lists of RestrictInfo nodes
 *			to be used as index qual conditions in the scan.
 * 'pathkeys' describes the ordering of the path.
 * 'indexscandir' is ForwardScanDirection or BackwardScanDirection
 *			for an ordered index, or NoMovementScanDirection for
 *			an unordered index.
 * 'outer_rel' is the outer relation if this is a join inner indexscan path.
 *			(pathkeys and indexscandir are ignored if so.)	NULL if not.
 *
 * Returns the new path node.
 */
IndexPath *
create_index_path(PlannerInfo *root,
				  IndexOptInfo *index,
				  List *clause_groups,
				  List *pathkeys,
				  ScanDirection indexscandir,
				  RelOptInfo *outer_rel)
{
	IndexPath  *pathnode = makeNode(IndexPath);
	RelOptInfo *rel = index->rel;
	List	   *indexquals,
			   *allclauses;

	/*
	 * For a join inner scan, there's no point in marking the path with any
	 * pathkeys, since it will only ever be used as the inner path of a
	 * nestloop, and so its ordering does not matter.  For the same reason we
	 * don't really care what order it's scanned in.  (We could expect the
	 * caller to supply the correct values, but it's easier to force it here.)
	 */
	if (outer_rel != NULL)
	{
		pathkeys = NIL;
		indexscandir = NoMovementScanDirection;
	}

	pathnode->path.pathtype = T_IndexScan;
	pathnode->path.parent = rel;
	pathnode->path.pathkeys = pathkeys;

	/* Convert clauses to indexquals the executor can handle */
	indexquals = expand_indexqual_conditions(index, clause_groups);

	/* Flatten the clause-groups list to produce indexclauses list */
	allclauses = flatten_clausegroups_list(clause_groups);

	/* Fill in the pathnode */
	pathnode->indexinfo = index;
	pathnode->indexclauses = allclauses;
	pathnode->indexquals = indexquals;

	pathnode->isjoininner = (outer_rel != NULL);
	pathnode->indexscandir = indexscandir;

	if (outer_rel != NULL)
	{
		/*
		 * We must compute the estimated number of output rows for the
		 * indexscan.  This is less than rel->rows because of the additional
		 * selectivity of the join clauses.  Since clause_groups may contain
		 * both restriction and join clauses, we have to do a set union to get
		 * the full set of clauses that must be considered to compute the
		 * correct selectivity.  (Without the union operation, we might have
		 * some restriction clauses appearing twice, which'd mislead
		 * clauselist_selectivity into double-counting their selectivity.
		 * However, since RestrictInfo nodes aren't copied when linking them
		 * into different lists, it should be sufficient to use pointer
		 * comparison to remove duplicates.)
		 *
		 * Note that we force the clauses to be treated as non-join clauses
		 * during selectivity estimation.
		 */
		allclauses = list_union_ptr(rel->baserestrictinfo, allclauses);
		pathnode->rows = rel->tuples *
			clauselist_selectivity(root,
								   allclauses,
								   rel->relid,	/* do not use 0! */
								   JOIN_INNER,
								   NULL);
		/* Like costsize.c, force estimate to be at least one row */
		pathnode->rows = clamp_row_est(pathnode->rows);
	}
	else
	{
		/*
		 * The number of rows is the same as the parent rel's estimate, since
		 * this isn't a join inner indexscan.
		 */
		pathnode->rows = rel->rows;
	}

	cost_index(pathnode, root, index, indexquals, outer_rel);

	return pathnode;
}

/*
 * create_bitmap_heap_path
 *	  Creates a path node for a bitmap scan.
 *
 * 'bitmapqual' is a tree of IndexPath, BitmapAndPath, and BitmapOrPath nodes.
 *
 * If this is a join inner indexscan path, 'outer_rel' is the outer relation,
 * and all the component IndexPaths should have been costed accordingly.
 */
BitmapHeapPath *
create_bitmap_heap_path(PlannerInfo *root,
						RelOptInfo *rel,
						Path *bitmapqual,
						RelOptInfo *outer_rel)
{
	BitmapHeapPath *pathnode = makeNode(BitmapHeapPath);

	pathnode->path.pathtype = T_BitmapHeapScan;
	pathnode->path.parent = rel;
	pathnode->path.pathkeys = NIL;		/* always unordered */

	pathnode->bitmapqual = bitmapqual;
	pathnode->isjoininner = (outer_rel != NULL);

	if (pathnode->isjoininner)
	{
		/*
		 * We must compute the estimated number of output rows for the
		 * indexscan.  This is less than rel->rows because of the additional
		 * selectivity of the join clauses.  We make use of the selectivity
		 * estimated for the bitmap to do this; this isn't really quite right
		 * since there may be restriction conditions not included in the
		 * bitmap ...
		 */
		Cost		indexTotalCost;
		Selectivity indexSelectivity;

		cost_bitmap_tree_node(bitmapqual, &indexTotalCost, &indexSelectivity);
		pathnode->rows = rel->tuples * indexSelectivity;
		if (pathnode->rows > rel->rows)
			pathnode->rows = rel->rows;
		/* Like costsize.c, force estimate to be at least one row */
		pathnode->rows = clamp_row_est(pathnode->rows);
	}
	else
	{
		/*
		 * The number of rows is the same as the parent rel's estimate, since
		 * this isn't a join inner indexscan.
		 */
		pathnode->rows = rel->rows;
	}

	cost_bitmap_heap_scan(&pathnode->path, root, rel, bitmapqual, outer_rel);

	return pathnode;
}

/*
 * create_bitmap_and_path
 *	  Creates a path node representing a BitmapAnd.
 */
BitmapAndPath *
create_bitmap_and_path(PlannerInfo *root,
					   RelOptInfo *rel,
					   List *bitmapquals)
{
	BitmapAndPath *pathnode = makeNode(BitmapAndPath);

	pathnode->path.pathtype = T_BitmapAnd;
	pathnode->path.parent = rel;
	pathnode->path.pathkeys = NIL;		/* always unordered */

	pathnode->bitmapquals = bitmapquals;

	/* this sets bitmapselectivity as well as the regular cost fields: */
	cost_bitmap_and_node(pathnode, root);

	return pathnode;
}

/*
 * create_bitmap_or_path
 *	  Creates a path node representing a BitmapOr.
 */
BitmapOrPath *
create_bitmap_or_path(PlannerInfo *root,
					  RelOptInfo *rel,
					  List *bitmapquals)
{
	BitmapOrPath *pathnode = makeNode(BitmapOrPath);

	pathnode->path.pathtype = T_BitmapOr;
	pathnode->path.parent = rel;
	pathnode->path.pathkeys = NIL;		/* always unordered */

	pathnode->bitmapquals = bitmapquals;

	/* this sets bitmapselectivity as well as the regular cost fields: */
	cost_bitmap_or_node(pathnode, root);

	return pathnode;
}

/*
 * create_tidscan_path
 *	  Creates a path corresponding to a scan by TID, returning the pathnode.
 */
TidPath *
create_tidscan_path(PlannerInfo *root, RelOptInfo *rel, List *tidquals)
{
	TidPath    *pathnode = makeNode(TidPath);

	pathnode->path.pathtype = T_TidScan;
	pathnode->path.parent = rel;
	pathnode->path.pathkeys = NIL;

	pathnode->tidquals = tidquals;

	cost_tidscan(&pathnode->path, root, rel, tidquals);

	return pathnode;
}

/*
 * create_append_path
 *	  Creates a path corresponding to an Append plan, returning the
 *	  pathnode.
 */
AppendPath *
create_append_path(RelOptInfo *rel, List *subpaths)
{
	AppendPath *pathnode = makeNode(AppendPath);
	ListCell   *l;

	pathnode->path.pathtype = T_Append;
	pathnode->path.parent = rel;
	pathnode->path.pathkeys = NIL;		/* result is always considered
										 * unsorted */
	pathnode->subpaths = subpaths;

	pathnode->path.startup_cost = 0;
	pathnode->path.total_cost = 0;
	foreach(l, subpaths)
	{
		Path	   *subpath = (Path *) lfirst(l);

		if (l == list_head(subpaths))	/* first node? */
			pathnode->path.startup_cost = subpath->startup_cost;
		pathnode->path.total_cost += subpath->total_cost;
	}

	return pathnode;
}

/*
 * create_result_path
 *	  Creates a path representing a Result-and-nothing-else plan.
 *	  This is only used for the case of a query with an empty jointree.
 */
ResultPath *
create_result_path(List *quals)
{
	ResultPath *pathnode = makeNode(ResultPath);

	pathnode->path.pathtype = T_Result;
	pathnode->path.parent = NULL;
	pathnode->path.pathkeys = NIL;
	pathnode->quals = quals;

	/* Ideally should define cost_result(), but I'm too lazy */
	pathnode->path.startup_cost = 0;
	pathnode->path.total_cost = cpu_tuple_cost;

	/*
	 * In theory we should include the qual eval cost as well, but at present
	 * that doesn't accomplish much except duplicate work that will be done
	 * again in make_result; since this is only used for degenerate cases,
	 * nothing interesting will be done with the path cost values...
	 */

	return pathnode;
}

/*
 * create_material_path
 *	  Creates a path corresponding to a Material plan, returning the
 *	  pathnode.
 */
MaterialPath *
create_material_path(RelOptInfo *rel, Path *subpath)
{
	MaterialPath *pathnode = makeNode(MaterialPath);

	pathnode->path.pathtype = T_Material;
	pathnode->path.parent = rel;

	pathnode->path.pathkeys = subpath->pathkeys;

	pathnode->subpath = subpath;

	cost_material(&pathnode->path,
				  subpath->total_cost,
				  rel->rows,
				  rel->width);

	return pathnode;
}

/*
 * create_unique_path
 *	  Creates a path representing elimination of distinct rows from the
 *	  input data.  Distinct-ness is defined according to the needs of the
 *	  semijoin represented by sjinfo.  If it is not possible to identify
 *	  how to make the data unique, NULL is returned.
 *
 * If used at all, this is likely to be called repeatedly on the same rel;
 * and the input subpath should always be the same (the cheapest_total path
 * for the rel).  So we cache the result.
 */
UniquePath *
create_unique_path(PlannerInfo *root, RelOptInfo *rel, Path *subpath,
				   SpecialJoinInfo *sjinfo)
{
	UniquePath *pathnode;
	Path		sort_path;		/* dummy for result of cost_sort */
	Path		agg_path;		/* dummy for result of cost_agg */
	MemoryContext oldcontext;
	List	   *in_operators;
	List	   *uniq_exprs;
	bool		all_btree;
	bool		all_hash;
	int			numCols;
	ListCell   *lc;

	/* Caller made a mistake if subpath isn't cheapest_total ... */
	Assert(subpath == rel->cheapest_total_path);
	/* ... or if SpecialJoinInfo is the wrong one */
	Assert(sjinfo->jointype == JOIN_SEMI);
	Assert(bms_equal(rel->relids, sjinfo->syn_righthand));

	/* If result already cached, return it */
	if (rel->cheapest_unique_path)
		return (UniquePath *) rel->cheapest_unique_path;

	/* If we previously failed, return NULL quickly */
	if (sjinfo->join_quals == NIL)
		return NULL;

	/*
	 * We must ensure path struct and subsidiary data are allocated in main
	 * planning context; otherwise GEQO memory management causes trouble.
	 * (Compare best_inner_indexscan().)
	 */
	oldcontext = MemoryContextSwitchTo(root->planner_cxt);

	/*
	 * Look to see whether the semijoin's join quals consist of AND'ed
	 * equality operators, with (only) RHS variables on only one side of
	 * each one.  If so, we can figure out how to enforce uniqueness for
	 * the RHS.
	 *
	 * Note that the in_operators list consists of the joinqual operators
	 * themselves (but commuted if needed to put the RHS value on the right).
	 * These could be cross-type operators, in which case the operator
	 * actually needed for uniqueness is a related single-type operator.
	 * We assume here that that operator will be available from the btree
	 * or hash opclass when the time comes ... if not, create_unique_plan()
	 * will fail.
	 */
	in_operators = NIL;
	uniq_exprs = NIL;
	all_btree = true;
	all_hash = enable_hashagg;		/* don't consider hash if not enabled */
	foreach(lc, sjinfo->join_quals)
	{
		OpExpr	   *op = (OpExpr *) lfirst(lc);
		Oid			opno;
		Node	   *left_expr;
		Node	   *right_expr;
		Relids		left_varnos;
		Relids		right_varnos;

		/* must be binary opclause... */
		if (!IsA(op, OpExpr))
			goto no_unique_path;
		if (list_length(op->args) != 2)
			goto no_unique_path;
		opno = op->opno;
		left_expr = linitial(op->args);
		right_expr = lsecond(op->args);

		/* check rel membership of arguments */
		left_varnos = pull_varnos(left_expr);
		right_varnos = pull_varnos(right_expr);
		if (!bms_is_empty(right_varnos) &&
			bms_is_subset(right_varnos, sjinfo->syn_righthand) &&
			!bms_overlap(left_varnos, sjinfo->syn_righthand))
		{
			/* typical case, right_expr is RHS variable */
		}
		else if (!bms_is_empty(left_varnos) &&
				 bms_is_subset(left_varnos, sjinfo->syn_righthand) &&
				 !bms_overlap(right_varnos, sjinfo->syn_righthand))
		{
			/* flipped case, left_expr is RHS variable */
			opno = get_commutator(opno);
			if (!OidIsValid(opno))
				goto no_unique_path;
			right_expr = left_expr;
		}
		else
			goto no_unique_path;

		/* all operators must be btree equality or hash equality */
		if (all_btree)
		{
			/* oprcanmerge is considered a hint... */
			if (!op_mergejoinable(opno) ||
				get_mergejoin_opfamilies(opno) == NIL)
				all_btree = false;
		}
		if (all_hash)
		{
			/* ... but oprcanhash had better be correct */
			if (!op_hashjoinable(opno))
				all_hash = false;
		}
		if (!(all_btree || all_hash))
			goto no_unique_path;

		/* so far so good, keep building lists */
		in_operators = lappend_oid(in_operators, opno);
		uniq_exprs = lappend(uniq_exprs, copyObject(right_expr));
	}

	/*
	 * The expressions we'd need to unique-ify mustn't be volatile.
	 */
	if (contain_volatile_functions((Node *) uniq_exprs))
		goto no_unique_path;

	/*
	 * If we get here, we can unique-ify using at least one of sorting
	 * and hashing.  Start building the result Path object.
	 */
	pathnode = makeNode(UniquePath);

	pathnode->path.pathtype = T_Unique;
	pathnode->path.parent = rel;

	/*
	 * Treat the output as always unsorted, since we don't necessarily have
	 * pathkeys to represent it.
	 */
	pathnode->path.pathkeys = NIL;

	pathnode->subpath = subpath;
	pathnode->in_operators = in_operators;
	pathnode->uniq_exprs = uniq_exprs;

	/*
	 * If the input is a subquery whose output must be unique already, then we
	 * don't need to do anything.  The test for uniqueness has to consider
	 * exactly which columns we are extracting; for example "SELECT DISTINCT
	 * x,y" doesn't guarantee that x alone is distinct. So we cannot check for
	 * this optimization unless uniq_exprs consists only of simple Vars
	 * referencing subquery outputs.  (Possibly we could do something with
	 * expressions in the subquery outputs, too, but for now keep it simple.)
	 */
	if (rel->rtekind == RTE_SUBQUERY)
	{
		RangeTblEntry *rte = planner_rt_fetch(rel->relid, root);
		List	   *sub_tlist_colnos;

		sub_tlist_colnos = translate_sub_tlist(uniq_exprs, rel->relid);

		if (sub_tlist_colnos &&
			query_is_distinct_for(rte->subquery,
								  sub_tlist_colnos, in_operators))
		{
			pathnode->umethod = UNIQUE_PATH_NOOP;
			pathnode->rows = rel->rows;
			pathnode->path.startup_cost = subpath->startup_cost;
			pathnode->path.total_cost = subpath->total_cost;
			pathnode->path.pathkeys = subpath->pathkeys;

			rel->cheapest_unique_path = (Path *) pathnode;

			MemoryContextSwitchTo(oldcontext);

			return pathnode;
		}
	}

	/* Estimate number of output rows */
	pathnode->rows = estimate_num_groups(root, uniq_exprs, rel->rows);
	numCols = list_length(uniq_exprs);

	if (all_btree)
	{
		/*
		 * Estimate cost for sort+unique implementation
		 */
		cost_sort(&sort_path, root, NIL,
				  subpath->total_cost,
				  rel->rows,
				  rel->width,
				  -1.0);

		/*
		 * Charge one cpu_operator_cost per comparison per input tuple.
		 * We assume all columns get compared at most of the tuples. (XXX
		 * probably this is an overestimate.)  This should agree with
		 * make_unique.
		 */
		sort_path.total_cost += cpu_operator_cost * rel->rows * numCols;
	}

	if (all_hash)
	{
		/*
		 * Estimate the overhead per hashtable entry at 64 bytes (same as in
		 * planner.c).
		 */
		int			hashentrysize = rel->width + 64;

		if (hashentrysize * pathnode->rows > work_mem * 1024L)
			all_hash = false;	/* don't try to hash */
		else
			cost_agg(&agg_path, root,
					 AGG_HASHED, 0,
					 numCols, pathnode->rows,
					 subpath->startup_cost,
					 subpath->total_cost,
					 rel->rows);
	}

	if (all_btree && all_hash)
	{
		if (agg_path.total_cost < sort_path.total_cost)
			pathnode->umethod = UNIQUE_PATH_HASH;
		else
			pathnode->umethod = UNIQUE_PATH_SORT;
	}
	else if (all_btree)
		pathnode->umethod = UNIQUE_PATH_SORT;
	else if (all_hash)
		pathnode->umethod = UNIQUE_PATH_HASH;
	else
		goto no_unique_path;

	if (pathnode->umethod == UNIQUE_PATH_HASH)
	{
		pathnode->path.startup_cost = agg_path.startup_cost;
		pathnode->path.total_cost = agg_path.total_cost;
	}
	else
	{
		pathnode->path.startup_cost = sort_path.startup_cost;
		pathnode->path.total_cost = sort_path.total_cost;
	}

	rel->cheapest_unique_path = (Path *) pathnode;

	MemoryContextSwitchTo(oldcontext);

	return pathnode;

no_unique_path:					/* failure exit */

	/* Mark the SpecialJoinInfo as not unique-able */
	sjinfo->join_quals = NIL;

	MemoryContextSwitchTo(oldcontext);

	return NULL;
}

/*
 * translate_sub_tlist - get subquery column numbers represented by tlist
 *
 * The given targetlist usually contains only Vars referencing the given relid.
 * Extract their varattnos (ie, the column numbers of the subquery) and return
 * as an integer List.
 *
 * If any of the tlist items is not a simple Var, we cannot determine whether
 * the subquery's uniqueness condition (if any) matches ours, so punt and
 * return NIL.
 */
static List *
translate_sub_tlist(List *tlist, int relid)
{
	List	   *result = NIL;
	ListCell   *l;

	foreach(l, tlist)
	{
		Var		   *var = (Var *) lfirst(l);

		if (!var || !IsA(var, Var) ||
			var->varno != relid)
			return NIL;			/* punt */

		result = lappend_int(result, var->varattno);
	}
	return result;
}

/*
 * query_is_distinct_for - does query never return duplicates of the
 *		specified columns?
 *
 * colnos is an integer list of output column numbers (resno's).  We are
 * interested in whether rows consisting of just these columns are certain
 * to be distinct.	"Distinctness" is defined according to whether the
 * corresponding upper-level equality operators listed in opids would think
 * the values are distinct.  (Note: the opids entries could be cross-type
 * operators, and thus not exactly the equality operators that the subquery
 * would use itself.  We use equality_ops_are_compatible() to check
 * compatibility.  That looks at btree or hash opfamily membership, and so
 * should give trustworthy answers for all operators that we might need
 * to deal with here.)
 */
static bool
query_is_distinct_for(Query *query, List *colnos, List *opids)
{
	ListCell   *l;
	Oid			opid;

	Assert(list_length(colnos) == list_length(opids));

	/*
	 * DISTINCT (including DISTINCT ON) guarantees uniqueness if all the
	 * columns in the DISTINCT clause appear in colnos and operator semantics
	 * match.
	 */
	if (query->distinctClause)
	{
		foreach(l, query->distinctClause)
		{
			SortGroupClause *sgc = (SortGroupClause *) lfirst(l);
			TargetEntry *tle = get_sortgroupclause_tle(sgc,
													   query->targetList);

			opid = distinct_col_search(tle->resno, colnos, opids);
			if (!OidIsValid(opid) ||
				!equality_ops_are_compatible(opid, sgc->eqop))
				break;			/* exit early if no match */
		}
		if (l == NULL)			/* had matches for all? */
			return true;
	}

	/*
	 * Similarly, GROUP BY guarantees uniqueness if all the grouped columns
	 * appear in colnos and operator semantics match.
	 */
	if (query->groupClause)
	{
		foreach(l, query->groupClause)
		{
			SortGroupClause *sgc = (SortGroupClause *) lfirst(l);
			TargetEntry *tle = get_sortgroupclause_tle(sgc,
													   query->targetList);

			opid = distinct_col_search(tle->resno, colnos, opids);
			if (!OidIsValid(opid) ||
				!equality_ops_are_compatible(opid, sgc->eqop))
				break;			/* exit early if no match */
		}
		if (l == NULL)			/* had matches for all? */
			return true;
	}
	else
	{
		/*
		 * If we have no GROUP BY, but do have aggregates or HAVING, then the
		 * result is at most one row so it's surely unique, for any operators.
		 */
		if (query->hasAggs || query->havingQual)
			return true;
	}

	/*
	 * UNION, INTERSECT, EXCEPT guarantee uniqueness of the whole output row,
	 * except with ALL.
	 */
	if (query->setOperations)
	{
		SetOperationStmt *topop = (SetOperationStmt *) query->setOperations;

		Assert(IsA(topop, SetOperationStmt));
		Assert(topop->op != SETOP_NONE);

		if (!topop->all)
		{
			ListCell   *lg;

			/* We're good if all the nonjunk output columns are in colnos */
			lg = list_head(topop->groupClauses);
			foreach(l, query->targetList)
			{
				TargetEntry *tle = (TargetEntry *) lfirst(l);
				SortGroupClause *sgc;

				if (tle->resjunk)
					continue;	/* ignore resjunk columns */

				/* non-resjunk columns should have grouping clauses */
				Assert(lg != NULL);
				sgc = (SortGroupClause *) lfirst(lg);
				lg = lnext(lg);

				opid = distinct_col_search(tle->resno, colnos, opids);
				if (!OidIsValid(opid) ||
					!equality_ops_are_compatible(opid, sgc->eqop))
					break;		/* exit early if no match */
			}
			if (l == NULL)		/* had matches for all? */
				return true;
		}
	}

	/*
	 * XXX Are there any other cases in which we can easily see the result
	 * must be distinct?
	 */

	return false;
}

/*
 * distinct_col_search - subroutine for query_is_distinct_for
 *
 * If colno is in colnos, return the corresponding element of opids,
 * else return InvalidOid.	(We expect colnos does not contain duplicates,
 * so the result is well-defined.)
 */
static Oid
distinct_col_search(int colno, List *colnos, List *opids)
{
	ListCell   *lc1,
			   *lc2;

	forboth(lc1, colnos, lc2, opids)
	{
		if (colno == lfirst_int(lc1))
			return lfirst_oid(lc2);
	}
	return InvalidOid;
}

/*
 * create_subqueryscan_path
 *	  Creates a path corresponding to a sequential scan of a subquery,
 *	  returning the pathnode.
 */
Path *
create_subqueryscan_path(RelOptInfo *rel, List *pathkeys)
{
	Path	   *pathnode = makeNode(Path);

	pathnode->pathtype = T_SubqueryScan;
	pathnode->parent = rel;
	pathnode->pathkeys = pathkeys;

	cost_subqueryscan(pathnode, rel);

	return pathnode;
}

/*
 * create_functionscan_path
 *	  Creates a path corresponding to a sequential scan of a function,
 *	  returning the pathnode.
 */
Path *
create_functionscan_path(PlannerInfo *root, RelOptInfo *rel)
{
	Path	   *pathnode = makeNode(Path);

	pathnode->pathtype = T_FunctionScan;
	pathnode->parent = rel;
	pathnode->pathkeys = NIL;	/* for now, assume unordered result */

	cost_functionscan(pathnode, root, rel);

	return pathnode;
}

/*
 * create_valuesscan_path
 *	  Creates a path corresponding to a scan of a VALUES list,
 *	  returning the pathnode.
 */
Path *
create_valuesscan_path(PlannerInfo *root, RelOptInfo *rel)
{
	Path	   *pathnode = makeNode(Path);

	pathnode->pathtype = T_ValuesScan;
	pathnode->parent = rel;
	pathnode->pathkeys = NIL;	/* result is always unordered */

	cost_valuesscan(pathnode, root, rel);

	return pathnode;
}

/*
 * create_nestloop_path
 *	  Creates a pathnode corresponding to a nestloop join between two
 *	  relations.
 *
 * 'joinrel' is the join relation.
 * 'jointype' is the type of join required
 * 'sjinfo' is extra info about the join for selectivity estimation
 * 'outer_path' is the outer path
 * 'inner_path' is the inner path
 * 'restrict_clauses' are the RestrictInfo nodes to apply at the join
 * 'pathkeys' are the path keys of the new join path
 *
 * Returns the resulting path node.
 */
NestPath *
create_nestloop_path(PlannerInfo *root,
					 RelOptInfo *joinrel,
					 JoinType jointype,
					 SpecialJoinInfo *sjinfo,
					 Path *outer_path,
					 Path *inner_path,
					 List *restrict_clauses,
					 List *pathkeys)
{
	NestPath   *pathnode = makeNode(NestPath);

	pathnode->path.pathtype = T_NestLoop;
	pathnode->path.parent = joinrel;
	pathnode->jointype = jointype;
	pathnode->outerjoinpath = outer_path;
	pathnode->innerjoinpath = inner_path;
	pathnode->joinrestrictinfo = restrict_clauses;
	pathnode->path.pathkeys = pathkeys;

	cost_nestloop(pathnode, root, sjinfo);

	return pathnode;
}

/*
 * create_mergejoin_path
 *	  Creates a pathnode corresponding to a mergejoin join between
 *	  two relations
 *
 * 'joinrel' is the join relation
 * 'jointype' is the type of join required
 * 'sjinfo' is extra info about the join for selectivity estimation
 * 'outer_path' is the outer path
 * 'inner_path' is the inner path
 * 'restrict_clauses' are the RestrictInfo nodes to apply at the join
 * 'pathkeys' are the path keys of the new join path
 * 'mergeclauses' are the RestrictInfo nodes to use as merge clauses
 *		(this should be a subset of the restrict_clauses list)
 * 'outersortkeys' are the sort varkeys for the outer relation
 * 'innersortkeys' are the sort varkeys for the inner relation
 */
MergePath *
create_mergejoin_path(PlannerInfo *root,
					  RelOptInfo *joinrel,
					  JoinType jointype,
					  SpecialJoinInfo *sjinfo,
					  Path *outer_path,
					  Path *inner_path,
					  List *restrict_clauses,
					  List *pathkeys,
					  List *mergeclauses,
					  List *outersortkeys,
					  List *innersortkeys)
{
	MergePath  *pathnode = makeNode(MergePath);

	/*
	 * If the given paths are already well enough ordered, we can skip doing
	 * an explicit sort.
	 */
	if (outersortkeys &&
		pathkeys_contained_in(outersortkeys, outer_path->pathkeys))
		outersortkeys = NIL;
	if (innersortkeys &&
		pathkeys_contained_in(innersortkeys, inner_path->pathkeys))
		innersortkeys = NIL;

	/*
	 * If we are not sorting the inner path, we may need a materialize node to
	 * ensure it can be marked/restored.  (Sort does support mark/restore, so
	 * no materialize is needed in that case.)
	 *
	 * Since the inner side must be ordered, and only Sorts and IndexScans can
	 * create order to begin with, you might think there's no problem --- but
	 * you'd be wrong.  Nestloop and merge joins can *preserve* the order of
	 * their inputs, so they can be selected as the input of a mergejoin, and
	 * they don't support mark/restore at present.
	 */
	if (innersortkeys == NIL &&
		!ExecSupportsMarkRestore(inner_path->pathtype))
		inner_path = (Path *)
			create_material_path(inner_path->parent, inner_path);

	pathnode->jpath.path.pathtype = T_MergeJoin;
	pathnode->jpath.path.parent = joinrel;
	pathnode->jpath.jointype = jointype;
	pathnode->jpath.outerjoinpath = outer_path;
	pathnode->jpath.innerjoinpath = inner_path;
	pathnode->jpath.joinrestrictinfo = restrict_clauses;
	pathnode->jpath.path.pathkeys = pathkeys;
	pathnode->path_mergeclauses = mergeclauses;
	pathnode->outersortkeys = outersortkeys;
	pathnode->innersortkeys = innersortkeys;

	cost_mergejoin(pathnode, root, sjinfo);

	return pathnode;
}

/*
 * create_hashjoin_path
 *	  Creates a pathnode corresponding to a hash join between two relations.
 *
 * 'joinrel' is the join relation
 * 'jointype' is the type of join required
 * 'sjinfo' is extra info about the join for selectivity estimation
 * 'outer_path' is the cheapest outer path
 * 'inner_path' is the cheapest inner path
 * 'restrict_clauses' are the RestrictInfo nodes to apply at the join
 * 'hashclauses' are the RestrictInfo nodes to use as hash clauses
 *		(this should be a subset of the restrict_clauses list)
 */
HashPath *
create_hashjoin_path(PlannerInfo *root,
					 RelOptInfo *joinrel,
					 JoinType jointype,
					 SpecialJoinInfo *sjinfo,
					 Path *outer_path,
					 Path *inner_path,
					 List *restrict_clauses,
					 List *hashclauses)
{
	HashPath   *pathnode = makeNode(HashPath);

	pathnode->jpath.path.pathtype = T_HashJoin;
	pathnode->jpath.path.parent = joinrel;
	pathnode->jpath.jointype = jointype;
	pathnode->jpath.outerjoinpath = outer_path;
	pathnode->jpath.innerjoinpath = inner_path;
	pathnode->jpath.joinrestrictinfo = restrict_clauses;
	/* A hashjoin never has pathkeys, since its ordering is unpredictable */
	pathnode->jpath.path.pathkeys = NIL;
	pathnode->path_hashclauses = hashclauses;

	cost_hashjoin(pathnode, root, sjinfo);

	return pathnode;
}