/*-------------------------------------------------------------------------
 *
 * joinrels.c
 *	  Routines to determine which relations should be joined
 *
 * Portions Copyright (c) 1996-2011, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *	  src/backend/optimizer/path/joinrels.c
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"

#include "optimizer/joininfo.h"
#include "optimizer/pathnode.h"
#include "optimizer/paths.h"
#include "utils/memutils.h"


static void make_rels_by_clause_joins(PlannerInfo *root,
						  RelOptInfo *old_rel,
						  ListCell *other_rels);
static void make_rels_by_clauseless_joins(PlannerInfo *root,
							  RelOptInfo *old_rel,
							  ListCell *other_rels);
static bool has_join_restriction(PlannerInfo *root, RelOptInfo *rel);
static bool has_legal_joinclause(PlannerInfo *root, RelOptInfo *rel);
static bool is_dummy_rel(RelOptInfo *rel);
static void mark_dummy_rel(RelOptInfo *rel);
static bool restriction_is_constant_false(List *restrictlist,
							  bool only_pushed_down);


/*
 * join_search_one_level
 *	  Consider ways to produce join relations containing exactly 'level'
 *	  jointree items.  (This is one step of the dynamic-programming method
 *	  embodied in standard_join_search.)  Join rel nodes for each feasible
 *	  combination of lower-level rels are created and returned in a list.
 *	  Implementation paths are created for each such joinrel, too.
 *
 * level: level of rels we want to make this time
 * root->join_rel_level[j], 1 <= j < level, is a list of rels containing j items
 *
 * The result is returned in root->join_rel_level[level].
 */
void
join_search_one_level(PlannerInfo *root, int level)
{
	List	  **joinrels = root->join_rel_level;
	ListCell   *r;
	int			k;

	Assert(joinrels[level] == NIL);

	/* Set join_cur_level so that new joinrels are added to proper list */
	root->join_cur_level = level;

	/*
	 * First, consider left-sided and right-sided plans, in which rels of
	 * exactly level-1 member relations are joined against initial relations.
	 * We prefer to join using join clauses, but if we find a rel of level-1
	 * members that has no join clauses, we will generate Cartesian-product
	 * joins against all initial rels not already contained in it.
	 *
	 * In the first pass (level == 2), we try to join each initial rel to each
	 * initial rel that appears later in joinrels[1].  (The mirror-image joins
	 * are handled automatically by make_join_rel.)  In later passes, we try
	 * to join rels of size level-1 from joinrels[level-1] to each initial rel
	 * in joinrels[1].
	 */
	foreach(r, joinrels[level - 1])
	{
		RelOptInfo *old_rel = (RelOptInfo *) lfirst(r);
		ListCell   *other_rels;

		if (level == 2)
			other_rels = lnext(r);		/* only consider remaining initial
										 * rels */
		else
			other_rels = list_head(joinrels[1]);		/* consider all initial
														 * rels */

		if (old_rel->joininfo != NIL || old_rel->has_eclass_joins ||
			has_join_restriction(root, old_rel))
		{
			/*
			 * Note that if all available join clauses for this rel require
			 * more than one other rel, we will fail to make any joins against
			 * it here.  In most cases that's OK; it'll be considered by
			 * "bushy plan" join code in a higher-level pass where we have
			 * those other rels collected into a join rel.
			 *
			 * See also the last-ditch case below.
			 */
			make_rels_by_clause_joins(root,
									  old_rel,
									  other_rels);
		}
		else
		{
			/*
			 * Oops, we have a relation that is not joined to any other
			 * relation, either directly or by join-order restrictions.
			 * Cartesian product time.
			 */
			make_rels_by_clauseless_joins(root,
										  old_rel,
										  other_rels);
		}
	}

	/*
	 * Now, consider "bushy plans" in which relations of k initial rels are
	 * joined to relations of level-k initial rels, for 2 <= k <= level-2.
	 *
	 * We only consider bushy-plan joins for pairs of rels where there is a
	 * suitable join clause (or join order restriction), in order to avoid
	 * unreasonable growth of planning time.
	 */
	for (k = 2;; k++)
	{
		int			other_level = level - k;

		/*
		 * Since make_join_rel(x, y) handles both x,y and y,x cases, we only
		 * need to go as far as the halfway point.
		 */
		if (k > other_level)
			break;

		foreach(r, joinrels[k])
		{
			RelOptInfo *old_rel = (RelOptInfo *) lfirst(r);
			ListCell   *other_rels;
			ListCell   *r2;

			/*
			 * We can ignore clauseless joins here, *except* when they
			 * participate in join-order restrictions --- then we might have
			 * to force a bushy join plan.
			 */
			if (old_rel->joininfo == NIL && !old_rel->has_eclass_joins &&
				!has_join_restriction(root, old_rel))
				continue;

			if (k == other_level)
				other_rels = lnext(r);	/* only consider remaining rels */
			else
				other_rels = list_head(joinrels[other_level]);

			for_each_cell(r2, other_rels)
			{
				RelOptInfo *new_rel = (RelOptInfo *) lfirst(r2);

				if (!bms_overlap(old_rel->relids, new_rel->relids))
				{
					/*
					 * OK, we can build a rel of the right level from this
					 * pair of rels.  Do so if there is at least one usable
					 * join clause or a relevant join restriction.
					 */
					if (have_relevant_joinclause(root, old_rel, new_rel) ||
						have_join_order_restriction(root, old_rel, new_rel))
					{
						(void) make_join_rel(root, old_rel, new_rel);
					}
				}
			}
		}
	}

	/*
	 * Last-ditch effort: if we failed to find any usable joins so far, force
	 * a set of cartesian-product joins to be generated.  This handles the
	 * special case where all the available rels have join clauses but we
	 * cannot use any of those clauses yet.  An example is
	 *
	 * SELECT * FROM a,b,c WHERE (a.f1 + b.f2 + c.f3) = 0;
	 *
	 * The join clause will be usable at level 3, but at level 2 we have no
	 * choice but to make cartesian joins.	We consider only left-sided and
	 * right-sided cartesian joins in this case (no bushy).
	 */
	if (joinrels[level] == NIL)
	{
		/*
		 * This loop is just like the first one, except we always call
		 * make_rels_by_clauseless_joins().
		 */
		foreach(r, joinrels[level - 1])
		{
			RelOptInfo *old_rel = (RelOptInfo *) lfirst(r);
			ListCell   *other_rels;

			if (level == 2)
				other_rels = lnext(r);	/* only consider remaining initial
										 * rels */
			else
				other_rels = list_head(joinrels[1]);	/* consider all initial
														 * rels */

			make_rels_by_clauseless_joins(root,
										  old_rel,
										  other_rels);
		}

		/*----------
		 * When special joins are involved, there may be no legal way
		 * to make an N-way join for some values of N.	For example consider
		 *
		 * SELECT ... FROM t1 WHERE
		 *	 x IN (SELECT ... FROM t2,t3 WHERE ...) AND
		 *	 y IN (SELECT ... FROM t4,t5 WHERE ...)
		 *
		 * We will flatten this query to a 5-way join problem, but there are
		 * no 4-way joins that join_is_legal() will consider legal.  We have
		 * to accept failure at level 4 and go on to discover a workable
		 * bushy plan at level 5.
		 *
		 * However, if there are no special joins then join_is_legal() should
		 * never fail, and so the following sanity check is useful.
		 *----------
		 */
		if (joinrels[level] == NIL && root->join_info_list == NIL)
			elog(ERROR, "failed to build any %d-way joins", level);
	}
}

/*
 * make_rels_by_clause_joins
 *	  Build joins between the given relation 'old_rel' and other relations
 *	  that participate in join clauses that 'old_rel' also participates in
 *	  (or participate in join-order restrictions with it).
 *	  The join rels are returned in root->join_rel_level[join_cur_level].
 *
 * Note: at levels above 2 we will generate the same joined relation in
 * multiple ways --- for example (a join b) join c is the same RelOptInfo as
 * (b join c) join a, though the second case will add a different set of Paths
 * to it.  This is the reason for using the join_rel_level mechanism, which
 * automatically ensures that each new joinrel is only added to the list once.
 *
 * 'old_rel' is the relation entry for the relation to be joined
 * 'other_rels': the first cell in a linked list containing the other
 * rels to be considered for joining
 *
 * Currently, this is only used with initial rels in other_rels, but it
 * will work for joining to joinrels too.
 */
static void
make_rels_by_clause_joins(PlannerInfo *root,
						  RelOptInfo *old_rel,
						  ListCell *other_rels)
{
	ListCell   *l;

	for_each_cell(l, other_rels)
	{
		RelOptInfo *other_rel = (RelOptInfo *) lfirst(l);

		if (!bms_overlap(old_rel->relids, other_rel->relids) &&
			(have_relevant_joinclause(root, old_rel, other_rel) ||
			 have_join_order_restriction(root, old_rel, other_rel)))
		{
			(void) make_join_rel(root, old_rel, other_rel);
		}
	}
}

/*
 * make_rels_by_clauseless_joins
 *	  Given a relation 'old_rel' and a list of other relations
 *	  'other_rels', create a join relation between 'old_rel' and each
 *	  member of 'other_rels' that isn't already included in 'old_rel'.
 *	  The join rels are returned in root->join_rel_level[join_cur_level].
 *
 * 'old_rel' is the relation entry for the relation to be joined
 * 'other_rels': the first cell of a linked list containing the
 * other rels to be considered for joining
 *
 * Currently, this is only used with initial rels in other_rels, but it would
 * work for joining to joinrels too.
 */
static void
make_rels_by_clauseless_joins(PlannerInfo *root,
							  RelOptInfo *old_rel,
							  ListCell *other_rels)
{
	ListCell   *l;

	for_each_cell(l, other_rels)
	{
		RelOptInfo *other_rel = (RelOptInfo *) lfirst(l);

		if (!bms_overlap(other_rel->relids, old_rel->relids))
		{
			(void) make_join_rel(root, old_rel, other_rel);
		}
	}
}


/*
 * join_is_legal
 *	   Determine whether a proposed join is legal given the query's
 *	   join order constraints; and if it is, determine the join type.
 *
 * Caller must supply not only the two rels, but the union of their relids.
 * (We could simplify the API by computing joinrelids locally, but this
 * would be redundant work in the normal path through make_join_rel.)
 *
 * On success, *sjinfo_p is set to NULL if this is to be a plain inner join,
 * else it's set to point to the associated SpecialJoinInfo node.  Also,
 * *reversed_p is set TRUE if the given relations need to be swapped to
 * match the SpecialJoinInfo node.
 */
static bool
join_is_legal(PlannerInfo *root, RelOptInfo *rel1, RelOptInfo *rel2,
			  Relids joinrelids,
			  SpecialJoinInfo **sjinfo_p, bool *reversed_p)
{
	SpecialJoinInfo *match_sjinfo;
	bool		reversed;
	bool		unique_ified;
	bool		is_valid_inner;
	ListCell   *l;

	/*
	 * Ensure output params are set on failure return.	This is just to
	 * suppress uninitialized-variable warnings from overly anal compilers.
	 */
	*sjinfo_p = NULL;
	*reversed_p = false;

	/*
	 * If we have any special joins, the proposed join might be illegal; and
	 * in any case we have to determine its join type.	Scan the join info
	 * list for conflicts.
	 */
	match_sjinfo = NULL;
	reversed = false;
	unique_ified = false;
	is_valid_inner = true;

	foreach(l, root->join_info_list)
	{
		SpecialJoinInfo *sjinfo = (SpecialJoinInfo *) lfirst(l);

		/*
		 * This special join is not relevant unless its RHS overlaps the
		 * proposed join.  (Check this first as a fast path for dismissing
		 * most irrelevant SJs quickly.)
		 */
		if (!bms_overlap(sjinfo->min_righthand, joinrelids))
			continue;

		/*
		 * Also, not relevant if proposed join is fully contained within RHS
		 * (ie, we're still building up the RHS).
		 */
		if (bms_is_subset(joinrelids, sjinfo->min_righthand))
			continue;

		/*
		 * Also, not relevant if SJ is already done within either input.
		 */
		if (bms_is_subset(sjinfo->min_lefthand, rel1->relids) &&
			bms_is_subset(sjinfo->min_righthand, rel1->relids))
			continue;
		if (bms_is_subset(sjinfo->min_lefthand, rel2->relids) &&
			bms_is_subset(sjinfo->min_righthand, rel2->relids))
			continue;

		/*
		 * If it's a semijoin and we already joined the RHS to any other rels
		 * within either input, then we must have unique-ified the RHS at that
		 * point (see below).  Therefore the semijoin is no longer relevant in
		 * this join path.
		 */
		if (sjinfo->jointype == JOIN_SEMI)
		{
			if (bms_is_subset(sjinfo->syn_righthand, rel1->relids) &&
				!bms_equal(sjinfo->syn_righthand, rel1->relids))
				continue;
			if (bms_is_subset(sjinfo->syn_righthand, rel2->relids) &&
				!bms_equal(sjinfo->syn_righthand, rel2->relids))
				continue;
		}

		/*
		 * If one input contains min_lefthand and the other contains
		 * min_righthand, then we can perform the SJ at this join.
		 *
		 * Barf if we get matches to more than one SJ (is that possible?)
		 */
		if (bms_is_subset(sjinfo->min_lefthand, rel1->relids) &&
			bms_is_subset(sjinfo->min_righthand, rel2->relids))
		{
			if (match_sjinfo)
				return false;	/* invalid join path */
			match_sjinfo = sjinfo;
			reversed = false;
		}
		else if (bms_is_subset(sjinfo->min_lefthand, rel2->relids) &&
				 bms_is_subset(sjinfo->min_righthand, rel1->relids))
		{
			if (match_sjinfo)
				return false;	/* invalid join path */
			match_sjinfo = sjinfo;
			reversed = true;
		}
		else if (sjinfo->jointype == JOIN_SEMI &&
				 bms_equal(sjinfo->syn_righthand, rel2->relids) &&
				 create_unique_path(root, rel2, rel2->cheapest_total_path,
									sjinfo) != NULL)
		{
			/*----------
			 * For a semijoin, we can join the RHS to anything else by
			 * unique-ifying the RHS (if the RHS can be unique-ified).
			 * We will only get here if we have the full RHS but less
			 * than min_lefthand on the LHS.
			 *
			 * The reason to consider such a join path is exemplified by
			 *	SELECT ... FROM a,b WHERE (a.x,b.y) IN (SELECT c1,c2 FROM c)
			 * If we insist on doing this as a semijoin we will first have
			 * to form the cartesian product of A*B.  But if we unique-ify
			 * C then the semijoin becomes a plain innerjoin and we can join
			 * in any order, eg C to A and then to B.  When C is much smaller
			 * than A and B this can be a huge win.  So we allow C to be
			 * joined to just A or just B here, and then make_join_rel has
			 * to handle the case properly.
			 *
			 * Note that actually we'll allow unique-ified C to be joined to
			 * some other relation D here, too.  That is legal, if usually not
			 * very sane, and this routine is only concerned with legality not
			 * with whether the join is good strategy.
			 *----------
			 */
			if (match_sjinfo)
				return false;	/* invalid join path */
			match_sjinfo = sjinfo;
			reversed = false;
			unique_ified = true;
		}
		else if (sjinfo->jointype == JOIN_SEMI &&
				 bms_equal(sjinfo->syn_righthand, rel1->relids) &&
				 create_unique_path(root, rel1, rel1->cheapest_total_path,
									sjinfo) != NULL)
		{
			/* Reversed semijoin case */
			if (match_sjinfo)
				return false;	/* invalid join path */
			match_sjinfo = sjinfo;
			reversed = true;
			unique_ified = true;
		}
		else
		{
			/*----------
			 * Otherwise, the proposed join overlaps the RHS but isn't
			 * a valid implementation of this SJ.  It might still be
			 * a legal join, however.  If both inputs overlap the RHS,
			 * assume that it's OK.  Since the inputs presumably got past
			 * this function's checks previously, they can't overlap the
			 * LHS and their violations of the RHS boundary must represent
			 * SJs that have been determined to commute with this one.
			 * We have to allow this to work correctly in cases like
			 *		(a LEFT JOIN (b JOIN (c LEFT JOIN d)))
			 * when the c/d join has been determined to commute with the join
			 * to a, and hence d is not part of min_righthand for the upper
			 * join.  It should be legal to join b to c/d but this will appear
			 * as a violation of the upper join's RHS.
			 * Furthermore, if one input overlaps the RHS and the other does
			 * not, we should still allow the join if it is a valid
			 * implementation of some other SJ.  We have to allow this to
			 * support the associative identity
			 *		(a LJ b on Pab) LJ c ON Pbc = a LJ (b LJ c ON Pbc) on Pab
			 * since joining B directly to C violates the lower SJ's RHS.
			 * We assume that make_outerjoininfo() set things up correctly
			 * so that we'll only match to some SJ if the join is valid.
			 * Set flag here to check at bottom of loop.
			 *----------
			 */
			if (sjinfo->jointype != JOIN_SEMI &&
				bms_overlap(rel1->relids, sjinfo->min_righthand) &&
				bms_overlap(rel2->relids, sjinfo->min_righthand))
			{
				/* seems OK */
				Assert(!bms_overlap(joinrelids, sjinfo->min_lefthand));
			}
			else
				is_valid_inner = false;
		}
	}

	/*
	 * Fail if violated some SJ's RHS and didn't match to another SJ. However,
	 * "matching" to a semijoin we are implementing by unique-ification
	 * doesn't count (think: it's really an inner join).
	 */
	if (!is_valid_inner &&
		(match_sjinfo == NULL || unique_ified))
		return false;			/* invalid join path */

	/* Otherwise, it's a valid join */
	*sjinfo_p = match_sjinfo;
	*reversed_p = reversed;
	return true;
}


/*
 * make_join_rel
 *	   Find or create a join RelOptInfo that represents the join of
 *	   the two given rels, and add to it path information for paths
 *	   created with the two rels as outer and inner rel.
 *	   (The join rel may already contain paths generated from other
 *	   pairs of rels that add up to the same set of base rels.)
 *
 * NB: will return NULL if attempted join is not valid.  This can happen
 * when working with outer joins, or with IN or EXISTS clauses that have been
 * turned into joins.
 */
RelOptInfo *
make_join_rel(PlannerInfo *root, RelOptInfo *rel1, RelOptInfo *rel2)
{
	Relids		joinrelids;
	SpecialJoinInfo *sjinfo;
	bool		reversed;
	SpecialJoinInfo sjinfo_data;
	RelOptInfo *joinrel;
	List	   *restrictlist;

	/* We should never try to join two overlapping sets of rels. */
	Assert(!bms_overlap(rel1->relids, rel2->relids));

	/* Construct Relids set that identifies the joinrel. */
	joinrelids = bms_union(rel1->relids, rel2->relids);

	/* Check validity and determine join type. */
	if (!join_is_legal(root, rel1, rel2, joinrelids,
					   &sjinfo, &reversed))
	{
		/* invalid join path */
		bms_free(joinrelids);
		return NULL;
	}

	/* Swap rels if needed to match the join info. */
	if (reversed)
	{
		RelOptInfo *trel = rel1;

		rel1 = rel2;
		rel2 = trel;
	}

	/*
	 * If it's a plain inner join, then we won't have found anything in
	 * join_info_list.	Make up a SpecialJoinInfo so that selectivity
	 * estimation functions will know what's being joined.
	 */
	if (sjinfo == NULL)
	{
		sjinfo = &sjinfo_data;
		sjinfo->type = T_SpecialJoinInfo;
		sjinfo->min_lefthand = rel1->relids;
		sjinfo->min_righthand = rel2->relids;
		sjinfo->syn_lefthand = rel1->relids;
		sjinfo->syn_righthand = rel2->relids;
		sjinfo->jointype = JOIN_INNER;
		/* we don't bother trying to make the remaining fields valid */
		sjinfo->lhs_strict = false;
		sjinfo->delay_upper_joins = false;
		sjinfo->join_quals = NIL;
	}

	/*
	 * Find or build the join RelOptInfo, and compute the restrictlist that
	 * goes with this particular joining.
	 */
	joinrel = build_join_rel(root, joinrelids, rel1, rel2, sjinfo,
							 &restrictlist);

	/*
	 * If we've already proven this join is empty, we needn't consider any
	 * more paths for it.
	 */
	if (is_dummy_rel(joinrel))
	{
		bms_free(joinrelids);
		return joinrel;
	}

	/*
	 * Consider paths using each rel as both outer and inner.  Depending on
	 * the join type, a provably empty outer or inner rel might mean the join
	 * is provably empty too; in which case throw away any previously computed
	 * paths and mark the join as dummy.  (We do it this way since it's
	 * conceivable that dummy-ness of a multi-element join might only be
	 * noticeable for certain construction paths.)
	 *
	 * Also, a provably constant-false join restriction typically means that
	 * we can skip evaluating one or both sides of the join.  We do this by
	 * marking the appropriate rel as dummy.  For outer joins, a
	 * constant-false restriction that is pushed down still means the whole
	 * join is dummy, while a non-pushed-down one means that no inner rows
	 * will join so we can treat the inner rel as dummy.
	 *
	 * We need only consider the jointypes that appear in join_info_list, plus
	 * JOIN_INNER.
	 */
	switch (sjinfo->jointype)
	{
		case JOIN_INNER:
			if (is_dummy_rel(rel1) || is_dummy_rel(rel2) ||
				restriction_is_constant_false(restrictlist, false))
			{
				mark_dummy_rel(joinrel);
				break;
			}
			add_paths_to_joinrel(root, joinrel, rel1, rel2,
								 JOIN_INNER, sjinfo,
								 restrictlist);
			add_paths_to_joinrel(root, joinrel, rel2, rel1,
								 JOIN_INNER, sjinfo,
								 restrictlist);
			break;
		case JOIN_LEFT:
			if (is_dummy_rel(rel1) ||
				restriction_is_constant_false(restrictlist, true))
			{
				mark_dummy_rel(joinrel);
				break;
			}
			if (restriction_is_constant_false(restrictlist, false) &&
				bms_is_subset(rel2->relids, sjinfo->syn_righthand))
				mark_dummy_rel(rel2);
			add_paths_to_joinrel(root, joinrel, rel1, rel2,
								 JOIN_LEFT, sjinfo,
								 restrictlist);
			add_paths_to_joinrel(root, joinrel, rel2, rel1,
								 JOIN_RIGHT, sjinfo,
								 restrictlist);
			break;
		case JOIN_FULL:
			if ((is_dummy_rel(rel1) && is_dummy_rel(rel2)) ||
				restriction_is_constant_false(restrictlist, true))
			{
				mark_dummy_rel(joinrel);
				break;
			}
			add_paths_to_joinrel(root, joinrel, rel1, rel2,
								 JOIN_FULL, sjinfo,
								 restrictlist);
			add_paths_to_joinrel(root, joinrel, rel2, rel1,
								 JOIN_FULL, sjinfo,
								 restrictlist);

			/*
			 * If there are join quals that aren't mergeable or hashable, we
			 * may not be able to build any valid plan.  Complain here so that
			 * we can give a somewhat-useful error message.  (Since we have no
			 * flexibility of planning for a full join, there's no chance of
			 * succeeding later with another pair of input rels.)
			 */
			if (joinrel->pathlist == NIL)
				ereport(ERROR,
						(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
						 errmsg("FULL JOIN is only supported with merge-joinable or hash-joinable join conditions")));
			break;
		case JOIN_SEMI:

			/*
			 * We might have a normal semijoin, or a case where we don't have
			 * enough rels to do the semijoin but can unique-ify the RHS and
			 * then do an innerjoin (see comments in join_is_legal).  In the
			 * latter case we can't apply JOIN_SEMI joining.
			 */
			if (bms_is_subset(sjinfo->min_lefthand, rel1->relids) &&
				bms_is_subset(sjinfo->min_righthand, rel2->relids))
			{
				if (is_dummy_rel(rel1) || is_dummy_rel(rel2) ||
					restriction_is_constant_false(restrictlist, false))
				{
					mark_dummy_rel(joinrel);
					break;
				}
				add_paths_to_joinrel(root, joinrel, rel1, rel2,
									 JOIN_SEMI, sjinfo,
									 restrictlist);
			}

			/*
			 * If we know how to unique-ify the RHS and one input rel is
			 * exactly the RHS (not a superset) we can consider unique-ifying
			 * it and then doing a regular join.  (The create_unique_path
			 * check here is probably redundant with what join_is_legal did,
			 * but if so the check is cheap because it's cached.  So test
			 * anyway to be sure.)
			 */
			if (bms_equal(sjinfo->syn_righthand, rel2->relids) &&
				create_unique_path(root, rel2, rel2->cheapest_total_path,
								   sjinfo) != NULL)
			{
				if (is_dummy_rel(rel1) || is_dummy_rel(rel2) ||
					restriction_is_constant_false(restrictlist, false))
				{
					mark_dummy_rel(joinrel);
					break;
				}
				add_paths_to_joinrel(root, joinrel, rel1, rel2,
									 JOIN_UNIQUE_INNER, sjinfo,
									 restrictlist);
				add_paths_to_joinrel(root, joinrel, rel2, rel1,
									 JOIN_UNIQUE_OUTER, sjinfo,
									 restrictlist);
			}
			break;
		case JOIN_ANTI:
			if (is_dummy_rel(rel1) ||
				restriction_is_constant_false(restrictlist, true))
			{
				mark_dummy_rel(joinrel);
				break;
			}
			if (restriction_is_constant_false(restrictlist, false) &&
				bms_is_subset(rel2->relids, sjinfo->syn_righthand))
				mark_dummy_rel(rel2);
			add_paths_to_joinrel(root, joinrel, rel1, rel2,
								 JOIN_ANTI, sjinfo,
								 restrictlist);
			break;
		default:
			/* other values not expected here */
			elog(ERROR, "unrecognized join type: %d", (int) sjinfo->jointype);
			break;
	}

	bms_free(joinrelids);

	return joinrel;
}


/*
 * have_join_order_restriction
 *		Detect whether the two relations should be joined to satisfy
 *		a join-order restriction arising from special joins.
 *
 * In practice this is always used with have_relevant_joinclause(), and so
 * could be merged with that function, but it seems clearer to separate the
 * two concerns.  We need this test because there are degenerate cases where
 * a clauseless join must be performed to satisfy join-order restrictions.
 *
 * Note: this is only a problem if one side of a degenerate outer join
 * contains multiple rels, or a clauseless join is required within an
 * IN/EXISTS RHS; else we will find a join path via the "last ditch" case in
 * join_search_one_level().  We could dispense with this test if we were
 * willing to try bushy plans in the "last ditch" case, but that seems much
 * less efficient.
 */
bool
have_join_order_restriction(PlannerInfo *root,
							RelOptInfo *rel1, RelOptInfo *rel2)
{
	bool		result = false;
	ListCell   *l;

	/*
	 * It's possible that the rels correspond to the left and right sides of a
	 * degenerate outer join, that is, one with no joinclause mentioning the
	 * non-nullable side; in which case we should force the join to occur.
	 *
	 * Also, the two rels could represent a clauseless join that has to be
	 * completed to build up the LHS or RHS of an outer join.
	 */
	foreach(l, root->join_info_list)
	{
		SpecialJoinInfo *sjinfo = (SpecialJoinInfo *) lfirst(l);

		/* ignore full joins --- other mechanisms handle them */
		if (sjinfo->jointype == JOIN_FULL)
			continue;

		/* Can we perform the SJ with these rels? */
		if (bms_is_subset(sjinfo->min_lefthand, rel1->relids) &&
			bms_is_subset(sjinfo->min_righthand, rel2->relids))
		{
			result = true;
			break;
		}
		if (bms_is_subset(sjinfo->min_lefthand, rel2->relids) &&
			bms_is_subset(sjinfo->min_righthand, rel1->relids))
		{
			result = true;
			break;
		}

		/*
		 * Might we need to join these rels to complete the RHS?  We have to
		 * use "overlap" tests since either rel might include a lower SJ that
		 * has been proven to commute with this one.
		 */
		if (bms_overlap(sjinfo->min_righthand, rel1->relids) &&
			bms_overlap(sjinfo->min_righthand, rel2->relids))
		{
			result = true;
			break;
		}

		/* Likewise for the LHS. */
		if (bms_overlap(sjinfo->min_lefthand, rel1->relids) &&
			bms_overlap(sjinfo->min_lefthand, rel2->relids))
		{
			result = true;
			break;
		}
	}

	/*
	 * We do not force the join to occur if either input rel can legally be
	 * joined to anything else using joinclauses.  This essentially means that
	 * clauseless bushy joins are put off as long as possible. The reason is
	 * that when there is a join order restriction high up in the join tree
	 * (that is, with many rels inside the LHS or RHS), we would otherwise
	 * expend lots of effort considering very stupid join combinations within
	 * its LHS or RHS.
	 */
	if (result)
	{
		if (has_legal_joinclause(root, rel1) ||
			has_legal_joinclause(root, rel2))
			result = false;
	}

	return result;
}


/*
 * has_join_restriction
 *		Detect whether the specified relation has join-order restrictions
 *		due to being inside an outer join or an IN (sub-SELECT).
 *
 * Essentially, this tests whether have_join_order_restriction() could
 * succeed with this rel and some other one.  It's OK if we sometimes
 * say "true" incorrectly.	(Therefore, we don't bother with the relatively
 * expensive has_legal_joinclause test.)
 */
static bool
has_join_restriction(PlannerInfo *root, RelOptInfo *rel)
{
	ListCell   *l;

	foreach(l, root->join_info_list)
	{
		SpecialJoinInfo *sjinfo = (SpecialJoinInfo *) lfirst(l);

		/* ignore full joins --- other mechanisms preserve their ordering */
		if (sjinfo->jointype == JOIN_FULL)
			continue;

		/* ignore if SJ is already contained in rel */
		if (bms_is_subset(sjinfo->min_lefthand, rel->relids) &&
			bms_is_subset(sjinfo->min_righthand, rel->relids))
			continue;

		/* restricted if it overlaps LHS or RHS, but doesn't contain SJ */
		if (bms_overlap(sjinfo->min_lefthand, rel->relids) ||
			bms_overlap(sjinfo->min_righthand, rel->relids))
			return true;
	}

	return false;
}


/*
 * has_legal_joinclause
 *		Detect whether the specified relation can legally be joined
 *		to any other rels using join clauses.
 *
 * We consider only joins to single other relations in the current
 * initial_rels list.  This is sufficient to get a "true" result in most real
 * queries, and an occasional erroneous "false" will only cost a bit more
 * planning time.  The reason for this limitation is that considering joins to
 * other joins would require proving that the other join rel can legally be
 * formed, which seems like too much trouble for something that's only a
 * heuristic to save planning time.  (Note: we must look at initial_rels
 * and not all of the query, since when we are planning a sub-joinlist we
 * may be forced to make clauseless joins within initial_rels even though
 * there are join clauses linking to other parts of the query.)
 */
static bool
has_legal_joinclause(PlannerInfo *root, RelOptInfo *rel)
{
	ListCell   *lc;

	foreach(lc, root->initial_rels)
	{
		RelOptInfo *rel2 = (RelOptInfo *) lfirst(lc);

		/* ignore rels that are already in "rel" */
		if (bms_overlap(rel->relids, rel2->relids))
			continue;

		if (have_relevant_joinclause(root, rel, rel2))
		{
			Relids		joinrelids;
			SpecialJoinInfo *sjinfo;
			bool		reversed;

			/* join_is_legal needs relids of the union */
			joinrelids = bms_union(rel->relids, rel2->relids);

			if (join_is_legal(root, rel, rel2, joinrelids,
							  &sjinfo, &reversed))
			{
				/* Yes, this will work */
				bms_free(joinrelids);
				return true;
			}

			bms_free(joinrelids);
		}
	}

	return false;
}


/*
 * is_dummy_rel --- has relation been proven empty?
 *
 * If so, it will have a single path that is dummy.
 */
static bool
is_dummy_rel(RelOptInfo *rel)
{
	return (rel->cheapest_total_path != NULL &&
			IS_DUMMY_PATH(rel->cheapest_total_path));
}

/*
 * Mark a relation as proven empty.
 *
 * During GEQO planning, this can get invoked more than once on the same
 * baserel struct, so it's worth checking to see if the rel is already marked
 * dummy.
 *
 * Also, when called during GEQO join planning, we are in a short-lived
 * memory context.  We must make sure that the dummy path attached to a
 * baserel survives the GEQO cycle, else the baserel is trashed for future
 * GEQO cycles.  On the other hand, when we are marking a joinrel during GEQO,
 * we don't want the dummy path to clutter the main planning context.  Upshot
 * is that the best solution is to explicitly make the dummy path in the same
 * context the given RelOptInfo is in.
 */
static void
mark_dummy_rel(RelOptInfo *rel)
{
	MemoryContext oldcontext;

	/* Already marked? */
	if (is_dummy_rel(rel))
		return;

	/* No, so choose correct context to make the dummy path in */
	oldcontext = MemoryContextSwitchTo(GetMemoryChunkContext(rel));

	/* Set dummy size estimate */
	rel->rows = 0;

	/* Evict any previously chosen paths */
	rel->pathlist = NIL;

	/* Set up the dummy path */
	add_path(rel, (Path *) create_append_path(rel, NIL));

	/* Set or update cheapest_total_path */
	set_cheapest(rel);

	MemoryContextSwitchTo(oldcontext);
}


/*
 * restriction_is_constant_false --- is a restrictlist just FALSE?
 *
 * In cases where a qual is provably constant FALSE, eval_const_expressions
 * will generally have thrown away anything that's ANDed with it.  In outer
 * join situations this will leave us computing cartesian products only to
 * decide there's no match for an outer row, which is pretty stupid.  So,
 * we need to detect the case.
 *
 * If only_pushed_down is TRUE, then consider only pushed-down quals.
 */
static bool
restriction_is_constant_false(List *restrictlist, bool only_pushed_down)
{
	ListCell   *lc;

	/*
	 * Despite the above comment, the restriction list we see here might
	 * possibly have other members besides the FALSE constant, since other
	 * quals could get "pushed down" to the outer join level.  So we check
	 * each member of the list.
	 */
	foreach(lc, restrictlist)
	{
		RestrictInfo *rinfo = (RestrictInfo *) lfirst(lc);

		Assert(IsA(rinfo, RestrictInfo));
		if (only_pushed_down && !rinfo->is_pushed_down)
			continue;

		if (rinfo->clause && IsA(rinfo->clause, Const))
		{
			Const	   *con = (Const *) rinfo->clause;

			/* constant NULL is as good as constant FALSE for our purposes */
			if (con->constisnull)
				return true;
			if (!DatumGetBool(con->constvalue))
				return true;
		}
	}
	return false;
}
