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Sourcecode: postgresql-8.4 version File versions

joinrels.c

/*-------------------------------------------------------------------------
 *
 * joinrels.c
 *      Routines to determine which relations should be joined
 *
 * Portions Copyright (c) 1996-2009, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *      $PostgreSQL: pgsql/src/backend/optimizer/path/joinrels.c,v 1.100 2009/06/11 14:48:59 momjian Exp $
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"

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


static List *make_rels_by_clause_joins(PlannerInfo *root,
                                      RelOptInfo *old_rel,
                                      ListCell *other_rels);
static List *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);


/*
 * 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.
 * joinrels[j], 1 <= j < level, is a list of rels containing j items.
 */
List *
join_search_one_level(PlannerInfo *root, int level, List **joinrels)
{
      List     *result_rels = NIL;
      List     *new_rels;
      ListCell   *r;
      int               k;

      /*
       * 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.
                   */
                  new_rels = 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.
                   */
                  new_rels = make_rels_by_clauseless_joins(root,
                                                                               old_rel,
                                                                               other_rels);
            }

            /*
             * 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.  To avoid making extra work for
             * subsequent passes, do not enter the same RelOptInfo into our output
             * list multiple times.
             */
            result_rels = list_concat_unique_ptr(result_rels, new_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))
                              {
                                    RelOptInfo *jrel;

                                    jrel = make_join_rel(root, old_rel, new_rel);
                                    /* Avoid making duplicate entries ... */
                                    if (jrel)
                                          result_rels = list_append_unique_ptr(result_rels,
                                                                                                 jrel);
                              }
                        }
                  }
            }
      }

      /*
       * 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 (result_rels == 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 */

                  new_rels = make_rels_by_clauseless_joins(root,
                                                                               old_rel,
                                                                               other_rels);

                  result_rels = list_concat_unique_ptr(result_rels, new_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 (result_rels == NIL && root->join_info_list == NIL)
                  elog(ERROR, "failed to build any %d-way joins", level);
      }

      return result_rels;
}

/*
 * 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 rel nodes are returned in a list.
 *
 * '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 List *
make_rels_by_clause_joins(PlannerInfo *root,
                                      RelOptInfo *old_rel,
                                      ListCell *other_rels)
{
      List     *result = NIL;
      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)))
            {
                  RelOptInfo *jrel;

                  jrel = make_join_rel(root, old_rel, other_rel);
                  if (jrel)
                        result = lcons(jrel, result);
            }
      }

      return result;
}

/*
 * 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 rel nodes are returned in a list.
 *
 * '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 List *
make_rels_by_clauseless_joins(PlannerInfo *root,
                                            RelOptInfo *old_rel,
                                            ListCell *other_rels)
{
      List     *result = NIL;
      ListCell   *i;

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

            if (!bms_overlap(other_rel->relids, old_rel->relids))
            {
                  RelOptInfo *jrel;

                  jrel = make_join_rel(root, old_rel, other_rel);

                  /*
                   * As long as given other_rels are distinct, don't need to test to
                   * see if jrel is already part of output list.
                   */
                  if (jrel)
                        result = lcons(jrel, result);
            }
      }

      return result;
}


/*
 * 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        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;
      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 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;
            }
            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;
            }
            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.
                   *
                   * For a semijoin, assume it's okay if either side fully contains
                   * the RHS (per the unique-ification case above).
                   *----------
                   */
                  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 if (sjinfo->jointype == JOIN_SEMI &&
                               (bms_is_subset(sjinfo->syn_righthand, rel1->relids) ||
                                bms_is_subset(sjinfo->syn_righthand, rel2->relids)))
                  {
                        /* seems OK */
                  }
                  else
                        is_valid_inner = false;
            }
      }

      /* Fail if violated some SJ's RHS and didn't match to another SJ */
      if (match_sjinfo == NULL && !is_valid_inner)
            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.
       *
       * 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))
                  {
                        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))
                  {
                        mark_dummy_rel(joinrel);
                        break;
                  }
                  if (restriction_is_constant_false(restrictlist) &&
                        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))
                  {
                        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);
                  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))
                        {
                              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)
                  {
                        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))
                  {
                        mark_dummy_rel(joinrel);
                        break;
                  }
                  if (restriction_is_constant_false(restrictlist) &&
                        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 rel as proven empty.
 */
static void
mark_dummy_rel(RelOptInfo *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);
}


/*
 * 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.
 */
static bool
restriction_is_constant_false(List *restrictlist)
{
      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 (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;
}

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