joinrels.c

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/*-------------------------------------------------------------------------
 *
 * joinrels.c
 *    Routines to determine which relations should be joined
 *
 * Portions Copyright (c) 1996-2021, 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 "miscadmin.h"
#include "optimizer/appendinfo.h"
#include "optimizer/joininfo.h"
#include "optimizer/pathnode.h"
#include "optimizer/paths.h"
#include "partitioning/partbounds.h"
#include "utils/memutils.h"


static void make_rels_by_clause_joins(PlannerInfo *root,
                                      RelOptInfo *old_rel,
                                      List *other_rels_list,
                                      ListCell *other_rels);
static void make_rels_by_clauseless_joins(PlannerInfo *root,
                                          RelOptInfo *old_rel,
                                          List *other_rels);
static bool has_join_restriction(PlannerInfo *root, RelOptInfo *rel);
static bool has_legal_joinclause(PlannerInfo *root, RelOptInfo *rel);
static bool restriction_is_constant_false(List *restrictlist,
                                          RelOptInfo *joinrel,
                                          bool only_pushed_down);
static void populate_joinrel_with_paths(PlannerInfo *root, RelOptInfo *rel1,
                                        RelOptInfo *rel2, RelOptInfo *joinrel,
                                        SpecialJoinInfo *sjinfo, List *restrictlist);
static void try_partitionwise_join(PlannerInfo *root, RelOptInfo *rel1,
                                   RelOptInfo *rel2, RelOptInfo *joinrel,
                                   SpecialJoinInfo *parent_sjinfo,
                                   List *parent_restrictlist);
static SpecialJoinInfo *build_child_join_sjinfo(PlannerInfo *root,
                                                SpecialJoinInfo *parent_sjinfo,
                                                Relids left_relids, Relids right_relids);
static void compute_partition_bounds(PlannerInfo *root, RelOptInfo *rel1,
                                     RelOptInfo *rel2, RelOptInfo *joinrel,
                                     SpecialJoinInfo *parent_sjinfo,
                                     List **parts1, List **parts2);
static void get_matching_part_pairs(PlannerInfo *root, RelOptInfo *joinrel,
                                    RelOptInfo *rel1, RelOptInfo *rel2,
                                    List **parts1, List **parts2);


/*
 * 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.
     */
    foreach(r, joinrels[level - 1])
    {
        RelOptInfo *old_rel = (RelOptInfo *) lfirst(r);

        if (old_rel->joininfo != NIL || old_rel->has_eclass_joins ||
            has_join_restriction(root, old_rel))
        {
            /*
             * There are join clauses or join order restrictions relevant to
             * this rel, so consider joins between this rel and (only) those
             * initial rels it is linked to by a clause or restriction.
             *
             * At level 2 this condition is symmetric, so there is no need to
             * look at initial rels before this one in the list; we already
             * considered such joins when we were at the earlier rel.  (The
             * mirror-image joins are handled automatically by make_join_rel.)
             * In later passes (level > 2), we join rels of the previous level
             * to each initial rel they don't already include but have a join
             * clause or restriction with.
             */
            List       *other_rels_list;
            ListCell   *other_rels;

            if (level == 2)     /* consider remaining initial rels */
            {
                other_rels_list = joinrels[level - 1];
                other_rels = lnext(other_rels_list, r);
            }
            else                /* consider all initial rels */
            {
                other_rels_list = joinrels[1];
                other_rels = list_head(other_rels_list);
            }

            make_rels_by_clause_joins(root,
                                      old_rel,
                                      other_rels_list,
                                      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.
             *
             * We consider a cartesian product with each not-already-included
             * initial rel, whether it has other join clauses or not.  At
             * level 2, if there are two or more clauseless initial rels, we
             * will redundantly consider joining them in both directions; but
             * such cases aren't common enough to justify adding complexity to
             * avoid the duplicated effort.
             */
            make_rels_by_clauseless_joins(root,
                                          old_rel,
                                          joinrels[1]);
        }
    }

    /*
     * 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);
            List       *other_rels_list;
            ListCell   *other_rels;
            ListCell   *r2;

            /*
             * We can ignore relations without join clauses here, unless 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)
            {
                /* only consider remaining rels */
                other_rels_list = joinrels[k];
                other_rels = lnext(other_rels_list, r);
            }
            else
            {
                other_rels_list = joinrels[other_level];
                other_rels = list_head(other_rels_list);
            }

            for_each_cell(r2, other_rels_list, 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 relevant
                     * join clause or join order 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.  This can only happen when we are
     * considering a join sub-problem (a sub-joinlist) and all the rels in the
     * sub-problem have only join clauses with rels outside the sub-problem.
     * An example is
     *
     *      SELECT ... FROM a INNER JOIN b ON TRUE, c, d, ...
     *      WHERE a.w = c.x and b.y = d.z;
     *
     * If the "a INNER JOIN b" sub-problem does not get flattened into the
     * upper level, we must be willing to make a cartesian join of a and b;
     * but the code above will not have done so, because it thought that both
     * a and b have joinclauses.  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);

            make_rels_by_clauseless_joins(root,
                                          old_rel,
                                          joinrels[1]);
        }

        /*----------
         * 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 and no lateral references
         * then join_is_legal() should never fail, and so the following sanity
         * check is useful.
         *----------
         */
        if (joinrels[level] == NIL &&
            root->join_info_list == NIL &&
            !root->hasLateralRTEs)
            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_list': a list containing the other
 * rels to be considered for joining
 * 'other_rels': the first cell to be considered
 *
 * 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,
                          List *other_rels_list,
                          ListCell *other_rels)
{
    ListCell   *l;

    for_each_cell(l, other_rels_list, 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': a 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,
                              List *other_rels)
{
    ListCell   *l;

    foreach(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        must_be_leftjoin;
    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 matches and conflicts.
     */
    match_sjinfo = NULL;
    reversed = false;
    unique_ified = false;
    must_be_leftjoin = false;

    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.
         *
         * Reject if we get matches to more than one SJ; that implies we're
         * considering something that's not really valid.
         */
        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.  But don't panic quite yet: the RHS
             * violation might have occurred previously, in one or both input
             * relations, in which case we must have previously decided that
             * it was OK to commute some other SJ with this one.  If we need
             * to perform this join to finish building up the RHS, rejecting
             * it could lead to not finding any plan at all.  (This can occur
             * because of the heuristics elsewhere in this file that postpone
             * clauseless joins: we might not consider doing a clauseless join
             * within the RHS until after we've performed other, validly
             * commutable SJs with one or both sides of the clauseless join.)
             * This consideration boils down to the rule that if both inputs
             * overlap the RHS, we can allow the join --- they are either
             * fully within the RHS, or represent previously-allowed joins to
             * rels outside it.
             */
            if (bms_overlap(rel1->relids, sjinfo->min_righthand) &&
                bms_overlap(rel2->relids, sjinfo->min_righthand))
                continue;       /* assume valid previous violation of RHS */

            /*
             * The proposed join could still be legal, but only if we're
             * allowed to associate it into the RHS of this SJ.  That means
             * this SJ must be a LEFT join (not SEMI or ANTI, and certainly
             * not FULL) and the proposed join must not overlap the LHS.
             */
            if (sjinfo->jointype != JOIN_LEFT ||
                bms_overlap(joinrelids, sjinfo->min_lefthand))
                return false;   /* invalid join path */

            /*
             * To be valid, the proposed join must be a LEFT join; otherwise
             * it can't associate into this SJ's RHS.  But we may not yet have
             * found the SpecialJoinInfo matching the proposed join, so we
             * can't test that yet.  Remember the requirement for later.
             */
            must_be_leftjoin = true;
        }
    }

    /*
     * Fail if violated any SJ's RHS and didn't match to a LEFT SJ: the
     * proposed join can't associate into an SJ's RHS.
     *
     * Also, fail if the proposed join's predicate isn't strict; we're
     * essentially checking to see if we can apply outer-join identity 3, and
     * that's a requirement.  (This check may be redundant with checks in
     * make_outerjoininfo, but I'm not quite sure, and it's cheap to test.)
     */
    if (must_be_leftjoin &&
        (match_sjinfo == NULL ||
         match_sjinfo->jointype != JOIN_LEFT ||
         !match_sjinfo->lhs_strict))
        return false;           /* invalid join path */

    /*
     * We also have to check for constraints imposed by LATERAL references.
     */
    if (root->hasLateralRTEs)
    {
        bool        lateral_fwd;
        bool        lateral_rev;
        Relids      join_lateral_rels;

        /*
         * The proposed rels could each contain lateral references to the
         * other, in which case the join is impossible.  If there are lateral
         * references in just one direction, then the join has to be done with
         * a nestloop with the lateral referencer on the inside.  If the join
         * matches an SJ that cannot be implemented by such a nestloop, the
         * join is impossible.
         *
         * Also, if the lateral reference is only indirect, we should reject
         * the join; whatever rel(s) the reference chain goes through must be
         * joined to first.
         *
         * Another case that might keep us from building a valid plan is the
         * implementation restriction described by have_dangerous_phv().
         */
        lateral_fwd = bms_overlap(rel1->relids, rel2->lateral_relids);
        lateral_rev = bms_overlap(rel2->relids, rel1->lateral_relids);
        if (lateral_fwd && lateral_rev)
            return false;       /* have lateral refs in both directions */
        if (lateral_fwd)
        {
            /* has to be implemented as nestloop with rel1 on left */
            if (match_sjinfo &&
                (reversed ||
                 unique_ified ||
                 match_sjinfo->jointype == JOIN_FULL))
                return false;   /* not implementable as nestloop */
            /* check there is a direct reference from rel2 to rel1 */
            if (!bms_overlap(rel1->relids, rel2->direct_lateral_relids))
                return false;   /* only indirect refs, so reject */
            /* check we won't have a dangerous PHV */
            if (have_dangerous_phv(root, rel1->relids, rel2->lateral_relids))
                return false;   /* might be unable to handle required PHV */
        }
        else if (lateral_rev)
        {
            /* has to be implemented as nestloop with rel2 on left */
            if (match_sjinfo &&
                (!reversed ||
                 unique_ified ||
                 match_sjinfo->jointype == JOIN_FULL))
                return false;   /* not implementable as nestloop */
            /* check there is a direct reference from rel1 to rel2 */
            if (!bms_overlap(rel2->relids, rel1->direct_lateral_relids))
                return false;   /* only indirect refs, so reject */
            /* check we won't have a dangerous PHV */
            if (have_dangerous_phv(root, rel2->relids, rel1->lateral_relids))
                return false;   /* might be unable to handle required PHV */
        }

        /*
         * LATERAL references could also cause problems later on if we accept
         * this join: if the join's minimum parameterization includes any rels
         * that would have to be on the inside of an outer join with this join
         * rel, then it's never going to be possible to build the complete
         * query using this join.  We should reject this join not only because
         * it'll save work, but because if we don't, the clauseless-join
         * heuristics might think that legality of this join means that some
         * other join rel need not be formed, and that could lead to failure
         * to find any plan at all.  We have to consider not only rels that
         * are directly on the inner side of an OJ with the joinrel, but also
         * ones that are indirectly so, so search to find all such rels.
         */
        join_lateral_rels = min_join_parameterization(root, joinrelids,
                                                      rel1, rel2);
        if (join_lateral_rels)
        {
            Relids      join_plus_rhs = bms_copy(joinrelids);
            bool        more;

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

                    /* ignore full joins --- their ordering is predetermined */
                    if (sjinfo->jointype == JOIN_FULL)
                        continue;

                    if (bms_overlap(sjinfo->min_lefthand, join_plus_rhs) &&
                        !bms_is_subset(sjinfo->min_righthand, join_plus_rhs))
                    {
                        join_plus_rhs = bms_add_members(join_plus_rhs,
                                                        sjinfo->min_righthand);
                        more = true;
                    }
                }
            } while (more);
            if (bms_overlap(join_plus_rhs, join_lateral_rels))
                return false;   /* will not be able to join to some RHS rel */
        }
    }

    /* 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->semi_can_btree = false;
        sjinfo->semi_can_hash = false;
        sjinfo->semi_operators = NIL;
        sjinfo->semi_rhs_exprs = 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;
    }

    /* Add paths to the join relation. */
    populate_joinrel_with_paths(root, rel1, rel2, joinrel, sjinfo,
                                restrictlist);

    bms_free(joinrelids);

    return joinrel;
}

/*
 * populate_joinrel_with_paths
 *    Add paths to the given joinrel for given pair of joining relations. The
 *    SpecialJoinInfo provides details about the join and the restrictlist
 *    contains the join clauses and the other clauses applicable for given pair
 *    of the joining relations.
 */
static void
populate_joinrel_with_paths(PlannerInfo *root, RelOptInfo *rel1,
                            RelOptInfo *rel2, RelOptInfo *joinrel,
                            SpecialJoinInfo *sjinfo, List *restrictlist)
{
    /*
     * 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, joinrel, 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, joinrel, true))
            {
                mark_dummy_rel(joinrel);
                break;
            }
            if (restriction_is_constant_false(restrictlist, joinrel, 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, joinrel, 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, joinrel, 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, joinrel, 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, joinrel, true))
            {
                mark_dummy_rel(joinrel);
                break;
            }
            if (restriction_is_constant_false(restrictlist, joinrel, 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;
    }

    /* Apply partitionwise join technique, if possible. */
    try_partitionwise_join(root, rel1, rel2, joinrel, sjinfo, restrictlist);
}


/*
 * have_join_order_restriction
 *      Detect whether the two relations should be joined to satisfy
 *      a join-order restriction arising from special or lateral 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.
 * Also, if one rel has a lateral reference to the other, or both are needed
 * to compute some PHV, we should consider joining them even if the join would
 * be clauseless.
 *
 * 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;

    /*
     * If either side has a direct lateral reference to the other, attempt the
     * join regardless of outer-join considerations.
     */
    if (bms_overlap(rel1->relids, rel2->direct_lateral_relids) ||
        bms_overlap(rel2->relids, rel1->direct_lateral_relids))
        return true;

    /*
     * Likewise, if both rels are needed to compute some PlaceHolderVar,
     * attempt the join regardless of outer-join considerations.  (This is not
     * very desirable, because a PHV with a large eval_at set will cause a lot
     * of probably-useless joins to be considered, but failing to do this can
     * cause us to fail to construct a plan at all.)
     */
    foreach(l, root->placeholder_list)
    {
        PlaceHolderInfo *phinfo = (PlaceHolderInfo *) lfirst(l);

        if (bms_is_subset(rel1->relids, phinfo->ph_eval_at) &&
            bms_is_subset(rel2->relids, phinfo->ph_eval_at))
            return true;
    }

    /*
     * 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),
 *      or participating in any LATERAL references or multi-rel PHVs.
 *
 * 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;

    if (rel->lateral_relids != NULL || rel->lateral_referencers != NULL)
        return true;

    foreach(l, root->placeholder_list)
    {
        PlaceHolderInfo *phinfo = (PlaceHolderInfo *) lfirst(l);

        if (bms_is_subset(rel->relids, phinfo->ph_eval_at) &&
            !bms_equal(rel->relids, phinfo->ph_eval_at))
            return true;
    }

    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;
}


/*
 * There's a pitfall for creating parameterized nestloops: suppose the inner
 * rel (call it A) has a parameter that is a PlaceHolderVar, and that PHV's
 * minimum eval_at set includes the outer rel (B) and some third rel (C).
 * We might think we could create a B/A nestloop join that's parameterized by
 * C.  But we would end up with a plan in which the PHV's expression has to be
 * evaluated as a nestloop parameter at the B/A join; and the executor is only
 * set up to handle simple Vars as NestLoopParams.  Rather than add complexity
 * and overhead to the executor for such corner cases, it seems better to
 * forbid the join.  (Note that we can still make use of A's parameterized
 * path with pre-joined B+C as the outer rel.  have_join_order_restriction()
 * ensures that we will consider making such a join even if there are not
 * other reasons to do so.)
 *
 * So we check whether any PHVs used in the query could pose such a hazard.
 * We don't have any simple way of checking whether a risky PHV would actually
 * be used in the inner plan, and the case is so unusual that it doesn't seem
 * worth working very hard on it.
 *
 * This needs to be checked in two places.  If the inner rel's minimum
 * parameterization would trigger the restriction, then join_is_legal() should
 * reject the join altogether, because there will be no workable paths for it.
 * But joinpath.c has to check again for every proposed nestloop path, because
 * the inner path might have more than the minimum parameterization, causing
 * some PHV to be dangerous for it that otherwise wouldn't be.
 */
bool
have_dangerous_phv(PlannerInfo *root,
                   Relids outer_relids, Relids inner_params)
{
    ListCell   *lc;

    foreach(lc, root->placeholder_list)
    {
        PlaceHolderInfo *phinfo = (PlaceHolderInfo *) lfirst(lc);

        if (!bms_is_subset(phinfo->ph_eval_at, inner_params))
            continue;           /* ignore, could not be a nestloop param */
        if (!bms_overlap(phinfo->ph_eval_at, outer_relids))
            continue;           /* ignore, not relevant to this join */
        if (bms_is_subset(phinfo->ph_eval_at, outer_relids))
            continue;           /* safe, it can be eval'd within outerrel */
        /* Otherwise, it's potentially unsafe, so reject the join */
        return true;
    }

    /* OK to perform the join */
    return false;
}


/*
 * is_dummy_rel --- has relation been proven empty?
 */
bool
is_dummy_rel(RelOptInfo *rel)
{
    Path       *path;

    /*
     * A rel that is known dummy will have just one path that is a childless
     * Append.  (Even if somehow it has more paths, a childless Append will
     * have cost zero and hence should be at the front of the pathlist.)
     */
    if (rel->pathlist == NIL)
        return false;
    path = (Path *) linitial(rel->pathlist);

    /*
     * Initially, a dummy path will just be a childless Append.  But in later
     * planning stages we might stick a ProjectSetPath and/or ProjectionPath
     * on top, since Append can't project.  Rather than make assumptions about
     * which combinations can occur, just descend through whatever we find.
     */
    for (;;)
    {
        if (IsA(path, ProjectionPath))
            path = ((ProjectionPath *) path)->subpath;
        else if (IsA(path, ProjectSetPath))
            path = ((ProjectSetPath *) path)->subpath;
        else
            break;
    }
    if (IS_DUMMY_APPEND(path))
        return true;
    return false;
}

/*
 * 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.
 */
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;
    rel->partial_pathlist = NIL;

    /* Set up the dummy path */
    add_path(rel, (Path *) create_append_path(NULL, rel, NIL, NIL,
                                              NIL, rel->lateral_relids,
                                              0, false, -1));

    /* Set or update cheapest_total_path and related fields */
    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 quals that are pushed-down
 * from the point of view of the joinrel.
 */
static bool
restriction_is_constant_false(List *restrictlist,
                              RelOptInfo *joinrel,
                              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 = lfirst_node(RestrictInfo, lc);

        if (only_pushed_down && !RINFO_IS_PUSHED_DOWN(rinfo, joinrel->relids))
            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;
}

/*
 * Assess whether join between given two partitioned relations can be broken
 * down into joins between matching partitions; a technique called
 * "partitionwise join"
 *
 * Partitionwise join is possible when a. Joining relations have same
 * partitioning scheme b. There exists an equi-join between the partition keys
 * of the two relations.
 *
 * Partitionwise join is planned as follows (details: optimizer/README.)
 *
 * 1. Create the RelOptInfos for joins between matching partitions i.e
 * child-joins and add paths to them.
 *
 * 2. Construct Append or MergeAppend paths across the set of child joins.
 * This second phase is implemented by generate_partitionwise_join_paths().
 *
 * The RelOptInfo, SpecialJoinInfo and restrictlist for each child join are
 * obtained by translating the respective parent join structures.
 */
static void
try_partitionwise_join(PlannerInfo *root, RelOptInfo *rel1, RelOptInfo *rel2,
                       RelOptInfo *joinrel, SpecialJoinInfo *parent_sjinfo,
                       List *parent_restrictlist)
{
    bool        rel1_is_simple = IS_SIMPLE_REL(rel1);
    bool        rel2_is_simple = IS_SIMPLE_REL(rel2);
    List       *parts1 = NIL;
    List       *parts2 = NIL;
    ListCell   *lcr1 = NULL;
    ListCell   *lcr2 = NULL;
    int         cnt_parts;

    /* Guard against stack overflow due to overly deep partition hierarchy. */
    check_stack_depth();

    /* Nothing to do, if the join relation is not partitioned. */
    if (joinrel->part_scheme == NULL || joinrel->nparts == 0)
        return;

    /* The join relation should have consider_partitionwise_join set. */
    Assert(joinrel->consider_partitionwise_join);

    /*
     * We can not perform partitionwise join if either of the joining
     * relations is not partitioned.
     */
    if (!IS_PARTITIONED_REL(rel1) || !IS_PARTITIONED_REL(rel2))
        return;

    Assert(REL_HAS_ALL_PART_PROPS(rel1) && REL_HAS_ALL_PART_PROPS(rel2));

    /* The joining relations should have consider_partitionwise_join set. */
    Assert(rel1->consider_partitionwise_join &&
           rel2->consider_partitionwise_join);

    /*
     * The partition scheme of the join relation should match that of the
     * joining relations.
     */
    Assert(joinrel->part_scheme == rel1->part_scheme &&
           joinrel->part_scheme == rel2->part_scheme);

    Assert(!(joinrel->partbounds_merged && (joinrel->nparts <= 0)));

    compute_partition_bounds(root, rel1, rel2, joinrel, parent_sjinfo,
                             &parts1, &parts2);

    if (joinrel->partbounds_merged)
    {
        lcr1 = list_head(parts1);
        lcr2 = list_head(parts2);
    }

    /*
     * Create child-join relations for this partitioned join, if those don't
     * exist. Add paths to child-joins for a pair of child relations
     * corresponding to the given pair of parent relations.
     */
    for (cnt_parts = 0; cnt_parts < joinrel->nparts; cnt_parts++)
    {
        RelOptInfo *child_rel1;
        RelOptInfo *child_rel2;
        bool        rel1_empty;
        bool        rel2_empty;
        SpecialJoinInfo *child_sjinfo;
        List       *child_restrictlist;
        RelOptInfo *child_joinrel;
        Relids      child_joinrelids;
        AppendRelInfo **appinfos;
        int         nappinfos;

        if (joinrel->partbounds_merged)
        {
            child_rel1 = lfirst_node(RelOptInfo, lcr1);
            child_rel2 = lfirst_node(RelOptInfo, lcr2);
            lcr1 = lnext(parts1, lcr1);
            lcr2 = lnext(parts2, lcr2);
        }
        else
        {
            child_rel1 = rel1->part_rels[cnt_parts];
            child_rel2 = rel2->part_rels[cnt_parts];
        }

        rel1_empty = (child_rel1 == NULL || IS_DUMMY_REL(child_rel1));
        rel2_empty = (child_rel2 == NULL || IS_DUMMY_REL(child_rel2));

        /*
         * Check for cases where we can prove that this segment of the join
         * returns no rows, due to one or both inputs being empty (including
         * inputs that have been pruned away entirely).  If so just ignore it.
         * These rules are equivalent to populate_joinrel_with_paths's rules
         * for dummy input relations.
         */
        switch (parent_sjinfo->jointype)
        {
            case JOIN_INNER:
            case JOIN_SEMI:
                if (rel1_empty || rel2_empty)
                    continue;   /* ignore this join segment */
                break;
            case JOIN_LEFT:
            case JOIN_ANTI:
                if (rel1_empty)
                    continue;   /* ignore this join segment */
                break;
            case JOIN_FULL:
                if (rel1_empty && rel2_empty)
                    continue;   /* ignore this join segment */
                break;
            default:
                /* other values not expected here */
                elog(ERROR, "unrecognized join type: %d",
                     (int) parent_sjinfo->jointype);
                break;
        }

        /*
         * If a child has been pruned entirely then we can't generate paths
         * for it, so we have to reject partitionwise joining unless we were
         * able to eliminate this partition above.
         */
        if (child_rel1 == NULL || child_rel2 == NULL)
        {
            /*
             * Mark the joinrel as unpartitioned so that later functions treat
             * it correctly.
             */
            joinrel->nparts = 0;
            return;
        }

        /*
         * If a leaf relation has consider_partitionwise_join=false, it means
         * that it's a dummy relation for which we skipped setting up tlist
         * expressions and adding EC members in set_append_rel_size(), so
         * again we have to fail here.
         */
        if (rel1_is_simple && !child_rel1->consider_partitionwise_join)
        {
            Assert(child_rel1->reloptkind == RELOPT_OTHER_MEMBER_REL);
            Assert(IS_DUMMY_REL(child_rel1));
            joinrel->nparts = 0;
            return;
        }
        if (rel2_is_simple && !child_rel2->consider_partitionwise_join)
        {
            Assert(child_rel2->reloptkind == RELOPT_OTHER_MEMBER_REL);
            Assert(IS_DUMMY_REL(child_rel2));
            joinrel->nparts = 0;
            return;
        }

        /* We should never try to join two overlapping sets of rels. */
        Assert(!bms_overlap(child_rel1->relids, child_rel2->relids));
        child_joinrelids = bms_union(child_rel1->relids, child_rel2->relids);
        appinfos = find_appinfos_by_relids(root, child_joinrelids, &nappinfos);

        /*
         * Construct SpecialJoinInfo from parent join relations's
         * SpecialJoinInfo.
         */
        child_sjinfo = build_child_join_sjinfo(root, parent_sjinfo,
                                               child_rel1->relids,
                                               child_rel2->relids);

        /*
         * Construct restrictions applicable to the child join from those
         * applicable to the parent join.
         */
        child_restrictlist =
            (List *) adjust_appendrel_attrs(root,
                                            (Node *) parent_restrictlist,
                                            nappinfos, appinfos);
        pfree(appinfos);

        child_joinrel = joinrel->part_rels[cnt_parts];
        if (!child_joinrel)
        {
            child_joinrel = build_child_join_rel(root, child_rel1, child_rel2,
                                                 joinrel, child_restrictlist,
                                                 child_sjinfo,
                                                 child_sjinfo->jointype);
            joinrel->part_rels[cnt_parts] = child_joinrel;
            joinrel->all_partrels = bms_add_members(joinrel->all_partrels,
                                                    child_joinrel->relids);
        }

        Assert(bms_equal(child_joinrel->relids, child_joinrelids));

        populate_joinrel_with_paths(root, child_rel1, child_rel2,
                                    child_joinrel, child_sjinfo,
                                    child_restrictlist);
    }
}

/*
 * Construct the SpecialJoinInfo for a child-join by translating
 * SpecialJoinInfo for the join between parents. left_relids and right_relids
 * are the relids of left and right side of the join respectively.
 */
static SpecialJoinInfo *
build_child_join_sjinfo(PlannerInfo *root, SpecialJoinInfo *parent_sjinfo,
                        Relids left_relids, Relids right_relids)
{
    SpecialJoinInfo *sjinfo = makeNode(SpecialJoinInfo);
    AppendRelInfo **left_appinfos;
    int         left_nappinfos;
    AppendRelInfo **right_appinfos;
    int         right_nappinfos;

    memcpy(sjinfo, parent_sjinfo, sizeof(SpecialJoinInfo));
    left_appinfos = find_appinfos_by_relids(root, left_relids,
                                            &left_nappinfos);
    right_appinfos = find_appinfos_by_relids(root, right_relids,
                                             &right_nappinfos);

    sjinfo->min_lefthand = adjust_child_relids(sjinfo->min_lefthand,
                                               left_nappinfos, left_appinfos);
    sjinfo->min_righthand = adjust_child_relids(sjinfo->min_righthand,
                                                right_nappinfos,
                                                right_appinfos);
    sjinfo->syn_lefthand = adjust_child_relids(sjinfo->syn_lefthand,
                                               left_nappinfos, left_appinfos);
    sjinfo->syn_righthand = adjust_child_relids(sjinfo->syn_righthand,
                                                right_nappinfos,
                                                right_appinfos);
    sjinfo->semi_rhs_exprs = (List *) adjust_appendrel_attrs(root,
                                                             (Node *) sjinfo->semi_rhs_exprs,
                                                             right_nappinfos,
                                                             right_appinfos);

    pfree(left_appinfos);
    pfree(right_appinfos);

    return sjinfo;
}

/*
 * compute_partition_bounds
 *      Compute the partition bounds for a join rel from those for inputs
 */
static void
compute_partition_bounds(PlannerInfo *root, RelOptInfo *rel1,
                         RelOptInfo *rel2, RelOptInfo *joinrel,
                         SpecialJoinInfo *parent_sjinfo,
                         List **parts1, List **parts2)
{
    /*
     * If we don't have the partition bounds for the join rel yet, try to
     * compute those along with pairs of partitions to be joined.
     */
    if (joinrel->nparts == -1)
    {
        PartitionScheme part_scheme = joinrel->part_scheme;
        PartitionBoundInfo boundinfo = NULL;
        int         nparts = 0;

        Assert(joinrel->boundinfo == NULL);
        Assert(joinrel->part_rels == NULL);

        /*
         * See if the partition bounds for inputs are exactly the same, in
         * which case we don't need to work hard: the join rel have the same
         * partition bounds as inputs, and the partitions with the same
         * cardinal positions form the pairs.
         *
         * Note: even in cases where one or both inputs have merged bounds, it
         * would be possible for both the bounds to be exactly the same, but
         * it seems unlikely to be worth the cycles to check.
         */
        if (!rel1->partbounds_merged &&
            !rel2->partbounds_merged &&
            rel1->nparts == rel2->nparts &&
            partition_bounds_equal(part_scheme->partnatts,
                                   part_scheme->parttyplen,
                                   part_scheme->parttypbyval,
                                   rel1->boundinfo, rel2->boundinfo))
        {
            boundinfo = rel1->boundinfo;
            nparts = rel1->nparts;
        }
        else
        {
            /* Try merging the partition bounds for inputs. */
            boundinfo = partition_bounds_merge(part_scheme->partnatts,
                                               part_scheme->partsupfunc,
                                               part_scheme->partcollation,
                                               rel1, rel2,
                                               parent_sjinfo->jointype,
                                               parts1, parts2);
            if (boundinfo == NULL)
            {
                joinrel->nparts = 0;
                return;
            }
            nparts = list_length(*parts1);
            joinrel->partbounds_merged = true;
        }

        Assert(nparts > 0);
        joinrel->boundinfo = boundinfo;
        joinrel->nparts = nparts;
        joinrel->part_rels =
            (RelOptInfo **) palloc0(sizeof(RelOptInfo *) * nparts);
    }
    else
    {
        Assert(joinrel->nparts > 0);
        Assert(joinrel->boundinfo);
        Assert(joinrel->part_rels);

        /*
         * If the join rel's partbounds_merged flag is true, it means inputs
         * are not guaranteed to have the same partition bounds, therefore we
         * can't assume that the partitions at the same cardinal positions
         * form the pairs; let get_matching_part_pairs() generate the pairs.
         * Otherwise, nothing to do since we can assume that.
         */
        if (joinrel->partbounds_merged)
        {
            get_matching_part_pairs(root, joinrel, rel1, rel2,
                                    parts1, parts2);
            Assert(list_length(*parts1) == joinrel->nparts);
            Assert(list_length(*parts2) == joinrel->nparts);
        }
    }
}

/*
 * get_matching_part_pairs
 *      Generate pairs of partitions to be joined from inputs
 */
static void
get_matching_part_pairs(PlannerInfo *root, RelOptInfo *joinrel,
                        RelOptInfo *rel1, RelOptInfo *rel2,
                        List **parts1, List **parts2)
{
    bool        rel1_is_simple = IS_SIMPLE_REL(rel1);
    bool        rel2_is_simple = IS_SIMPLE_REL(rel2);
    int         cnt_parts;

    *parts1 = NIL;
    *parts2 = NIL;

    for (cnt_parts = 0; cnt_parts < joinrel->nparts; cnt_parts++)
    {
        RelOptInfo *child_joinrel = joinrel->part_rels[cnt_parts];
        RelOptInfo *child_rel1;
        RelOptInfo *child_rel2;
        Relids      child_relids1;
        Relids      child_relids2;

        /*
         * If this segment of the join is empty, it means that this segment
         * was ignored when previously creating child-join paths for it in
         * try_partitionwise_join() as it would not contribute to the join
         * result, due to one or both inputs being empty; add NULL to each of
         * the given lists so that this segment will be ignored again in that
         * function.
         */
        if (!child_joinrel)
        {
            *parts1 = lappend(*parts1, NULL);
            *parts2 = lappend(*parts2, NULL);
            continue;
        }

        /*
         * Get a relids set of partition(s) involved in this join segment that
         * are from the rel1 side.
         */
        child_relids1 = bms_intersect(child_joinrel->relids,
                                      rel1->all_partrels);
        Assert(bms_num_members(child_relids1) == bms_num_members(rel1->relids));

        /*
         * Get a child rel for rel1 with the relids.  Note that we should have
         * the child rel even if rel1 is a join rel, because in that case the
         * partitions specified in the relids would have matching/overlapping
         * boundaries, so the specified partitions should be considered as
         * ones to be joined when planning partitionwise joins of rel1,
         * meaning that the child rel would have been built by the time we get
         * here.
         */
        if (rel1_is_simple)
        {
            int         varno = bms_singleton_member(child_relids1);

            child_rel1 = find_base_rel(root, varno);
        }
        else
            child_rel1 = find_join_rel(root, child_relids1);
        Assert(child_rel1);

        /*
         * Get a relids set of partition(s) involved in this join segment that
         * are from the rel2 side.
         */
        child_relids2 = bms_intersect(child_joinrel->relids,
                                      rel2->all_partrels);
        Assert(bms_num_members(child_relids2) == bms_num_members(rel2->relids));

        /*
         * Get a child rel for rel2 with the relids.  See above comments.
         */
        if (rel2_is_simple)
        {
            int         varno = bms_singleton_member(child_relids2);

            child_rel2 = find_base_rel(root, varno);
        }
        else
            child_rel2 = find_join_rel(root, child_relids2);
        Assert(child_rel2);

        /*
         * The join of rel1 and rel2 is legal, so is the join of the child
         * rels obtained above; add them to the given lists as a join pair
         * producing this join segment.
         */
        *parts1 = lappend(*parts1, child_rel1);
        *parts2 = lappend(*parts2, child_rel2);
    }
}