joinpath.c

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/*-------------------------------------------------------------------------
 *
 * joinpath.c
 *    Routines to find all possible paths for processing a set of joins
 *
 * Portions Copyright (c) 1996-2021, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *    src/backend/optimizer/path/joinpath.c
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"

#include <math.h>

#include "executor/executor.h"
#include "foreign/fdwapi.h"
#include "nodes/nodeFuncs.h"
#include "optimizer/cost.h"
#include "optimizer/optimizer.h"
#include "optimizer/pathnode.h"
#include "optimizer/paths.h"
#include "optimizer/planmain.h"
#include "utils/typcache.h"

/* Hook for plugins to get control in add_paths_to_joinrel() */
set_join_pathlist_hook_type set_join_pathlist_hook = NULL;

/*
 * Paths parameterized by the parent can be considered to be parameterized by
 * any of its child.
 */
#define PATH_PARAM_BY_PARENT(path, rel) \
    ((path)->param_info && bms_overlap(PATH_REQ_OUTER(path),    \
                                       (rel)->top_parent_relids))
#define PATH_PARAM_BY_REL_SELF(path, rel)  \
    ((path)->param_info && bms_overlap(PATH_REQ_OUTER(path), (rel)->relids))

#define PATH_PARAM_BY_REL(path, rel)    \
    (PATH_PARAM_BY_REL_SELF(path, rel) || PATH_PARAM_BY_PARENT(path, rel))

static void try_partial_mergejoin_path(PlannerInfo *root,
                                       RelOptInfo *joinrel,
                                       Path *outer_path,
                                       Path *inner_path,
                                       List *pathkeys,
                                       List *mergeclauses,
                                       List *outersortkeys,
                                       List *innersortkeys,
                                       JoinType jointype,
                                       JoinPathExtraData *extra);
static void sort_inner_and_outer(PlannerInfo *root, RelOptInfo *joinrel,
                                 RelOptInfo *outerrel, RelOptInfo *innerrel,
                                 JoinType jointype, JoinPathExtraData *extra);
static inline bool clause_sides_match_join(RestrictInfo *rinfo,
                                           RelOptInfo *outerrel,
                                           RelOptInfo *innerrel);
static void match_unsorted_outer(PlannerInfo *root, RelOptInfo *joinrel,
                                 RelOptInfo *outerrel, RelOptInfo *innerrel,
                                 JoinType jointype, JoinPathExtraData *extra);
static void consider_parallel_nestloop(PlannerInfo *root,
                                       RelOptInfo *joinrel,
                                       RelOptInfo *outerrel,
                                       RelOptInfo *innerrel,
                                       JoinType jointype,
                                       JoinPathExtraData *extra);
static void consider_parallel_mergejoin(PlannerInfo *root,
                                        RelOptInfo *joinrel,
                                        RelOptInfo *outerrel,
                                        RelOptInfo *innerrel,
                                        JoinType jointype,
                                        JoinPathExtraData *extra,
                                        Path *inner_cheapest_total);
static void hash_inner_and_outer(PlannerInfo *root, RelOptInfo *joinrel,
                                 RelOptInfo *outerrel, RelOptInfo *innerrel,
                                 JoinType jointype, JoinPathExtraData *extra);
static List *select_mergejoin_clauses(PlannerInfo *root,
                                      RelOptInfo *joinrel,
                                      RelOptInfo *outerrel,
                                      RelOptInfo *innerrel,
                                      List *restrictlist,
                                      JoinType jointype,
                                      bool *mergejoin_allowed);
static void generate_mergejoin_paths(PlannerInfo *root,
                                     RelOptInfo *joinrel,
                                     RelOptInfo *innerrel,
                                     Path *outerpath,
                                     JoinType jointype,
                                     JoinPathExtraData *extra,
                                     bool useallclauses,
                                     Path *inner_cheapest_total,
                                     List *merge_pathkeys,
                                     bool is_partial);


/*
 * add_paths_to_joinrel
 *    Given a join relation and two component rels from which it can be made,
 *    consider all possible paths that use the two component rels as outer
 *    and inner rel respectively.  Add these paths to the join rel's pathlist
 *    if they survive comparison with other paths (and remove any existing
 *    paths that are dominated by these paths).
 *
 * Modifies the pathlist field of the joinrel node to contain the best
 * paths found so far.
 *
 * jointype is not necessarily the same as sjinfo->jointype; it might be
 * "flipped around" if we are considering joining the rels in the opposite
 * direction from what's indicated in sjinfo.
 *
 * Also, this routine and others in this module accept the special JoinTypes
 * JOIN_UNIQUE_OUTER and JOIN_UNIQUE_INNER to indicate that we should
 * unique-ify the outer or inner relation and then apply a regular inner
 * join.  These values are not allowed to propagate outside this module,
 * however.  Path cost estimation code may need to recognize that it's
 * dealing with such a case --- the combination of nominal jointype INNER
 * with sjinfo->jointype == JOIN_SEMI indicates that.
 */
void
add_paths_to_joinrel(PlannerInfo *root,
                     RelOptInfo *joinrel,
                     RelOptInfo *outerrel,
                     RelOptInfo *innerrel,
                     JoinType jointype,
                     SpecialJoinInfo *sjinfo,
                     List *restrictlist)
{
    JoinPathExtraData extra;
    bool        mergejoin_allowed = true;
    ListCell   *lc;
    Relids      joinrelids;

    /*
     * PlannerInfo doesn't contain the SpecialJoinInfos created for joins
     * between child relations, even if there is a SpecialJoinInfo node for
     * the join between the topmost parents. So, while calculating Relids set
     * representing the restriction, consider relids of topmost parent of
     * partitions.
     */
    if (joinrel->reloptkind == RELOPT_OTHER_JOINREL)
        joinrelids = joinrel->top_parent_relids;
    else
        joinrelids = joinrel->relids;

    extra.restrictlist = restrictlist;
    extra.mergeclause_list = NIL;
    extra.sjinfo = sjinfo;
    extra.param_source_rels = NULL;

    /*
     * See if the inner relation is provably unique for this outer rel.
     *
     * We have some special cases: for JOIN_SEMI and JOIN_ANTI, it doesn't
     * matter since the executor can make the equivalent optimization anyway;
     * we need not expend planner cycles on proofs.  For JOIN_UNIQUE_INNER, we
     * must be considering a semijoin whose inner side is not provably unique
     * (else reduce_unique_semijoins would've simplified it), so there's no
     * point in calling innerrel_is_unique.  However, if the LHS covers all of
     * the semijoin's min_lefthand, then it's appropriate to set inner_unique
     * because the path produced by create_unique_path will be unique relative
     * to the LHS.  (If we have an LHS that's only part of the min_lefthand,
     * that is *not* true.)  For JOIN_UNIQUE_OUTER, pass JOIN_INNER to avoid
     * letting that value escape this module.
     */
    switch (jointype)
    {
        case JOIN_SEMI:
        case JOIN_ANTI:

            /*
             * XXX it may be worth proving this to allow a Memoize to be
             * considered for Nested Loop Semi/Anti Joins.
             */
            extra.inner_unique = false; /* well, unproven */
            break;
        case JOIN_UNIQUE_INNER:
            extra.inner_unique = bms_is_subset(sjinfo->min_lefthand,
                                               outerrel->relids);
            break;
        case JOIN_UNIQUE_OUTER:
            extra.inner_unique = innerrel_is_unique(root,
                                                    joinrel->relids,
                                                    outerrel->relids,
                                                    innerrel,
                                                    JOIN_INNER,
                                                    restrictlist,
                                                    false);
            break;
        default:
            extra.inner_unique = innerrel_is_unique(root,
                                                    joinrel->relids,
                                                    outerrel->relids,
                                                    innerrel,
                                                    jointype,
                                                    restrictlist,
                                                    false);
            break;
    }

    /*
     * Find potential mergejoin clauses.  We can skip this if we are not
     * interested in doing a mergejoin.  However, mergejoin may be our only
     * way of implementing a full outer join, so override enable_mergejoin if
     * it's a full join.
     */
    if (enable_mergejoin || jointype == JOIN_FULL)
        extra.mergeclause_list = select_mergejoin_clauses(root,
                                                          joinrel,
                                                          outerrel,
                                                          innerrel,
                                                          restrictlist,
                                                          jointype,
                                                          &mergejoin_allowed);

    /*
     * If it's SEMI, ANTI, or inner_unique join, compute correction factors
     * for cost estimation.  These will be the same for all paths.
     */
    if (jointype == JOIN_SEMI || jointype == JOIN_ANTI || extra.inner_unique)
        compute_semi_anti_join_factors(root, joinrel, outerrel, innerrel,
                                       jointype, sjinfo, restrictlist,
                                       &extra.semifactors);

    /*
     * Decide whether it's sensible to generate parameterized paths for this
     * joinrel, and if so, which relations such paths should require.  There
     * is usually no need to create a parameterized result path unless there
     * is a join order restriction that prevents joining one of our input rels
     * directly to the parameter source rel instead of joining to the other
     * input rel.  (But see allow_star_schema_join().)  This restriction
     * reduces the number of parameterized paths we have to deal with at
     * higher join levels, without compromising the quality of the resulting
     * plan.  We express the restriction as a Relids set that must overlap the
     * parameterization of any proposed join path.
     */
    foreach(lc, root->join_info_list)
    {
        SpecialJoinInfo *sjinfo2 = (SpecialJoinInfo *) lfirst(lc);

        /*
         * SJ is relevant to this join if we have some part of its RHS
         * (possibly not all of it), and haven't yet joined to its LHS.  (This
         * test is pretty simplistic, but should be sufficient considering the
         * join has already been proven legal.)  If the SJ is relevant, it
         * presents constraints for joining to anything not in its RHS.
         */
        if (bms_overlap(joinrelids, sjinfo2->min_righthand) &&
            !bms_overlap(joinrelids, sjinfo2->min_lefthand))
            extra.param_source_rels = bms_join(extra.param_source_rels,
                                               bms_difference(root->all_baserels,
                                                              sjinfo2->min_righthand));

        /* full joins constrain both sides symmetrically */
        if (sjinfo2->jointype == JOIN_FULL &&
            bms_overlap(joinrelids, sjinfo2->min_lefthand) &&
            !bms_overlap(joinrelids, sjinfo2->min_righthand))
            extra.param_source_rels = bms_join(extra.param_source_rels,
                                               bms_difference(root->all_baserels,
                                                              sjinfo2->min_lefthand));
    }

    /*
     * However, when a LATERAL subquery is involved, there will simply not be
     * any paths for the joinrel that aren't parameterized by whatever the
     * subquery is parameterized by, unless its parameterization is resolved
     * within the joinrel.  So we might as well allow additional dependencies
     * on whatever residual lateral dependencies the joinrel will have.
     */
    extra.param_source_rels = bms_add_members(extra.param_source_rels,
                                              joinrel->lateral_relids);

    /*
     * 1. Consider mergejoin paths where both relations must be explicitly
     * sorted.  Skip this if we can't mergejoin.
     */
    if (mergejoin_allowed)
        sort_inner_and_outer(root, joinrel, outerrel, innerrel,
                             jointype, &extra);

    /*
     * 2. Consider paths where the outer relation need not be explicitly
     * sorted. This includes both nestloops and mergejoins where the outer
     * path is already ordered.  Again, skip this if we can't mergejoin.
     * (That's okay because we know that nestloop can't handle right/full
     * joins at all, so it wouldn't work in the prohibited cases either.)
     */
    if (mergejoin_allowed)
        match_unsorted_outer(root, joinrel, outerrel, innerrel,
                             jointype, &extra);

#ifdef NOT_USED

    /*
     * 3. Consider paths where the inner relation need not be explicitly
     * sorted.  This includes mergejoins only (nestloops were already built in
     * match_unsorted_outer).
     *
     * Diked out as redundant 2/13/2000 -- tgl.  There isn't any really
     * significant difference between the inner and outer side of a mergejoin,
     * so match_unsorted_inner creates no paths that aren't equivalent to
     * those made by match_unsorted_outer when add_paths_to_joinrel() is
     * invoked with the two rels given in the other order.
     */
    if (mergejoin_allowed)
        match_unsorted_inner(root, joinrel, outerrel, innerrel,
                             jointype, &extra);
#endif

    /*
     * 4. Consider paths where both outer and inner relations must be hashed
     * before being joined.  As above, disregard enable_hashjoin for full
     * joins, because there may be no other alternative.
     */
    if (enable_hashjoin || jointype == JOIN_FULL)
        hash_inner_and_outer(root, joinrel, outerrel, innerrel,
                             jointype, &extra);

    /*
     * 5. If inner and outer relations are foreign tables (or joins) belonging
     * to the same server and assigned to the same user to check access
     * permissions as, give the FDW a chance to push down joins.
     */
    if (joinrel->fdwroutine &&
        joinrel->fdwroutine->GetForeignJoinPaths)
        joinrel->fdwroutine->GetForeignJoinPaths(root, joinrel,
                                                 outerrel, innerrel,
                                                 jointype, &extra);

    /*
     * 6. Finally, give extensions a chance to manipulate the path list.
     */
    if (set_join_pathlist_hook)
        set_join_pathlist_hook(root, joinrel, outerrel, innerrel,
                               jointype, &extra);
}

/*
 * We override the param_source_rels heuristic to accept nestloop paths in
 * which the outer rel satisfies some but not all of the inner path's
 * parameterization.  This is necessary to get good plans for star-schema
 * scenarios, in which a parameterized path for a large table may require
 * parameters from multiple small tables that will not get joined directly to
 * each other.  We can handle that by stacking nestloops that have the small
 * tables on the outside; but this breaks the rule the param_source_rels
 * heuristic is based on, namely that parameters should not be passed down
 * across joins unless there's a join-order-constraint-based reason to do so.
 * So we ignore the param_source_rels restriction when this case applies.
 *
 * allow_star_schema_join() returns true if the param_source_rels restriction
 * should be overridden, ie, it's okay to perform this join.
 */
static inline bool
allow_star_schema_join(PlannerInfo *root,
                       Relids outerrelids,
                       Relids inner_paramrels)
{
    /*
     * It's a star-schema case if the outer rel provides some but not all of
     * the inner rel's parameterization.
     */
    return (bms_overlap(inner_paramrels, outerrelids) &&
            bms_nonempty_difference(inner_paramrels, outerrelids));
}

/*
 * paraminfo_get_equal_hashops
 *      Determine if param_info and innerrel's lateral_vars can be hashed.
 *      Returns true the hashing is possible, otherwise return false.
 *
 * Additionally we also collect the outer exprs and the hash operators for
 * each parameter to innerrel.  These set in 'param_exprs' and 'operators'
 * when we return true.
 */
static bool
paraminfo_get_equal_hashops(PlannerInfo *root, ParamPathInfo *param_info,
                            RelOptInfo *outerrel, RelOptInfo *innerrel,
                            List **param_exprs, List **operators)

{
    ListCell   *lc;

    *param_exprs = NIL;
    *operators = NIL;

    if (param_info != NULL)
    {
        List       *clauses = param_info->ppi_clauses;

        foreach(lc, clauses)
        {
            RestrictInfo *rinfo = (RestrictInfo *) lfirst(lc);
            OpExpr     *opexpr;
            Node       *expr;

            /* can't use a memoize node without a valid hash equals operator */
            if (!OidIsValid(rinfo->hasheqoperator) ||
                !clause_sides_match_join(rinfo, outerrel, innerrel))
            {
                list_free(*operators);
                list_free(*param_exprs);
                return false;
            }

            /*
             * We already checked that this is an OpExpr with 2 args when
             * setting hasheqoperator.
             */
            opexpr = (OpExpr *) rinfo->clause;
            if (rinfo->outer_is_left)
                expr = (Node *) linitial(opexpr->args);
            else
                expr = (Node *) lsecond(opexpr->args);

            *operators = lappend_oid(*operators, rinfo->hasheqoperator);
            *param_exprs = lappend(*param_exprs, expr);
        }
    }

    /* Now add any lateral vars to the cache key too */
    foreach(lc, innerrel->lateral_vars)
    {
        Node       *expr = (Node *) lfirst(lc);
        TypeCacheEntry *typentry;

        /* Reject if there are any volatile functions */
        if (contain_volatile_functions(expr))
        {
            list_free(*operators);
            list_free(*param_exprs);
            return false;
        }

        typentry = lookup_type_cache(exprType(expr),
                                     TYPECACHE_HASH_PROC | TYPECACHE_EQ_OPR);

        /* can't use a memoize node without a valid hash equals operator */
        if (!OidIsValid(typentry->hash_proc) || !OidIsValid(typentry->eq_opr))
        {
            list_free(*operators);
            list_free(*param_exprs);
            return false;
        }

        *operators = lappend_oid(*operators, typentry->eq_opr);
        *param_exprs = lappend(*param_exprs, expr);
    }

    /* We're okay to use memoize */
    return true;
}

/*
 * get_memoize_path
 *      If possible, make and return a Memoize path atop of 'inner_path'.
 *      Otherwise return NULL.
 */
static Path *
get_memoize_path(PlannerInfo *root, RelOptInfo *innerrel,
                 RelOptInfo *outerrel, Path *inner_path,
                 Path *outer_path, JoinType jointype,
                 JoinPathExtraData *extra)
{
    List       *param_exprs;
    List       *hash_operators;
    ListCell   *lc;

    /* Obviously not if it's disabled */
    if (!enable_memoize)
        return NULL;

    /*
     * We can safely not bother with all this unless we expect to perform more
     * than one inner scan.  The first scan is always going to be a cache
     * miss.  This would likely fail later anyway based on costs, so this is
     * really just to save some wasted effort.
     */
    if (outer_path->parent->rows < 2)
        return NULL;

    /*
     * We can only have a memoize node when there's some kind of cache key,
     * either parameterized path clauses or lateral Vars.  No cache key sounds
     * more like something a Materialize node might be more useful for.
     */
    if ((inner_path->param_info == NULL ||
         inner_path->param_info->ppi_clauses == NIL) &&
        innerrel->lateral_vars == NIL)
        return NULL;

    /*
     * Currently we don't do this for SEMI and ANTI joins unless they're
     * marked as inner_unique.  This is because nested loop SEMI/ANTI joins
     * don't scan the inner node to completion, which will mean memoize cannot
     * mark the cache entry as complete.
     *
     * XXX Currently we don't attempt to mark SEMI/ANTI joins as inner_unique
     * = true.  Should we?  See add_paths_to_joinrel()
     */
    if (!extra->inner_unique && (jointype == JOIN_SEMI ||
                                 jointype == JOIN_ANTI))
        return NULL;

    /*
     * Memoize normally marks cache entries as complete when it runs out of
     * tuples to read from its subplan.  However, with unique joins, Nested
     * Loop will skip to the next outer tuple after finding the first matching
     * inner tuple.  This means that we may not read the inner side of the
     * join to completion which leaves no opportunity to mark the cache entry
     * as complete.  To work around that, when the join is unique we
     * automatically mark cache entries as complete after fetching the first
     * tuple.  This works when the entire join condition is parameterized.
     * Otherwise, when the parameterization is only a subset of the join
     * condition, we can't be sure which part of it causes the join to be
     * unique.  This means there are no guarantees that only 1 tuple will be
     * read.  We cannot mark the cache entry as complete after reading the
     * first tuple without that guarantee.  This means the scope of Memoize
     * node's usefulness is limited to only outer rows that have no join
     * partner as this is the only case where Nested Loop would exhaust the
     * inner scan of a unique join.  Since the scope is limited to that, we
     * just don't bother making a memoize path in this case.
     *
     * Lateral vars needn't be considered here as they're not considered when
     * determining if the join is unique.
     *
     * XXX this could be enabled if the remaining join quals were made part of
     * the inner scan's filter instead of the join filter.  Maybe it's worth
     * considering doing that?
     */
    if (extra->inner_unique &&
        (inner_path->param_info == NULL ||
         list_length(inner_path->param_info->ppi_clauses) <
         list_length(extra->restrictlist)))
        return NULL;

    /*
     * We can't use a memoize node if there are volatile functions in the
     * inner rel's target list or restrict list.  A cache hit could reduce the
     * number of calls to these functions.
     */
    if (contain_volatile_functions((Node *) innerrel->reltarget))
        return NULL;

    foreach(lc, innerrel->baserestrictinfo)
    {
        RestrictInfo *rinfo = (RestrictInfo *) lfirst(lc);

        if (contain_volatile_functions((Node *) rinfo))
            return NULL;
    }

    /* Check if we have hash ops for each parameter to the path */
    if (paraminfo_get_equal_hashops(root,
                                    inner_path->param_info,
                                    outerrel,
                                    innerrel,
                                    &param_exprs,
                                    &hash_operators))
    {
        return (Path *) create_memoize_path(root,
                                            innerrel,
                                            inner_path,
                                            param_exprs,
                                            hash_operators,
                                            extra->inner_unique,
                                            outer_path->parent->rows);
    }

    return NULL;
}

/*
 * try_nestloop_path
 *    Consider a nestloop join path; if it appears useful, push it into
 *    the joinrel's pathlist via add_path().
 */
static void
try_nestloop_path(PlannerInfo *root,
                  RelOptInfo *joinrel,
                  Path *outer_path,
                  Path *inner_path,
                  List *pathkeys,
                  JoinType jointype,
                  JoinPathExtraData *extra)
{
    Relids      required_outer;
    JoinCostWorkspace workspace;
    RelOptInfo *innerrel = inner_path->parent;
    RelOptInfo *outerrel = outer_path->parent;
    Relids      innerrelids;
    Relids      outerrelids;
    Relids      inner_paramrels = PATH_REQ_OUTER(inner_path);
    Relids      outer_paramrels = PATH_REQ_OUTER(outer_path);

    /*
     * Paths are parameterized by top-level parents, so run parameterization
     * tests on the parent relids.
     */
    if (innerrel->top_parent_relids)
        innerrelids = innerrel->top_parent_relids;
    else
        innerrelids = innerrel->relids;

    if (outerrel->top_parent_relids)
        outerrelids = outerrel->top_parent_relids;
    else
        outerrelids = outerrel->relids;

    /*
     * Check to see if proposed path is still parameterized, and reject if the
     * parameterization wouldn't be sensible --- unless allow_star_schema_join
     * says to allow it anyway.  Also, we must reject if have_dangerous_phv
     * doesn't like the look of it, which could only happen if the nestloop is
     * still parameterized.
     */
    required_outer = calc_nestloop_required_outer(outerrelids, outer_paramrels,
                                                  innerrelids, inner_paramrels);
    if (required_outer &&
        ((!bms_overlap(required_outer, extra->param_source_rels) &&
          !allow_star_schema_join(root, outerrelids, inner_paramrels)) ||
         have_dangerous_phv(root, outerrelids, inner_paramrels)))
    {
        /* Waste no memory when we reject a path here */
        bms_free(required_outer);
        return;
    }

    /*
     * Do a precheck to quickly eliminate obviously-inferior paths.  We
     * calculate a cheap lower bound on the path's cost and then use
     * add_path_precheck() to see if the path is clearly going to be dominated
     * by some existing path for the joinrel.  If not, do the full pushup with
     * creating a fully valid path structure and submitting it to add_path().
     * The latter two steps are expensive enough to make this two-phase
     * methodology worthwhile.
     */
    initial_cost_nestloop(root, &workspace, jointype,
                          outer_path, inner_path, extra);

    if (add_path_precheck(joinrel,
                          workspace.startup_cost, workspace.total_cost,
                          pathkeys, required_outer))
    {
        /*
         * If the inner path is parameterized, it is parameterized by the
         * topmost parent of the outer rel, not the outer rel itself.  Fix
         * that.
         */
        if (PATH_PARAM_BY_PARENT(inner_path, outer_path->parent))
        {
            inner_path = reparameterize_path_by_child(root, inner_path,
                                                      outer_path->parent);

            /*
             * If we could not translate the path, we can't create nest loop
             * path.
             */
            if (!inner_path)
            {
                bms_free(required_outer);
                return;
            }
        }

        add_path(joinrel, (Path *)
                 create_nestloop_path(root,
                                      joinrel,
                                      jointype,
                                      &workspace,
                                      extra,
                                      outer_path,
                                      inner_path,
                                      extra->restrictlist,
                                      pathkeys,
                                      required_outer));
    }
    else
    {
        /* Waste no memory when we reject a path here */
        bms_free(required_outer);
    }
}

/*
 * try_partial_nestloop_path
 *    Consider a partial nestloop join path; if it appears useful, push it into
 *    the joinrel's partial_pathlist via add_partial_path().
 */
static void
try_partial_nestloop_path(PlannerInfo *root,
                          RelOptInfo *joinrel,
                          Path *outer_path,
                          Path *inner_path,
                          List *pathkeys,
                          JoinType jointype,
                          JoinPathExtraData *extra)
{
    JoinCostWorkspace workspace;

    /*
     * If the inner path is parameterized, the parameterization must be fully
     * satisfied by the proposed outer path.  Parameterized partial paths are
     * not supported.  The caller should already have verified that no lateral
     * rels are required here.
     */
    Assert(bms_is_empty(joinrel->lateral_relids));
    if (inner_path->param_info != NULL)
    {
        Relids      inner_paramrels = inner_path->param_info->ppi_req_outer;
        RelOptInfo *outerrel = outer_path->parent;
        Relids      outerrelids;

        /*
         * The inner and outer paths are parameterized, if at all, by the top
         * level parents, not the child relations, so we must use those relids
         * for our parameterization tests.
         */
        if (outerrel->top_parent_relids)
            outerrelids = outerrel->top_parent_relids;
        else
            outerrelids = outerrel->relids;

        if (!bms_is_subset(inner_paramrels, outerrelids))
            return;
    }

    /*
     * Before creating a path, get a quick lower bound on what it is likely to
     * cost.  Bail out right away if it looks terrible.
     */
    initial_cost_nestloop(root, &workspace, jointype,
                          outer_path, inner_path, extra);
    if (!add_partial_path_precheck(joinrel, workspace.total_cost, pathkeys))
        return;

    /*
     * If the inner path is parameterized, it is parameterized by the topmost
     * parent of the outer rel, not the outer rel itself.  Fix that.
     */
    if (PATH_PARAM_BY_PARENT(inner_path, outer_path->parent))
    {
        inner_path = reparameterize_path_by_child(root, inner_path,
                                                  outer_path->parent);

        /*
         * If we could not translate the path, we can't create nest loop path.
         */
        if (!inner_path)
            return;
    }

    /* Might be good enough to be worth trying, so let's try it. */
    add_partial_path(joinrel, (Path *)
                     create_nestloop_path(root,
                                          joinrel,
                                          jointype,
                                          &workspace,
                                          extra,
                                          outer_path,
                                          inner_path,
                                          extra->restrictlist,
                                          pathkeys,
                                          NULL));
}

/*
 * try_mergejoin_path
 *    Consider a merge join path; if it appears useful, push it into
 *    the joinrel's pathlist via add_path().
 */
static void
try_mergejoin_path(PlannerInfo *root,
                   RelOptInfo *joinrel,
                   Path *outer_path,
                   Path *inner_path,
                   List *pathkeys,
                   List *mergeclauses,
                   List *outersortkeys,
                   List *innersortkeys,
                   JoinType jointype,
                   JoinPathExtraData *extra,
                   bool is_partial)
{
    Relids      required_outer;
    JoinCostWorkspace workspace;

    if (is_partial)
    {
        try_partial_mergejoin_path(root,
                                   joinrel,
                                   outer_path,
                                   inner_path,
                                   pathkeys,
                                   mergeclauses,
                                   outersortkeys,
                                   innersortkeys,
                                   jointype,
                                   extra);
        return;
    }

    /*
     * Check to see if proposed path is still parameterized, and reject if the
     * parameterization wouldn't be sensible.
     */
    required_outer = calc_non_nestloop_required_outer(outer_path,
                                                      inner_path);
    if (required_outer &&
        !bms_overlap(required_outer, extra->param_source_rels))
    {
        /* Waste no memory when we reject a path here */
        bms_free(required_outer);
        return;
    }

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

    /*
     * See comments in try_nestloop_path().
     */
    initial_cost_mergejoin(root, &workspace, jointype, mergeclauses,
                           outer_path, inner_path,
                           outersortkeys, innersortkeys,
                           extra);

    if (add_path_precheck(joinrel,
                          workspace.startup_cost, workspace.total_cost,
                          pathkeys, required_outer))
    {
        add_path(joinrel, (Path *)
                 create_mergejoin_path(root,
                                       joinrel,
                                       jointype,
                                       &workspace,
                                       extra,
                                       outer_path,
                                       inner_path,
                                       extra->restrictlist,
                                       pathkeys,
                                       required_outer,
                                       mergeclauses,
                                       outersortkeys,
                                       innersortkeys));
    }
    else
    {
        /* Waste no memory when we reject a path here */
        bms_free(required_outer);
    }
}

/*
 * try_partial_mergejoin_path
 *    Consider a partial merge join path; if it appears useful, push it into
 *    the joinrel's pathlist via add_partial_path().
 */
static void
try_partial_mergejoin_path(PlannerInfo *root,
                           RelOptInfo *joinrel,
                           Path *outer_path,
                           Path *inner_path,
                           List *pathkeys,
                           List *mergeclauses,
                           List *outersortkeys,
                           List *innersortkeys,
                           JoinType jointype,
                           JoinPathExtraData *extra)
{
    JoinCostWorkspace workspace;

    /*
     * See comments in try_partial_hashjoin_path().
     */
    Assert(bms_is_empty(joinrel->lateral_relids));
    if (inner_path->param_info != NULL)
    {
        Relids      inner_paramrels = inner_path->param_info->ppi_req_outer;

        if (!bms_is_empty(inner_paramrels))
            return;
    }

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

    /*
     * See comments in try_partial_nestloop_path().
     */
    initial_cost_mergejoin(root, &workspace, jointype, mergeclauses,
                           outer_path, inner_path,
                           outersortkeys, innersortkeys,
                           extra);

    if (!add_partial_path_precheck(joinrel, workspace.total_cost, pathkeys))
        return;

    /* Might be good enough to be worth trying, so let's try it. */
    add_partial_path(joinrel, (Path *)
                     create_mergejoin_path(root,
                                           joinrel,
                                           jointype,
                                           &workspace,
                                           extra,
                                           outer_path,
                                           inner_path,
                                           extra->restrictlist,
                                           pathkeys,
                                           NULL,
                                           mergeclauses,
                                           outersortkeys,
                                           innersortkeys));
}

/*
 * try_hashjoin_path
 *    Consider a hash join path; if it appears useful, push it into
 *    the joinrel's pathlist via add_path().
 */
static void
try_hashjoin_path(PlannerInfo *root,
                  RelOptInfo *joinrel,
                  Path *outer_path,
                  Path *inner_path,
                  List *hashclauses,
                  JoinType jointype,
                  JoinPathExtraData *extra)
{
    Relids      required_outer;
    JoinCostWorkspace workspace;

    /*
     * Check to see if proposed path is still parameterized, and reject if the
     * parameterization wouldn't be sensible.
     */
    required_outer = calc_non_nestloop_required_outer(outer_path,
                                                      inner_path);
    if (required_outer &&
        !bms_overlap(required_outer, extra->param_source_rels))
    {
        /* Waste no memory when we reject a path here */
        bms_free(required_outer);
        return;
    }

    /*
     * See comments in try_nestloop_path().  Also note that hashjoin paths
     * never have any output pathkeys, per comments in create_hashjoin_path.
     */
    initial_cost_hashjoin(root, &workspace, jointype, hashclauses,
                          outer_path, inner_path, extra, false);

    if (add_path_precheck(joinrel,
                          workspace.startup_cost, workspace.total_cost,
                          NIL, required_outer))
    {
        add_path(joinrel, (Path *)
                 create_hashjoin_path(root,
                                      joinrel,
                                      jointype,
                                      &workspace,
                                      extra,
                                      outer_path,
                                      inner_path,
                                      false,    /* parallel_hash */
                                      extra->restrictlist,
                                      required_outer,
                                      hashclauses));
    }
    else
    {
        /* Waste no memory when we reject a path here */
        bms_free(required_outer);
    }
}

/*
 * try_partial_hashjoin_path
 *    Consider a partial hashjoin join path; if it appears useful, push it into
 *    the joinrel's partial_pathlist via add_partial_path().
 *    The outer side is partial.  If parallel_hash is true, then the inner path
 *    must be partial and will be run in parallel to create one or more shared
 *    hash tables; otherwise the inner path must be complete and a copy of it
 *    is run in every process to create separate identical private hash tables.
 */
static void
try_partial_hashjoin_path(PlannerInfo *root,
                          RelOptInfo *joinrel,
                          Path *outer_path,
                          Path *inner_path,
                          List *hashclauses,
                          JoinType jointype,
                          JoinPathExtraData *extra,
                          bool parallel_hash)
{
    JoinCostWorkspace workspace;

    /*
     * If the inner path is parameterized, the parameterization must be fully
     * satisfied by the proposed outer path.  Parameterized partial paths are
     * not supported.  The caller should already have verified that no lateral
     * rels are required here.
     */
    Assert(bms_is_empty(joinrel->lateral_relids));
    if (inner_path->param_info != NULL)
    {
        Relids      inner_paramrels = inner_path->param_info->ppi_req_outer;

        if (!bms_is_empty(inner_paramrels))
            return;
    }

    /*
     * Before creating a path, get a quick lower bound on what it is likely to
     * cost.  Bail out right away if it looks terrible.
     */
    initial_cost_hashjoin(root, &workspace, jointype, hashclauses,
                          outer_path, inner_path, extra, parallel_hash);
    if (!add_partial_path_precheck(joinrel, workspace.total_cost, NIL))
        return;

    /* Might be good enough to be worth trying, so let's try it. */
    add_partial_path(joinrel, (Path *)
                     create_hashjoin_path(root,
                                          joinrel,
                                          jointype,
                                          &workspace,
                                          extra,
                                          outer_path,
                                          inner_path,
                                          parallel_hash,
                                          extra->restrictlist,
                                          NULL,
                                          hashclauses));
}

/*
 * clause_sides_match_join
 *    Determine whether a join clause is of the right form to use in this join.
 *
 * We already know that the clause is a binary opclause referencing only the
 * rels in the current join.  The point here is to check whether it has the
 * form "outerrel_expr op innerrel_expr" or "innerrel_expr op outerrel_expr",
 * rather than mixing outer and inner vars on either side.  If it matches,
 * we set the transient flag outer_is_left to identify which side is which.
 */
static inline bool
clause_sides_match_join(RestrictInfo *rinfo, RelOptInfo *outerrel,
                        RelOptInfo *innerrel)
{
    if (bms_is_subset(rinfo->left_relids, outerrel->relids) &&
        bms_is_subset(rinfo->right_relids, innerrel->relids))
    {
        /* lefthand side is outer */
        rinfo->outer_is_left = true;
        return true;
    }
    else if (bms_is_subset(rinfo->left_relids, innerrel->relids) &&
             bms_is_subset(rinfo->right_relids, outerrel->relids))
    {
        /* righthand side is outer */
        rinfo->outer_is_left = false;
        return true;
    }
    return false;               /* no good for these input relations */
}

/*
 * sort_inner_and_outer
 *    Create mergejoin join paths by explicitly sorting both the outer and
 *    inner join relations on each available merge ordering.
 *
 * 'joinrel' is the join relation
 * 'outerrel' is the outer join relation
 * 'innerrel' is the inner join relation
 * 'jointype' is the type of join to do
 * 'extra' contains additional input values
 */
static void
sort_inner_and_outer(PlannerInfo *root,
                     RelOptInfo *joinrel,
                     RelOptInfo *outerrel,
                     RelOptInfo *innerrel,
                     JoinType jointype,
                     JoinPathExtraData *extra)
{
    JoinType    save_jointype = jointype;
    Path       *outer_path;
    Path       *inner_path;
    Path       *cheapest_partial_outer = NULL;
    Path       *cheapest_safe_inner = NULL;
    List       *all_pathkeys;
    ListCell   *l;

    /*
     * We only consider the cheapest-total-cost input paths, since we are
     * assuming here that a sort is required.  We will consider
     * cheapest-startup-cost input paths later, and only if they don't need a
     * sort.
     *
     * This function intentionally does not consider parameterized input
     * paths, except when the cheapest-total is parameterized.  If we did so,
     * we'd have a combinatorial explosion of mergejoin paths of dubious
     * value.  This interacts with decisions elsewhere that also discriminate
     * against mergejoins with parameterized inputs; see comments in
     * src/backend/optimizer/README.
     */
    outer_path = outerrel->cheapest_total_path;
    inner_path = innerrel->cheapest_total_path;

    /*
     * If either cheapest-total path is parameterized by the other rel, we
     * can't use a mergejoin.  (There's no use looking for alternative input
     * paths, since these should already be the least-parameterized available
     * paths.)
     */
    if (PATH_PARAM_BY_REL(outer_path, innerrel) ||
        PATH_PARAM_BY_REL(inner_path, outerrel))
        return;

    /*
     * If unique-ification is requested, do it and then handle as a plain
     * inner join.
     */
    if (jointype == JOIN_UNIQUE_OUTER)
    {
        outer_path = (Path *) create_unique_path(root, outerrel,
                                                 outer_path, extra->sjinfo);
        Assert(outer_path);
        jointype = JOIN_INNER;
    }
    else if (jointype == JOIN_UNIQUE_INNER)
    {
        inner_path = (Path *) create_unique_path(root, innerrel,
                                                 inner_path, extra->sjinfo);
        Assert(inner_path);
        jointype = JOIN_INNER;
    }

    /*
     * If the joinrel is parallel-safe, we may be able to consider a partial
     * merge join.  However, we can't handle JOIN_UNIQUE_OUTER, because the
     * outer path will be partial, and therefore we won't be able to properly
     * guarantee uniqueness.  Similarly, we can't handle JOIN_FULL and
     * JOIN_RIGHT, because they can produce false null extended rows.  Also,
     * the resulting path must not be parameterized.
     */
    if (joinrel->consider_parallel &&
        save_jointype != JOIN_UNIQUE_OUTER &&
        save_jointype != JOIN_FULL &&
        save_jointype != JOIN_RIGHT &&
        outerrel->partial_pathlist != NIL &&
        bms_is_empty(joinrel->lateral_relids))
    {
        cheapest_partial_outer = (Path *) linitial(outerrel->partial_pathlist);

        if (inner_path->parallel_safe)
            cheapest_safe_inner = inner_path;
        else if (save_jointype != JOIN_UNIQUE_INNER)
            cheapest_safe_inner =
                get_cheapest_parallel_safe_total_inner(innerrel->pathlist);
    }

    /*
     * Each possible ordering of the available mergejoin clauses will generate
     * a differently-sorted result path at essentially the same cost.  We have
     * no basis for choosing one over another at this level of joining, but
     * some sort orders may be more useful than others for higher-level
     * mergejoins, so it's worth considering multiple orderings.
     *
     * Actually, it's not quite true that every mergeclause ordering will
     * generate a different path order, because some of the clauses may be
     * partially redundant (refer to the same EquivalenceClasses).  Therefore,
     * what we do is convert the mergeclause list to a list of canonical
     * pathkeys, and then consider different orderings of the pathkeys.
     *
     * Generating a path for *every* permutation of the pathkeys doesn't seem
     * like a winning strategy; the cost in planning time is too high. For
     * now, we generate one path for each pathkey, listing that pathkey first
     * and the rest in random order.  This should allow at least a one-clause
     * mergejoin without re-sorting against any other possible mergejoin
     * partner path.  But if we've not guessed the right ordering of secondary
     * keys, we may end up evaluating clauses as qpquals when they could have
     * been done as mergeclauses.  (In practice, it's rare that there's more
     * than two or three mergeclauses, so expending a huge amount of thought
     * on that is probably not worth it.)
     *
     * The pathkey order returned by select_outer_pathkeys_for_merge() has
     * some heuristics behind it (see that function), so be sure to try it
     * exactly as-is as well as making variants.
     */
    all_pathkeys = select_outer_pathkeys_for_merge(root,
                                                   extra->mergeclause_list,
                                                   joinrel);

    foreach(l, all_pathkeys)
    {
        List       *front_pathkey = (List *) lfirst(l);
        List       *cur_mergeclauses;
        List       *outerkeys;
        List       *innerkeys;
        List       *merge_pathkeys;

        /* Make a pathkey list with this guy first */
        if (l != list_head(all_pathkeys))
            outerkeys = lcons(front_pathkey,
                              list_delete_nth_cell(list_copy(all_pathkeys),
                                                   foreach_current_index(l)));
        else
            outerkeys = all_pathkeys;   /* no work at first one... */

        /* Sort the mergeclauses into the corresponding ordering */
        cur_mergeclauses =
            find_mergeclauses_for_outer_pathkeys(root,
                                                 outerkeys,
                                                 extra->mergeclause_list);

        /* Should have used them all... */
        Assert(list_length(cur_mergeclauses) == list_length(extra->mergeclause_list));

        /* Build sort pathkeys for the inner side */
        innerkeys = make_inner_pathkeys_for_merge(root,
                                                  cur_mergeclauses,
                                                  outerkeys);

        /* Build pathkeys representing output sort order */
        merge_pathkeys = build_join_pathkeys(root, joinrel, jointype,
                                             outerkeys);

        /*
         * And now we can make the path.
         *
         * Note: it's possible that the cheapest paths will already be sorted
         * properly.  try_mergejoin_path will detect that case and suppress an
         * explicit sort step, so we needn't do so here.
         */
        try_mergejoin_path(root,
                           joinrel,
                           outer_path,
                           inner_path,
                           merge_pathkeys,
                           cur_mergeclauses,
                           outerkeys,
                           innerkeys,
                           jointype,
                           extra,
                           false);

        /*
         * If we have partial outer and parallel safe inner path then try
         * partial mergejoin path.
         */
        if (cheapest_partial_outer && cheapest_safe_inner)
            try_partial_mergejoin_path(root,
                                       joinrel,
                                       cheapest_partial_outer,
                                       cheapest_safe_inner,
                                       merge_pathkeys,
                                       cur_mergeclauses,
                                       outerkeys,
                                       innerkeys,
                                       jointype,
                                       extra);
    }
}

/*
 * generate_mergejoin_paths
 *  Creates possible mergejoin paths for input outerpath.
 *
 * We generate mergejoins if mergejoin clauses are available.  We have
 * two ways to generate the inner path for a mergejoin: sort the cheapest
 * inner path, or use an inner path that is already suitably ordered for the
 * merge.  If we have several mergeclauses, it could be that there is no inner
 * path (or only a very expensive one) for the full list of mergeclauses, but
 * better paths exist if we truncate the mergeclause list (thereby discarding
 * some sort key requirements).  So, we consider truncations of the
 * mergeclause list as well as the full list.  (Ideally we'd consider all
 * subsets of the mergeclause list, but that seems way too expensive.)
 */
static void
generate_mergejoin_paths(PlannerInfo *root,
                         RelOptInfo *joinrel,
                         RelOptInfo *innerrel,
                         Path *outerpath,
                         JoinType jointype,
                         JoinPathExtraData *extra,
                         bool useallclauses,
                         Path *inner_cheapest_total,
                         List *merge_pathkeys,
                         bool is_partial)
{
    List       *mergeclauses;
    List       *innersortkeys;
    List       *trialsortkeys;
    Path       *cheapest_startup_inner;
    Path       *cheapest_total_inner;
    JoinType    save_jointype = jointype;
    int         num_sortkeys;
    int         sortkeycnt;

    if (jointype == JOIN_UNIQUE_OUTER || jointype == JOIN_UNIQUE_INNER)
        jointype = JOIN_INNER;

    /* Look for useful mergeclauses (if any) */
    mergeclauses =
        find_mergeclauses_for_outer_pathkeys(root,
                                             outerpath->pathkeys,
                                             extra->mergeclause_list);

    /*
     * Done with this outer path if no chance for a mergejoin.
     *
     * Special corner case: for "x FULL JOIN y ON true", there will be no join
     * clauses at all.  Ordinarily we'd generate a clauseless nestloop path,
     * but since mergejoin is our only join type that supports FULL JOIN
     * without any join clauses, it's necessary to generate a clauseless
     * mergejoin path instead.
     */
    if (mergeclauses == NIL)
    {
        if (jointype == JOIN_FULL)
             /* okay to try for mergejoin */ ;
        else
            return;
    }
    if (useallclauses &&
        list_length(mergeclauses) != list_length(extra->mergeclause_list))
        return;

    /* Compute the required ordering of the inner path */
    innersortkeys = make_inner_pathkeys_for_merge(root,
                                                  mergeclauses,
                                                  outerpath->pathkeys);

    /*
     * Generate a mergejoin on the basis of sorting the cheapest inner. Since
     * a sort will be needed, only cheapest total cost matters. (But
     * try_mergejoin_path will do the right thing if inner_cheapest_total is
     * already correctly sorted.)
     */
    try_mergejoin_path(root,
                       joinrel,
                       outerpath,
                       inner_cheapest_total,
                       merge_pathkeys,
                       mergeclauses,
                       NIL,
                       innersortkeys,
                       jointype,
                       extra,
                       is_partial);

    /* Can't do anything else if inner path needs to be unique'd */
    if (save_jointype == JOIN_UNIQUE_INNER)
        return;

    /*
     * Look for presorted inner paths that satisfy the innersortkey list ---
     * or any truncation thereof, if we are allowed to build a mergejoin using
     * a subset of the merge clauses.  Here, we consider both cheap startup
     * cost and cheap total cost.
     *
     * Currently we do not consider parameterized inner paths here. This
     * interacts with decisions elsewhere that also discriminate against
     * mergejoins with parameterized inputs; see comments in
     * src/backend/optimizer/README.
     *
     * As we shorten the sortkey list, we should consider only paths that are
     * strictly cheaper than (in particular, not the same as) any path found
     * in an earlier iteration.  Otherwise we'd be intentionally using fewer
     * merge keys than a given path allows (treating the rest as plain
     * joinquals), which is unlikely to be a good idea.  Also, eliminating
     * paths here on the basis of compare_path_costs is a lot cheaper than
     * building the mergejoin path only to throw it away.
     *
     * If inner_cheapest_total is well enough sorted to have not required a
     * sort in the path made above, we shouldn't make a duplicate path with
     * it, either.  We handle that case with the same logic that handles the
     * previous consideration, by initializing the variables that track
     * cheapest-so-far properly.  Note that we do NOT reject
     * inner_cheapest_total if we find it matches some shorter set of
     * pathkeys.  That case corresponds to using fewer mergekeys to avoid
     * sorting inner_cheapest_total, whereas we did sort it above, so the
     * plans being considered are different.
     */
    if (pathkeys_contained_in(innersortkeys,
                              inner_cheapest_total->pathkeys))
    {
        /* inner_cheapest_total didn't require a sort */
        cheapest_startup_inner = inner_cheapest_total;
        cheapest_total_inner = inner_cheapest_total;
    }
    else
    {
        /* it did require a sort, at least for the full set of keys */
        cheapest_startup_inner = NULL;
        cheapest_total_inner = NULL;
    }
    num_sortkeys = list_length(innersortkeys);
    if (num_sortkeys > 1 && !useallclauses)
        trialsortkeys = list_copy(innersortkeys);   /* need modifiable copy */
    else
        trialsortkeys = innersortkeys;  /* won't really truncate */

    for (sortkeycnt = num_sortkeys; sortkeycnt > 0; sortkeycnt--)
    {
        Path       *innerpath;
        List       *newclauses = NIL;

        /*
         * Look for an inner path ordered well enough for the first
         * 'sortkeycnt' innersortkeys.  NB: trialsortkeys list is modified
         * destructively, which is why we made a copy...
         */
        trialsortkeys = list_truncate(trialsortkeys, sortkeycnt);
        innerpath = get_cheapest_path_for_pathkeys(innerrel->pathlist,
                                                   trialsortkeys,
                                                   NULL,
                                                   TOTAL_COST,
                                                   is_partial);
        if (innerpath != NULL &&
            (cheapest_total_inner == NULL ||
             compare_path_costs(innerpath, cheapest_total_inner,
                                TOTAL_COST) < 0))
        {
            /* Found a cheap (or even-cheaper) sorted path */
            /* Select the right mergeclauses, if we didn't already */
            if (sortkeycnt < num_sortkeys)
            {
                newclauses =
                    trim_mergeclauses_for_inner_pathkeys(root,
                                                         mergeclauses,
                                                         trialsortkeys);
                Assert(newclauses != NIL);
            }
            else
                newclauses = mergeclauses;
            try_mergejoin_path(root,
                               joinrel,
                               outerpath,
                               innerpath,
                               merge_pathkeys,
                               newclauses,
                               NIL,
                               NIL,
                               jointype,
                               extra,
                               is_partial);
            cheapest_total_inner = innerpath;
        }
        /* Same on the basis of cheapest startup cost ... */
        innerpath = get_cheapest_path_for_pathkeys(innerrel->pathlist,
                                                   trialsortkeys,
                                                   NULL,
                                                   STARTUP_COST,
                                                   is_partial);
        if (innerpath != NULL &&
            (cheapest_startup_inner == NULL ||
             compare_path_costs(innerpath, cheapest_startup_inner,
                                STARTUP_COST) < 0))
        {
            /* Found a cheap (or even-cheaper) sorted path */
            if (innerpath != cheapest_total_inner)
            {
                /*
                 * Avoid rebuilding clause list if we already made one; saves
                 * memory in big join trees...
                 */
                if (newclauses == NIL)
                {
                    if (sortkeycnt < num_sortkeys)
                    {
                        newclauses =
                            trim_mergeclauses_for_inner_pathkeys(root,
                                                                 mergeclauses,
                                                                 trialsortkeys);
                        Assert(newclauses != NIL);
                    }
                    else
                        newclauses = mergeclauses;
                }
                try_mergejoin_path(root,
                                   joinrel,
                                   outerpath,
                                   innerpath,
                                   merge_pathkeys,
                                   newclauses,
                                   NIL,
                                   NIL,
                                   jointype,
                                   extra,
                                   is_partial);
            }
            cheapest_startup_inner = innerpath;
        }

        /*
         * Don't consider truncated sortkeys if we need all clauses.
         */
        if (useallclauses)
            break;
    }
}

/*
 * match_unsorted_outer
 *    Creates possible join paths for processing a single join relation
 *    'joinrel' by employing either iterative substitution or
 *    mergejoining on each of its possible outer paths (considering
 *    only outer paths that are already ordered well enough for merging).
 *
 * We always generate a nestloop path for each available outer path.
 * In fact we may generate as many as five: one on the cheapest-total-cost
 * inner path, one on the same with materialization, one on the
 * cheapest-startup-cost inner path (if different), one on the
 * cheapest-total inner-indexscan path (if any), and one on the
 * cheapest-startup inner-indexscan path (if different).
 *
 * We also consider mergejoins if mergejoin clauses are available.  See
 * detailed comments in generate_mergejoin_paths.
 *
 * 'joinrel' is the join relation
 * 'outerrel' is the outer join relation
 * 'innerrel' is the inner join relation
 * 'jointype' is the type of join to do
 * 'extra' contains additional input values
 */
static void
match_unsorted_outer(PlannerInfo *root,
                     RelOptInfo *joinrel,
                     RelOptInfo *outerrel,
                     RelOptInfo *innerrel,
                     JoinType jointype,
                     JoinPathExtraData *extra)
{
    JoinType    save_jointype = jointype;
    bool        nestjoinOK;
    bool        useallclauses;
    Path       *inner_cheapest_total = innerrel->cheapest_total_path;
    Path       *matpath = NULL;
    ListCell   *lc1;

    /*
     * Nestloop only supports inner, left, semi, and anti joins.  Also, if we
     * are doing a right or full mergejoin, we must use *all* the mergeclauses
     * as join clauses, else we will not have a valid plan.  (Although these
     * two flags are currently inverses, keep them separate for clarity and
     * possible future changes.)
     */
    switch (jointype)
    {
        case JOIN_INNER:
        case JOIN_LEFT:
        case JOIN_SEMI:
        case JOIN_ANTI:
            nestjoinOK = true;
            useallclauses = false;
            break;
        case JOIN_RIGHT:
        case JOIN_FULL:
            nestjoinOK = false;
            useallclauses = true;
            break;
        case JOIN_UNIQUE_OUTER:
        case JOIN_UNIQUE_INNER:
            jointype = JOIN_INNER;
            nestjoinOK = true;
            useallclauses = false;
            break;
        default:
            elog(ERROR, "unrecognized join type: %d",
                 (int) jointype);
            nestjoinOK = false; /* keep compiler quiet */
            useallclauses = false;
            break;
    }

    /*
     * If inner_cheapest_total is parameterized by the outer rel, ignore it;
     * we will consider it below as a member of cheapest_parameterized_paths,
     * but the other possibilities considered in this routine aren't usable.
     */
    if (PATH_PARAM_BY_REL(inner_cheapest_total, outerrel))
        inner_cheapest_total = NULL;

    /*
     * If we need to unique-ify the inner path, we will consider only the
     * cheapest-total inner.
     */
    if (save_jointype == JOIN_UNIQUE_INNER)
    {
        /* No way to do this with an inner path parameterized by outer rel */
        if (inner_cheapest_total == NULL)
            return;
        inner_cheapest_total = (Path *)
            create_unique_path(root, innerrel, inner_cheapest_total, extra->sjinfo);
        Assert(inner_cheapest_total);
    }
    else if (nestjoinOK)
    {
        /*
         * Consider materializing the cheapest inner path, unless
         * enable_material is off or the path in question materializes its
         * output anyway.
         */
        if (enable_material && inner_cheapest_total != NULL &&
            !ExecMaterializesOutput(inner_cheapest_total->pathtype))
            matpath = (Path *)
                create_material_path(innerrel, inner_cheapest_total);
    }

    foreach(lc1, outerrel->pathlist)
    {
        Path       *outerpath = (Path *) lfirst(lc1);
        List       *merge_pathkeys;

        /*
         * We cannot use an outer path that is parameterized by the inner rel.
         */
        if (PATH_PARAM_BY_REL(outerpath, innerrel))
            continue;

        /*
         * If we need to unique-ify the outer path, it's pointless to consider
         * any but the cheapest outer.  (XXX we don't consider parameterized
         * outers, nor inners, for unique-ified cases.  Should we?)
         */
        if (save_jointype == JOIN_UNIQUE_OUTER)
        {
            if (outerpath != outerrel->cheapest_total_path)
                continue;
            outerpath = (Path *) create_unique_path(root, outerrel,
                                                    outerpath, extra->sjinfo);
            Assert(outerpath);
        }

        /*
         * The result will have this sort order (even if it is implemented as
         * a nestloop, and even if some of the mergeclauses are implemented by
         * qpquals rather than as true mergeclauses):
         */
        merge_pathkeys = build_join_pathkeys(root, joinrel, jointype,
                                             outerpath->pathkeys);

        if (save_jointype == JOIN_UNIQUE_INNER)
        {
            /*
             * Consider nestloop join, but only with the unique-ified cheapest
             * inner path
             */
            try_nestloop_path(root,
                              joinrel,
                              outerpath,
                              inner_cheapest_total,
                              merge_pathkeys,
                              jointype,
                              extra);
        }
        else if (nestjoinOK)
        {
            /*
             * Consider nestloop joins using this outer path and various
             * available paths for the inner relation.  We consider the
             * cheapest-total paths for each available parameterization of the
             * inner relation, including the unparameterized case.
             */
            ListCell   *lc2;

            foreach(lc2, innerrel->cheapest_parameterized_paths)
            {
                Path       *innerpath = (Path *) lfirst(lc2);
                Path       *mpath;

                try_nestloop_path(root,
                                  joinrel,
                                  outerpath,
                                  innerpath,
                                  merge_pathkeys,
                                  jointype,
                                  extra);

                /*
                 * Try generating a memoize path and see if that makes the
                 * nested loop any cheaper.
                 */
                mpath = get_memoize_path(root, innerrel, outerrel,
                                         innerpath, outerpath, jointype,
                                         extra);
                if (mpath != NULL)
                    try_nestloop_path(root,
                                      joinrel,
                                      outerpath,
                                      mpath,
                                      merge_pathkeys,
                                      jointype,
                                      extra);
            }

            /* Also consider materialized form of the cheapest inner path */
            if (matpath != NULL)
                try_nestloop_path(root,
                                  joinrel,
                                  outerpath,
                                  matpath,
                                  merge_pathkeys,
                                  jointype,
                                  extra);
        }

        /* Can't do anything else if outer path needs to be unique'd */
        if (save_jointype == JOIN_UNIQUE_OUTER)
            continue;

        /* Can't do anything else if inner rel is parameterized by outer */
        if (inner_cheapest_total == NULL)
            continue;

        /* Generate merge join paths */
        generate_mergejoin_paths(root, joinrel, innerrel, outerpath,
                                 save_jointype, extra, useallclauses,
                                 inner_cheapest_total, merge_pathkeys,
                                 false);
    }

    /*
     * Consider partial nestloop and mergejoin plan if outerrel has any
     * partial path and the joinrel is parallel-safe.  However, we can't
     * handle JOIN_UNIQUE_OUTER, because the outer path will be partial, and
     * therefore we won't be able to properly guarantee uniqueness.  Nor can
     * we handle joins needing lateral rels, since partial paths must not be
     * parameterized. Similarly, we can't handle JOIN_FULL and JOIN_RIGHT,
     * because they can produce false null extended rows.
     */
    if (joinrel->consider_parallel &&
        save_jointype != JOIN_UNIQUE_OUTER &&
        save_jointype != JOIN_FULL &&
        save_jointype != JOIN_RIGHT &&
        outerrel->partial_pathlist != NIL &&
        bms_is_empty(joinrel->lateral_relids))
    {
        if (nestjoinOK)
            consider_parallel_nestloop(root, joinrel, outerrel, innerrel,
                                       save_jointype, extra);

        /*
         * If inner_cheapest_total is NULL or non parallel-safe then find the
         * cheapest total parallel safe path.  If doing JOIN_UNIQUE_INNER, we
         * can't use any alternative inner path.
         */
        if (inner_cheapest_total == NULL ||
            !inner_cheapest_total->parallel_safe)
        {
            if (save_jointype == JOIN_UNIQUE_INNER)
                return;

            inner_cheapest_total = get_cheapest_parallel_safe_total_inner(innerrel->pathlist);
        }

        if (inner_cheapest_total)
            consider_parallel_mergejoin(root, joinrel, outerrel, innerrel,
                                        save_jointype, extra,
                                        inner_cheapest_total);
    }
}

/*
 * consider_parallel_mergejoin
 *    Try to build partial paths for a joinrel by joining a partial path
 *    for the outer relation to a complete path for the inner relation.
 *
 * 'joinrel' is the join relation
 * 'outerrel' is the outer join relation
 * 'innerrel' is the inner join relation
 * 'jointype' is the type of join to do
 * 'extra' contains additional input values
 * 'inner_cheapest_total' cheapest total path for innerrel
 */
static void
consider_parallel_mergejoin(PlannerInfo *root,
                            RelOptInfo *joinrel,
                            RelOptInfo *outerrel,
                            RelOptInfo *innerrel,
                            JoinType jointype,
                            JoinPathExtraData *extra,
                            Path *inner_cheapest_total)
{
    ListCell   *lc1;

    /* generate merge join path for each partial outer path */
    foreach(lc1, outerrel->partial_pathlist)
    {
        Path       *outerpath = (Path *) lfirst(lc1);
        List       *merge_pathkeys;

        /*
         * Figure out what useful ordering any paths we create will have.
         */
        merge_pathkeys = build_join_pathkeys(root, joinrel, jointype,
                                             outerpath->pathkeys);

        generate_mergejoin_paths(root, joinrel, innerrel, outerpath, jointype,
                                 extra, false, inner_cheapest_total,
                                 merge_pathkeys, true);
    }
}

/*
 * consider_parallel_nestloop
 *    Try to build partial paths for a joinrel by joining a partial path for the
 *    outer relation to a complete path for the inner relation.
 *
 * 'joinrel' is the join relation
 * 'outerrel' is the outer join relation
 * 'innerrel' is the inner join relation
 * 'jointype' is the type of join to do
 * 'extra' contains additional input values
 */
static void
consider_parallel_nestloop(PlannerInfo *root,
                           RelOptInfo *joinrel,
                           RelOptInfo *outerrel,
                           RelOptInfo *innerrel,
                           JoinType jointype,
                           JoinPathExtraData *extra)
{
    JoinType    save_jointype = jointype;
    ListCell   *lc1;

    if (jointype == JOIN_UNIQUE_INNER)
        jointype = JOIN_INNER;

    foreach(lc1, outerrel->partial_pathlist)
    {
        Path       *outerpath = (Path *) lfirst(lc1);
        List       *pathkeys;
        ListCell   *lc2;

        /* Figure out what useful ordering any paths we create will have. */
        pathkeys = build_join_pathkeys(root, joinrel, jointype,
                                       outerpath->pathkeys);

        /*
         * Try the cheapest parameterized paths; only those which will produce
         * an unparameterized path when joined to this outerrel will survive
         * try_partial_nestloop_path.  The cheapest unparameterized path is
         * also in this list.
         */
        foreach(lc2, innerrel->cheapest_parameterized_paths)
        {
            Path       *innerpath = (Path *) lfirst(lc2);
            Path       *mpath;

            /* Can't join to an inner path that is not parallel-safe */
            if (!innerpath->parallel_safe)
                continue;

            /*
             * If we're doing JOIN_UNIQUE_INNER, we can only use the inner's
             * cheapest_total_path, and we have to unique-ify it.  (We might
             * be able to relax this to allow other safe, unparameterized
             * inner paths, but right now create_unique_path is not on board
             * with that.)
             */
            if (save_jointype == JOIN_UNIQUE_INNER)
            {
                if (innerpath != innerrel->cheapest_total_path)
                    continue;
                innerpath = (Path *) create_unique_path(root, innerrel,
                                                        innerpath,
                                                        extra->sjinfo);
                Assert(innerpath);
            }

            try_partial_nestloop_path(root, joinrel, outerpath, innerpath,
                                      pathkeys, jointype, extra);

            /*
             * Try generating a memoize path and see if that makes the nested
             * loop any cheaper.
             */
            mpath = get_memoize_path(root, innerrel, outerrel,
                                     innerpath, outerpath, jointype,
                                     extra);
            if (mpath != NULL)
                try_partial_nestloop_path(root, joinrel, outerpath, mpath,
                                          pathkeys, jointype, extra);
        }
    }
}

/*
 * hash_inner_and_outer
 *    Create hashjoin join paths by explicitly hashing both the outer and
 *    inner keys of each available hash clause.
 *
 * 'joinrel' is the join relation
 * 'outerrel' is the outer join relation
 * 'innerrel' is the inner join relation
 * 'jointype' is the type of join to do
 * 'extra' contains additional input values
 */
static void
hash_inner_and_outer(PlannerInfo *root,
                     RelOptInfo *joinrel,
                     RelOptInfo *outerrel,
                     RelOptInfo *innerrel,
                     JoinType jointype,
                     JoinPathExtraData *extra)
{
    JoinType    save_jointype = jointype;
    bool        isouterjoin = IS_OUTER_JOIN(jointype);
    List       *hashclauses;
    ListCell   *l;

    /*
     * We need to build only one hashclauses list for any given pair of outer
     * and inner relations; all of the hashable clauses will be used as keys.
     *
     * Scan the join's restrictinfo list to find hashjoinable clauses that are
     * usable with this pair of sub-relations.
     */
    hashclauses = NIL;
    foreach(l, extra->restrictlist)
    {
        RestrictInfo *restrictinfo = (RestrictInfo *) lfirst(l);

        /*
         * If processing an outer join, only use its own join clauses for
         * hashing.  For inner joins we need not be so picky.
         */
        if (isouterjoin && RINFO_IS_PUSHED_DOWN(restrictinfo, joinrel->relids))
            continue;

        if (!restrictinfo->can_join ||
            restrictinfo->hashjoinoperator == InvalidOid)
            continue;           /* not hashjoinable */

        /*
         * Check if clause has the form "outer op inner" or "inner op outer".
         */
        if (!clause_sides_match_join(restrictinfo, outerrel, innerrel))
            continue;           /* no good for these input relations */

        hashclauses = lappend(hashclauses, restrictinfo);
    }

    /* If we found any usable hashclauses, make paths */
    if (hashclauses)
    {
        /*
         * We consider both the cheapest-total-cost and cheapest-startup-cost
         * outer paths.  There's no need to consider any but the
         * cheapest-total-cost inner path, however.
         */
        Path       *cheapest_startup_outer = outerrel->cheapest_startup_path;
        Path       *cheapest_total_outer = outerrel->cheapest_total_path;
        Path       *cheapest_total_inner = innerrel->cheapest_total_path;

        /*
         * If either cheapest-total path is parameterized by the other rel, we
         * can't use a hashjoin.  (There's no use looking for alternative
         * input paths, since these should already be the least-parameterized
         * available paths.)
         */
        if (PATH_PARAM_BY_REL(cheapest_total_outer, innerrel) ||
            PATH_PARAM_BY_REL(cheapest_total_inner, outerrel))
            return;

        /* Unique-ify if need be; we ignore parameterized possibilities */
        if (jointype == JOIN_UNIQUE_OUTER)
        {
            cheapest_total_outer = (Path *)
                create_unique_path(root, outerrel,
                                   cheapest_total_outer, extra->sjinfo);
            Assert(cheapest_total_outer);
            jointype = JOIN_INNER;
            try_hashjoin_path(root,
                              joinrel,
                              cheapest_total_outer,
                              cheapest_total_inner,
                              hashclauses,
                              jointype,
                              extra);
            /* no possibility of cheap startup here */
        }
        else if (jointype == JOIN_UNIQUE_INNER)
        {
            cheapest_total_inner = (Path *)
                create_unique_path(root, innerrel,
                                   cheapest_total_inner, extra->sjinfo);
            Assert(cheapest_total_inner);
            jointype = JOIN_INNER;
            try_hashjoin_path(root,
                              joinrel,
                              cheapest_total_outer,
                              cheapest_total_inner,
                              hashclauses,
                              jointype,
                              extra);
            if (cheapest_startup_outer != NULL &&
                cheapest_startup_outer != cheapest_total_outer)
                try_hashjoin_path(root,
                                  joinrel,
                                  cheapest_startup_outer,
                                  cheapest_total_inner,
                                  hashclauses,
                                  jointype,
                                  extra);
        }
        else
        {
            /*
             * For other jointypes, we consider the cheapest startup outer
             * together with the cheapest total inner, and then consider
             * pairings of cheapest-total paths including parameterized ones.
             * There is no use in generating parameterized paths on the basis
             * of possibly cheap startup cost, so this is sufficient.
             */
            ListCell   *lc1;
            ListCell   *lc2;

            if (cheapest_startup_outer != NULL)
                try_hashjoin_path(root,
                                  joinrel,
                                  cheapest_startup_outer,
                                  cheapest_total_inner,
                                  hashclauses,
                                  jointype,
                                  extra);

            foreach(lc1, outerrel->cheapest_parameterized_paths)
            {
                Path       *outerpath = (Path *) lfirst(lc1);

                /*
                 * We cannot use an outer path that is parameterized by the
                 * inner rel.
                 */
                if (PATH_PARAM_BY_REL(outerpath, innerrel))
                    continue;

                foreach(lc2, innerrel->cheapest_parameterized_paths)
                {
                    Path       *innerpath = (Path *) lfirst(lc2);

                    /*
                     * We cannot use an inner path that is parameterized by
                     * the outer rel, either.
                     */
                    if (PATH_PARAM_BY_REL(innerpath, outerrel))
                        continue;

                    if (outerpath == cheapest_startup_outer &&
                        innerpath == cheapest_total_inner)
                        continue;   /* already tried it */

                    try_hashjoin_path(root,
                                      joinrel,
                                      outerpath,
                                      innerpath,
                                      hashclauses,
                                      jointype,
                                      extra);
                }
            }
        }

        /*
         * If the joinrel is parallel-safe, we may be able to consider a
         * partial hash join.  However, we can't handle JOIN_UNIQUE_OUTER,
         * because the outer path will be partial, and therefore we won't be
         * able to properly guarantee uniqueness.  Similarly, we can't handle
         * JOIN_FULL and JOIN_RIGHT, because they can produce false null
         * extended rows.  Also, the resulting path must not be parameterized.
         * We would be able to support JOIN_FULL and JOIN_RIGHT for Parallel
         * Hash, since in that case we're back to a single hash table with a
         * single set of match bits for each batch, but that will require
         * figuring out a deadlock-free way to wait for the probe to finish.
         */
        if (joinrel->consider_parallel &&
            save_jointype != JOIN_UNIQUE_OUTER &&
            save_jointype != JOIN_FULL &&
            save_jointype != JOIN_RIGHT &&
            outerrel->partial_pathlist != NIL &&
            bms_is_empty(joinrel->lateral_relids))
        {
            Path       *cheapest_partial_outer;
            Path       *cheapest_partial_inner = NULL;
            Path       *cheapest_safe_inner = NULL;

            cheapest_partial_outer =
                (Path *) linitial(outerrel->partial_pathlist);

            /*
             * Can we use a partial inner plan too, so that we can build a
             * shared hash table in parallel?  We can't handle
             * JOIN_UNIQUE_INNER because we can't guarantee uniqueness.
             */
            if (innerrel->partial_pathlist != NIL &&
                save_jointype != JOIN_UNIQUE_INNER &&
                enable_parallel_hash)
            {
                cheapest_partial_inner =
                    (Path *) linitial(innerrel->partial_pathlist);
                try_partial_hashjoin_path(root, joinrel,
                                          cheapest_partial_outer,
                                          cheapest_partial_inner,
                                          hashclauses, jointype, extra,
                                          true /* parallel_hash */ );
            }

            /*
             * Normally, given that the joinrel is parallel-safe, the cheapest
             * total inner path will also be parallel-safe, but if not, we'll
             * have to search for the cheapest safe, unparameterized inner
             * path.  If doing JOIN_UNIQUE_INNER, we can't use any alternative
             * inner path.
             */
            if (cheapest_total_inner->parallel_safe)
                cheapest_safe_inner = cheapest_total_inner;
            else if (save_jointype != JOIN_UNIQUE_INNER)
                cheapest_safe_inner =
                    get_cheapest_parallel_safe_total_inner(innerrel->pathlist);

            if (cheapest_safe_inner != NULL)
                try_partial_hashjoin_path(root, joinrel,
                                          cheapest_partial_outer,
                                          cheapest_safe_inner,
                                          hashclauses, jointype, extra,
                                          false /* parallel_hash */ );
        }
    }
}

/*
 * select_mergejoin_clauses
 *    Select mergejoin clauses that are usable for a particular join.
 *    Returns a list of RestrictInfo nodes for those clauses.
 *
 * *mergejoin_allowed is normally set to true, but it is set to false if
 * this is a right/full join and there are nonmergejoinable join clauses.
 * The executor's mergejoin machinery cannot handle such cases, so we have
 * to avoid generating a mergejoin plan.  (Note that this flag does NOT
 * consider whether there are actually any mergejoinable clauses.  This is
 * correct because in some cases we need to build a clauseless mergejoin.
 * Simply returning NIL is therefore not enough to distinguish safe from
 * unsafe cases.)
 *
 * We also mark each selected RestrictInfo to show which side is currently
 * being considered as outer.  These are transient markings that are only
 * good for the duration of the current add_paths_to_joinrel() call!
 *
 * We examine each restrictinfo clause known for the join to see
 * if it is mergejoinable and involves vars from the two sub-relations
 * currently of interest.
 */
static List *
select_mergejoin_clauses(PlannerInfo *root,
                         RelOptInfo *joinrel,
                         RelOptInfo *outerrel,
                         RelOptInfo *innerrel,
                         List *restrictlist,
                         JoinType jointype,
                         bool *mergejoin_allowed)
{
    List       *result_list = NIL;
    bool        isouterjoin = IS_OUTER_JOIN(jointype);
    bool        have_nonmergeable_joinclause = false;
    ListCell   *l;

    foreach(l, restrictlist)
    {
        RestrictInfo *restrictinfo = (RestrictInfo *) lfirst(l);

        /*
         * If processing an outer join, only use its own join clauses in the
         * merge.  For inner joins we can use pushed-down clauses too. (Note:
         * we don't set have_nonmergeable_joinclause here because pushed-down
         * clauses will become otherquals not joinquals.)
         */
        if (isouterjoin && RINFO_IS_PUSHED_DOWN(restrictinfo, joinrel->relids))
            continue;

        /* Check that clause is a mergeable operator clause */
        if (!restrictinfo->can_join ||
            restrictinfo->mergeopfamilies == NIL)
        {
            /*
             * The executor can handle extra joinquals that are constants, but
             * not anything else, when doing right/full merge join.  (The
             * reason to support constants is so we can do FULL JOIN ON
             * FALSE.)
             */
            if (!restrictinfo->clause || !IsA(restrictinfo->clause, Const))
                have_nonmergeable_joinclause = true;
            continue;           /* not mergejoinable */
        }

        /*
         * Check if clause has the form "outer op inner" or "inner op outer".
         */
        if (!clause_sides_match_join(restrictinfo, outerrel, innerrel))
        {
            have_nonmergeable_joinclause = true;
            continue;           /* no good for these input relations */
        }

        /*
         * Insist that each side have a non-redundant eclass.  This
         * restriction is needed because various bits of the planner expect
         * that each clause in a merge be associable with some pathkey in a
         * canonical pathkey list, but redundant eclasses can't appear in
         * canonical sort orderings.  (XXX it might be worth relaxing this,
         * but not enough time to address it for 8.3.)
         *
         * Note: it would be bad if this condition failed for an otherwise
         * mergejoinable FULL JOIN clause, since that would result in
         * undesirable planner failure.  I believe that is not possible
         * however; a variable involved in a full join could only appear in
         * below_outer_join eclasses, which aren't considered redundant.
         *
         * This case *can* happen for left/right join clauses: the outer-side
         * variable could be equated to a constant.  Because we will propagate
         * that constant across the join clause, the loss of ability to do a
         * mergejoin is not really all that big a deal, and so it's not clear
         * that improving this is important.
         */
        update_mergeclause_eclasses(root, restrictinfo);

        if (EC_MUST_BE_REDUNDANT(restrictinfo->left_ec) ||
            EC_MUST_BE_REDUNDANT(restrictinfo->right_ec))
        {
            have_nonmergeable_joinclause = true;
            continue;           /* can't handle redundant eclasses */
        }

        result_list = lappend(result_list, restrictinfo);
    }

    /*
     * Report whether mergejoin is allowed (see comment at top of function).
     */
    switch (jointype)
    {
        case JOIN_RIGHT:
        case JOIN_FULL:
            *mergejoin_allowed = !have_nonmergeable_joinclause;
            break;
        default:
            *mergejoin_allowed = true;
            break;
    }

    return result_list;
}