analyzejoins.c

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/*-------------------------------------------------------------------------
 *
 * analyzejoins.c
 *    Routines for simplifying joins after initial query analysis
 *
 * While we do a great deal of join simplification in prep/prepjointree.c,
 * certain optimizations cannot be performed at that stage for lack of
 * detailed information about the query.  The routines here are invoked
 * after initsplan.c has done its work, and can do additional join removal
 * and simplification steps based on the information extracted.  The penalty
 * is that we have to work harder to clean up after ourselves when we modify
 * the query, since the derived data structures have to be updated too.
 *
 * Portions Copyright (c) 1996-2018, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *    src/backend/optimizer/plan/analyzejoins.c
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"

#include "nodes/nodeFuncs.h"
#include "optimizer/clauses.h"
#include "optimizer/joininfo.h"
#include "optimizer/pathnode.h"
#include "optimizer/paths.h"
#include "optimizer/planmain.h"
#include "optimizer/tlist.h"
#include "optimizer/var.h"
#include "utils/lsyscache.h"

/* local functions */
static bool join_is_removable(PlannerInfo *root, SpecialJoinInfo *sjinfo);
static void remove_rel_from_query(PlannerInfo *root, int relid,
                      Relids joinrelids);
static List *remove_rel_from_joinlist(List *joinlist, int relid, int *nremoved);
static bool rel_supports_distinctness(PlannerInfo *root, RelOptInfo *rel);
static bool rel_is_distinct_for(PlannerInfo *root, RelOptInfo *rel,
                    List *clause_list);
static Oid  distinct_col_search(int colno, List *colnos, List *opids);
static bool is_innerrel_unique_for(PlannerInfo *root,
                       Relids joinrelids,
                       Relids outerrelids,
                       RelOptInfo *innerrel,
                       JoinType jointype,
                       List *restrictlist);


/*
 * remove_useless_joins
 *      Check for relations that don't actually need to be joined at all,
 *      and remove them from the query.
 *
 * We are passed the current joinlist and return the updated list.  Other
 * data structures that have to be updated are accessible via "root".
 */
List *
remove_useless_joins(PlannerInfo *root, List *joinlist)
{
    ListCell   *lc;

    /*
     * We are only interested in relations that are left-joined to, so we can
     * scan the join_info_list to find them easily.
     */
restart:
    foreach(lc, root->join_info_list)
    {
        SpecialJoinInfo *sjinfo = (SpecialJoinInfo *) lfirst(lc);
        int         innerrelid;
        int         nremoved;

        /* Skip if not removable */
        if (!join_is_removable(root, sjinfo))
            continue;

        /*
         * Currently, join_is_removable can only succeed when the sjinfo's
         * righthand is a single baserel.  Remove that rel from the query and
         * joinlist.
         */
        innerrelid = bms_singleton_member(sjinfo->min_righthand);

        remove_rel_from_query(root, innerrelid,
                              bms_union(sjinfo->min_lefthand,
                                        sjinfo->min_righthand));

        /* We verify that exactly one reference gets removed from joinlist */
        nremoved = 0;
        joinlist = remove_rel_from_joinlist(joinlist, innerrelid, &nremoved);
        if (nremoved != 1)
            elog(ERROR, "failed to find relation %d in joinlist", innerrelid);

        /*
         * We can delete this SpecialJoinInfo from the list too, since it's no
         * longer of interest.
         */
        root->join_info_list = list_delete_ptr(root->join_info_list, sjinfo);

        /*
         * Restart the scan.  This is necessary to ensure we find all
         * removable joins independently of ordering of the join_info_list
         * (note that removal of attr_needed bits may make a join appear
         * removable that did not before).  Also, since we just deleted the
         * current list cell, we'd have to have some kluge to continue the
         * list scan anyway.
         */
        goto restart;
    }

    return joinlist;
}

/*
 * 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, Relids outerrelids,
                        Relids innerrelids)
{
    if (bms_is_subset(rinfo->left_relids, outerrelids) &&
        bms_is_subset(rinfo->right_relids, innerrelids))
    {
        /* lefthand side is outer */
        rinfo->outer_is_left = true;
        return true;
    }
    else if (bms_is_subset(rinfo->left_relids, innerrelids) &&
             bms_is_subset(rinfo->right_relids, outerrelids))
    {
        /* righthand side is outer */
        rinfo->outer_is_left = false;
        return true;
    }
    return false;               /* no good for these input relations */
}

/*
 * join_is_removable
 *    Check whether we need not perform this special join at all, because
 *    it will just duplicate its left input.
 *
 * This is true for a left join for which the join condition cannot match
 * more than one inner-side row.  (There are other possibly interesting
 * cases, but we don't have the infrastructure to prove them.)  We also
 * have to check that the inner side doesn't generate any variables needed
 * above the join.
 */
static bool
join_is_removable(PlannerInfo *root, SpecialJoinInfo *sjinfo)
{
    int         innerrelid;
    RelOptInfo *innerrel;
    Relids      joinrelids;
    List       *clause_list = NIL;
    ListCell   *l;
    int         attroff;

    /*
     * Must be a non-delaying left join to a single baserel, else we aren't
     * going to be able to do anything with it.
     */
    if (sjinfo->jointype != JOIN_LEFT ||
        sjinfo->delay_upper_joins)
        return false;

    if (!bms_get_singleton_member(sjinfo->min_righthand, &innerrelid))
        return false;

    innerrel = find_base_rel(root, innerrelid);

    /*
     * Before we go to the effort of checking whether any innerrel variables
     * are needed above the join, make a quick check to eliminate cases in
     * which we will surely be unable to prove uniqueness of the innerrel.
     */
    if (!rel_supports_distinctness(root, innerrel))
        return false;

    /* Compute the relid set for the join we are considering */
    joinrelids = bms_union(sjinfo->min_lefthand, sjinfo->min_righthand);

    /*
     * We can't remove the join if any inner-rel attributes are used above the
     * join.
     *
     * Note that this test only detects use of inner-rel attributes in higher
     * join conditions and the target list.  There might be such attributes in
     * pushed-down conditions at this join, too.  We check that case below.
     *
     * As a micro-optimization, it seems better to start with max_attr and
     * count down rather than starting with min_attr and counting up, on the
     * theory that the system attributes are somewhat less likely to be wanted
     * and should be tested last.
     */
    for (attroff = innerrel->max_attr - innerrel->min_attr;
         attroff >= 0;
         attroff--)
    {
        if (!bms_is_subset(innerrel->attr_needed[attroff], joinrelids))
            return false;
    }

    /*
     * Similarly check that the inner rel isn't needed by any PlaceHolderVars
     * that will be used above the join.  We only need to fail if such a PHV
     * actually references some inner-rel attributes; but the correct check
     * for that is relatively expensive, so we first check against ph_eval_at,
     * which must mention the inner rel if the PHV uses any inner-rel attrs as
     * non-lateral references.  Note that if the PHV's syntactic scope is just
     * the inner rel, we can't drop the rel even if the PHV is variable-free.
     */
    foreach(l, root->placeholder_list)
    {
        PlaceHolderInfo *phinfo = (PlaceHolderInfo *) lfirst(l);

        if (bms_overlap(phinfo->ph_lateral, innerrel->relids))
            return false;       /* it references innerrel laterally */
        if (bms_is_subset(phinfo->ph_needed, joinrelids))
            continue;           /* PHV is not used above the join */
        if (!bms_overlap(phinfo->ph_eval_at, innerrel->relids))
            continue;           /* it definitely doesn't reference innerrel */
        if (bms_is_subset(phinfo->ph_eval_at, innerrel->relids))
            return false;       /* there isn't any other place to eval PHV */
        if (bms_overlap(pull_varnos((Node *) phinfo->ph_var->phexpr),
                        innerrel->relids))
            return false;       /* it does reference innerrel */
    }

    /*
     * Search for mergejoinable clauses that constrain the inner rel against
     * either the outer rel or a pseudoconstant.  If an operator is
     * mergejoinable then it behaves like equality for some btree opclass, so
     * it's what we want.  The mergejoinability test also eliminates clauses
     * containing volatile functions, which we couldn't depend on.
     */
    foreach(l, innerrel->joininfo)
    {
        RestrictInfo *restrictinfo = (RestrictInfo *) lfirst(l);

        /*
         * If it's not a join clause for this outer join, we can't use it.
         * Note that if the clause is pushed-down, then it is logically from
         * above the outer join, even if it references no other rels (it might
         * be from WHERE, for example).
         */
        if (RINFO_IS_PUSHED_DOWN(restrictinfo, joinrelids))
        {
            /*
             * If such a clause actually references the inner rel then join
             * removal has to be disallowed.  We have to check this despite
             * the previous attr_needed checks because of the possibility of
             * pushed-down clauses referencing the rel.
             */
            if (bms_is_member(innerrelid, restrictinfo->clause_relids))
                return false;
            continue;           /* else, ignore; not useful here */
        }

        /* Ignore if it's not a mergejoinable clause */
        if (!restrictinfo->can_join ||
            restrictinfo->mergeopfamilies == NIL)
            continue;           /* not mergejoinable */

        /*
         * Check if clause has the form "outer op inner" or "inner op outer",
         * and if so mark which side is inner.
         */
        if (!clause_sides_match_join(restrictinfo, sjinfo->min_lefthand,
                                     innerrel->relids))
            continue;           /* no good for these input relations */

        /* OK, add to list */
        clause_list = lappend(clause_list, restrictinfo);
    }

    /*
     * Now that we have the relevant equality join clauses, try to prove the
     * innerrel distinct.
     */
    if (rel_is_distinct_for(root, innerrel, clause_list))
        return true;

    /*
     * Some day it would be nice to check for other methods of establishing
     * distinctness.
     */
    return false;
}


/*
 * Remove the target relid from the planner's data structures, having
 * determined that there is no need to include it in the query.
 *
 * We are not terribly thorough here.  We must make sure that the rel is
 * no longer treated as a baserel, and that attributes of other baserels
 * are no longer marked as being needed at joins involving this rel.
 * Also, join quals involving the rel have to be removed from the joininfo
 * lists, but only if they belong to the outer join identified by joinrelids.
 */
static void
remove_rel_from_query(PlannerInfo *root, int relid, Relids joinrelids)
{
    RelOptInfo *rel = find_base_rel(root, relid);
    List       *joininfos;
    Index       rti;
    ListCell   *l;
    ListCell   *nextl;

    /*
     * Mark the rel as "dead" to show it is no longer part of the join tree.
     * (Removing it from the baserel array altogether seems too risky.)
     */
    rel->reloptkind = RELOPT_DEADREL;

    /*
     * Remove references to the rel from other baserels' attr_needed arrays.
     */
    for (rti = 1; rti < root->simple_rel_array_size; rti++)
    {
        RelOptInfo *otherrel = root->simple_rel_array[rti];
        int         attroff;

        /* there may be empty slots corresponding to non-baserel RTEs */
        if (otherrel == NULL)
            continue;

        Assert(otherrel->relid == rti); /* sanity check on array */

        /* no point in processing target rel itself */
        if (otherrel == rel)
            continue;

        for (attroff = otherrel->max_attr - otherrel->min_attr;
             attroff >= 0;
             attroff--)
        {
            otherrel->attr_needed[attroff] =
                bms_del_member(otherrel->attr_needed[attroff], relid);
        }
    }

    /*
     * Likewise remove references from SpecialJoinInfo data structures.
     *
     * This is relevant in case the outer join we're deleting is nested inside
     * other outer joins: the upper joins' relid sets have to be adjusted. The
     * RHS of the target outer join will be made empty here, but that's OK
     * since caller will delete that SpecialJoinInfo entirely.
     */
    foreach(l, root->join_info_list)
    {
        SpecialJoinInfo *sjinfo = (SpecialJoinInfo *) lfirst(l);

        sjinfo->min_lefthand = bms_del_member(sjinfo->min_lefthand, relid);
        sjinfo->min_righthand = bms_del_member(sjinfo->min_righthand, relid);
        sjinfo->syn_lefthand = bms_del_member(sjinfo->syn_lefthand, relid);
        sjinfo->syn_righthand = bms_del_member(sjinfo->syn_righthand, relid);
    }

    /*
     * Likewise remove references from PlaceHolderVar data structures,
     * removing any no-longer-needed placeholders entirely.
     *
     * Removal is a bit tricker than it might seem: we can remove PHVs that
     * are used at the target rel and/or in the join qual, but not those that
     * are used at join partner rels or above the join.  It's not that easy to
     * distinguish PHVs used at partner rels from those used in the join qual,
     * since they will both have ph_needed sets that are subsets of
     * joinrelids.  However, a PHV used at a partner rel could not have the
     * target rel in ph_eval_at, so we check that while deciding whether to
     * remove or just update the PHV.  There is no corresponding test in
     * join_is_removable because it doesn't need to distinguish those cases.
     */
    for (l = list_head(root->placeholder_list); l != NULL; l = nextl)
    {
        PlaceHolderInfo *phinfo = (PlaceHolderInfo *) lfirst(l);

        nextl = lnext(l);
        Assert(!bms_is_member(relid, phinfo->ph_lateral));
        if (bms_is_subset(phinfo->ph_needed, joinrelids) &&
            bms_is_member(relid, phinfo->ph_eval_at))
            root->placeholder_list = list_delete_ptr(root->placeholder_list,
                                                     phinfo);
        else
        {
            phinfo->ph_eval_at = bms_del_member(phinfo->ph_eval_at, relid);
            Assert(!bms_is_empty(phinfo->ph_eval_at));
            phinfo->ph_needed = bms_del_member(phinfo->ph_needed, relid);
        }
    }

    /*
     * Remove any joinquals referencing the rel from the joininfo lists.
     *
     * In some cases, a joinqual has to be put back after deleting its
     * reference to the target rel.  This can occur for pseudoconstant and
     * outerjoin-delayed quals, which can get marked as requiring the rel in
     * order to force them to be evaluated at or above the join.  We can't
     * just discard them, though.  Only quals that logically belonged to the
     * outer join being discarded should be removed from the query.
     *
     * We must make a copy of the rel's old joininfo list before starting the
     * loop, because otherwise remove_join_clause_from_rels would destroy the
     * list while we're scanning it.
     */
    joininfos = list_copy(rel->joininfo);
    foreach(l, joininfos)
    {
        RestrictInfo *rinfo = (RestrictInfo *) lfirst(l);

        remove_join_clause_from_rels(root, rinfo, rinfo->required_relids);

        if (RINFO_IS_PUSHED_DOWN(rinfo, joinrelids))
        {
            /* Recheck that qual doesn't actually reference the target rel */
            Assert(!bms_is_member(relid, rinfo->clause_relids));

            /*
             * The required_relids probably aren't shared with anything else,
             * but let's copy them just to be sure.
             */
            rinfo->required_relids = bms_copy(rinfo->required_relids);
            rinfo->required_relids = bms_del_member(rinfo->required_relids,
                                                    relid);
            distribute_restrictinfo_to_rels(root, rinfo);
        }
    }

    /*
     * There may be references to the rel in root->fkey_list, but if so,
     * match_foreign_keys_to_quals() will get rid of them.
     */
}

/*
 * Remove any occurrences of the target relid from a joinlist structure.
 *
 * It's easiest to build a whole new list structure, so we handle it that
 * way.  Efficiency is not a big deal here.
 *
 * *nremoved is incremented by the number of occurrences removed (there
 * should be exactly one, but the caller checks that).
 */
static List *
remove_rel_from_joinlist(List *joinlist, int relid, int *nremoved)
{
    List       *result = NIL;
    ListCell   *jl;

    foreach(jl, joinlist)
    {
        Node       *jlnode = (Node *) lfirst(jl);

        if (IsA(jlnode, RangeTblRef))
        {
            int         varno = ((RangeTblRef *) jlnode)->rtindex;

            if (varno == relid)
                (*nremoved)++;
            else
                result = lappend(result, jlnode);
        }
        else if (IsA(jlnode, List))
        {
            /* Recurse to handle subproblem */
            List       *sublist;

            sublist = remove_rel_from_joinlist((List *) jlnode,
                                               relid, nremoved);
            /* Avoid including empty sub-lists in the result */
            if (sublist)
                result = lappend(result, sublist);
        }
        else
        {
            elog(ERROR, "unrecognized joinlist node type: %d",
                 (int) nodeTag(jlnode));
        }
    }

    return result;
}


/*
 * reduce_unique_semijoins
 *      Check for semijoins that can be simplified to plain inner joins
 *      because the inner relation is provably unique for the join clauses.
 *
 * Ideally this would happen during reduce_outer_joins, but we don't have
 * enough information at that point.
 *
 * To perform the strength reduction when applicable, we need only delete
 * the semijoin's SpecialJoinInfo from root->join_info_list.  (We don't
 * bother fixing the join type attributed to it in the query jointree,
 * since that won't be consulted again.)
 */
void
reduce_unique_semijoins(PlannerInfo *root)
{
    ListCell   *lc;
    ListCell   *next;

    /*
     * Scan the join_info_list to find semijoins.  We can't use foreach
     * because we may delete the current cell.
     */
    for (lc = list_head(root->join_info_list); lc != NULL; lc = next)
    {
        SpecialJoinInfo *sjinfo = (SpecialJoinInfo *) lfirst(lc);
        int         innerrelid;
        RelOptInfo *innerrel;
        Relids      joinrelids;
        List       *restrictlist;

        next = lnext(lc);

        /*
         * Must be a non-delaying semijoin to a single baserel, else we aren't
         * going to be able to do anything with it.  (It's probably not
         * possible for delay_upper_joins to be set on a semijoin, but we
         * might as well check.)
         */
        if (sjinfo->jointype != JOIN_SEMI ||
            sjinfo->delay_upper_joins)
            continue;

        if (!bms_get_singleton_member(sjinfo->min_righthand, &innerrelid))
            continue;

        innerrel = find_base_rel(root, innerrelid);

        /*
         * Before we trouble to run generate_join_implied_equalities, make a
         * quick check to eliminate cases in which we will surely be unable to
         * prove uniqueness of the innerrel.
         */
        if (!rel_supports_distinctness(root, innerrel))
            continue;

        /* Compute the relid set for the join we are considering */
        joinrelids = bms_union(sjinfo->min_lefthand, sjinfo->min_righthand);

        /*
         * Since we're only considering a single-rel RHS, any join clauses it
         * has must be clauses linking it to the semijoin's min_lefthand.  We
         * can also consider EC-derived join clauses.
         */
        restrictlist =
            list_concat(generate_join_implied_equalities(root,
                                                         joinrelids,
                                                         sjinfo->min_lefthand,
                                                         innerrel),
                        innerrel->joininfo);

        /* Test whether the innerrel is unique for those clauses. */
        if (!innerrel_is_unique(root,
                                joinrelids, sjinfo->min_lefthand, innerrel,
                                JOIN_SEMI, restrictlist, true))
            continue;

        /* OK, remove the SpecialJoinInfo from the list. */
        root->join_info_list = list_delete_ptr(root->join_info_list, sjinfo);
    }
}


/*
 * rel_supports_distinctness
 *      Could the relation possibly be proven distinct on some set of columns?
 *
 * This is effectively a pre-checking function for rel_is_distinct_for().
 * It must return true if rel_is_distinct_for() could possibly return true
 * with this rel, but it should not expend a lot of cycles.  The idea is
 * that callers can avoid doing possibly-expensive processing to compute
 * rel_is_distinct_for()'s argument lists if the call could not possibly
 * succeed.
 */
static bool
rel_supports_distinctness(PlannerInfo *root, RelOptInfo *rel)
{
    /* We only know about baserels ... */
    if (rel->reloptkind != RELOPT_BASEREL)
        return false;
    if (rel->rtekind == RTE_RELATION)
    {
        /*
         * For a plain relation, we only know how to prove uniqueness by
         * reference to unique indexes.  Make sure there's at least one
         * suitable unique index.  It must be immediately enforced, and if
         * it's a partial index, it must match the query.  (Keep these
         * conditions in sync with relation_has_unique_index_for!)
         */
        ListCell   *lc;

        foreach(lc, rel->indexlist)
        {
            IndexOptInfo *ind = (IndexOptInfo *) lfirst(lc);

            if (ind->unique && ind->immediate &&
                (ind->indpred == NIL || ind->predOK))
                return true;
        }
    }
    else if (rel->rtekind == RTE_SUBQUERY)
    {
        Query      *subquery = root->simple_rte_array[rel->relid]->subquery;

        /* Check if the subquery has any qualities that support distinctness */
        if (query_supports_distinctness(subquery))
            return true;
    }
    /* We have no proof rules for any other rtekinds. */
    return false;
}

/*
 * rel_is_distinct_for
 *      Does the relation return only distinct rows according to clause_list?
 *
 * clause_list is a list of join restriction clauses involving this rel and
 * some other one.  Return true if no two rows emitted by this rel could
 * possibly join to the same row of the other rel.
 *
 * The caller must have already determined that each condition is a
 * mergejoinable equality with an expression in this relation on one side, and
 * an expression not involving this relation on the other.  The transient
 * outer_is_left flag is used to identify which side references this relation:
 * left side if outer_is_left is false, right side if it is true.
 *
 * Note that the passed-in clause_list may be destructively modified!  This
 * is OK for current uses, because the clause_list is built by the caller for
 * the sole purpose of passing to this function.
 */
static bool
rel_is_distinct_for(PlannerInfo *root, RelOptInfo *rel, List *clause_list)
{
    /*
     * We could skip a couple of tests here if we assume all callers checked
     * rel_supports_distinctness first, but it doesn't seem worth taking any
     * risk for.
     */
    if (rel->reloptkind != RELOPT_BASEREL)
        return false;
    if (rel->rtekind == RTE_RELATION)
    {
        /*
         * Examine the indexes to see if we have a matching unique index.
         * relation_has_unique_index_for automatically adds any usable
         * restriction clauses for the rel, so we needn't do that here.
         */
        if (relation_has_unique_index_for(root, rel, clause_list, NIL, NIL))
            return true;
    }
    else if (rel->rtekind == RTE_SUBQUERY)
    {
        Index       relid = rel->relid;
        Query      *subquery = root->simple_rte_array[relid]->subquery;
        List       *colnos = NIL;
        List       *opids = NIL;
        ListCell   *l;

        /*
         * Build the argument lists for query_is_distinct_for: a list of
         * output column numbers that the query needs to be distinct over, and
         * a list of equality operators that the output columns need to be
         * distinct according to.
         *
         * (XXX we are not considering restriction clauses attached to the
         * subquery; is that worth doing?)
         */
        foreach(l, clause_list)
        {
            RestrictInfo *rinfo = lfirst_node(RestrictInfo, l);
            Oid         op;
            Var        *var;

            /*
             * Get the equality operator we need uniqueness according to.
             * (This might be a cross-type operator and thus not exactly the
             * same operator the subquery would consider; that's all right
             * since query_is_distinct_for can resolve such cases.)  The
             * caller's mergejoinability test should have selected only
             * OpExprs.
             */
            op = castNode(OpExpr, rinfo->clause)->opno;

            /* caller identified the inner side for us */
            if (rinfo->outer_is_left)
                var = (Var *) get_rightop(rinfo->clause);
            else
                var = (Var *) get_leftop(rinfo->clause);

            /*
             * We may ignore any RelabelType node above the operand.  (There
             * won't be more than one, since eval_const_expressions() has been
             * applied already.)
             */
            if (var && IsA(var, RelabelType))
                var = (Var *) ((RelabelType *) var)->arg;

            /*
             * If inner side isn't a Var referencing a subquery output column,
             * this clause doesn't help us.
             */
            if (!var || !IsA(var, Var) ||
                var->varno != relid || var->varlevelsup != 0)
                continue;

            colnos = lappend_int(colnos, var->varattno);
            opids = lappend_oid(opids, op);
        }

        if (query_is_distinct_for(subquery, colnos, opids))
            return true;
    }
    return false;
}


/*
 * query_supports_distinctness - could the query possibly be proven distinct
 *      on some set of output columns?
 *
 * This is effectively a pre-checking function for query_is_distinct_for().
 * It must return true if query_is_distinct_for() could possibly return true
 * with this query, but it should not expend a lot of cycles.  The idea is
 * that callers can avoid doing possibly-expensive processing to compute
 * query_is_distinct_for()'s argument lists if the call could not possibly
 * succeed.
 */
bool
query_supports_distinctness(Query *query)
{
    /* SRFs break distinctness except with DISTINCT, see below */
    if (query->hasTargetSRFs && query->distinctClause == NIL)
        return false;

    /* check for features we can prove distinctness with */
    if (query->distinctClause != NIL ||
        query->groupClause != NIL ||
        query->groupingSets != NIL ||
        query->hasAggs ||
        query->havingQual ||
        query->setOperations)
        return true;

    return false;
}

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

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

    /*
     * DISTINCT (including DISTINCT ON) guarantees uniqueness if all the
     * columns in the DISTINCT clause appear in colnos and operator semantics
     * match.  This is true even if there are SRFs in the DISTINCT columns or
     * elsewhere in the tlist.
     */
    if (query->distinctClause)
    {
        foreach(l, query->distinctClause)
        {
            SortGroupClause *sgc = (SortGroupClause *) lfirst(l);
            TargetEntry *tle = get_sortgroupclause_tle(sgc,
                                                       query->targetList);

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

    /*
     * Otherwise, a set-returning function in the query's targetlist can
     * result in returning duplicate rows, despite any grouping that might
     * occur before tlist evaluation.  (If all tlist SRFs are within GROUP BY
     * columns, it would be safe because they'd be expanded before grouping.
     * But it doesn't currently seem worth the effort to check for that.)
     */
    if (query->hasTargetSRFs)
        return false;

    /*
     * Similarly, GROUP BY without GROUPING SETS guarantees uniqueness if all
     * the grouped columns appear in colnos and operator semantics match.
     */
    if (query->groupClause && !query->groupingSets)
    {
        foreach(l, query->groupClause)
        {
            SortGroupClause *sgc = (SortGroupClause *) lfirst(l);
            TargetEntry *tle = get_sortgroupclause_tle(sgc,
                                                       query->targetList);

            opid = distinct_col_search(tle->resno, colnos, opids);
            if (!OidIsValid(opid) ||
                !equality_ops_are_compatible(opid, sgc->eqop))
                break;          /* exit early if no match */
        }
        if (l == NULL)          /* had matches for all? */
            return true;
    }
    else if (query->groupingSets)
    {
        /*
         * If we have grouping sets with expressions, we probably don't have
         * uniqueness and analysis would be hard. Punt.
         */
        if (query->groupClause)
            return false;

        /*
         * If we have no groupClause (therefore no grouping expressions), we
         * might have one or many empty grouping sets. If there's just one,
         * then we're returning only one row and are certainly unique. But
         * otherwise, we know we're certainly not unique.
         */
        if (list_length(query->groupingSets) == 1 &&
            ((GroupingSet *) linitial(query->groupingSets))->kind == GROUPING_SET_EMPTY)
            return true;
        else
            return false;
    }
    else
    {
        /*
         * If we have no GROUP BY, but do have aggregates or HAVING, then the
         * result is at most one row so it's surely unique, for any operators.
         */
        if (query->hasAggs || query->havingQual)
            return true;
    }

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

        Assert(topop->op != SETOP_NONE);

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

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

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

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

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

    /*
     * XXX Are there any other cases in which we can easily see the result
     * must be distinct?
     *
     * If you do add more smarts to this function, be sure to update
     * query_supports_distinctness() to match.
     */

    return false;
}

/*
 * distinct_col_search - subroutine for query_is_distinct_for
 *
 * If colno is in colnos, return the corresponding element of opids,
 * else return InvalidOid.  (Ordinarily colnos would not contain duplicates,
 * but if it does, we arbitrarily select the first match.)
 */
static Oid
distinct_col_search(int colno, List *colnos, List *opids)
{
    ListCell   *lc1,
               *lc2;

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


/*
 * innerrel_is_unique
 *    Check if the innerrel provably contains at most one tuple matching any
 *    tuple from the outerrel, based on join clauses in the 'restrictlist'.
 *
 * We need an actual RelOptInfo for the innerrel, but it's sufficient to
 * identify the outerrel by its Relids.  This asymmetry supports use of this
 * function before joinrels have been built.  (The caller is expected to
 * also supply the joinrelids, just to save recalculating that.)
 *
 * The proof must be made based only on clauses that will be "joinquals"
 * rather than "otherquals" at execution.  For an inner join there's no
 * difference; but if the join is outer, we must ignore pushed-down quals,
 * as those will become "otherquals".  Note that this means the answer might
 * vary depending on whether IS_OUTER_JOIN(jointype); since we cache the
 * answer without regard to that, callers must take care not to call this
 * with jointypes that would be classified differently by IS_OUTER_JOIN().
 *
 * The actual proof is undertaken by is_innerrel_unique_for(); this function
 * is a frontend that is mainly concerned with caching the answers.
 * In particular, the force_cache argument allows overriding the internal
 * heuristic about whether to cache negative answers; it should be "true"
 * if making an inquiry that is not part of the normal bottom-up join search
 * sequence.
 */
bool
innerrel_is_unique(PlannerInfo *root,
                   Relids joinrelids,
                   Relids outerrelids,
                   RelOptInfo *innerrel,
                   JoinType jointype,
                   List *restrictlist,
                   bool force_cache)
{
    MemoryContext old_context;
    ListCell   *lc;

    /* Certainly can't prove uniqueness when there are no joinclauses */
    if (restrictlist == NIL)
        return false;

    /*
     * Make a quick check to eliminate cases in which we will surely be unable
     * to prove uniqueness of the innerrel.
     */
    if (!rel_supports_distinctness(root, innerrel))
        return false;

    /*
     * Query the cache to see if we've managed to prove that innerrel is
     * unique for any subset of this outerrel.  We don't need an exact match,
     * as extra outerrels can't make the innerrel any less unique (or more
     * formally, the restrictlist for a join to a superset outerrel must be a
     * superset of the conditions we successfully used before).
     */
    foreach(lc, innerrel->unique_for_rels)
    {
        Relids      unique_for_rels = (Relids) lfirst(lc);

        if (bms_is_subset(unique_for_rels, outerrelids))
            return true;        /* Success! */
    }

    /*
     * Conversely, we may have already determined that this outerrel, or some
     * superset thereof, cannot prove this innerrel to be unique.
     */
    foreach(lc, innerrel->non_unique_for_rels)
    {
        Relids      unique_for_rels = (Relids) lfirst(lc);

        if (bms_is_subset(outerrelids, unique_for_rels))
            return false;
    }

    /* No cached information, so try to make the proof. */
    if (is_innerrel_unique_for(root, joinrelids, outerrelids, innerrel,
                               jointype, restrictlist))
    {
        /*
         * Cache the positive result for future probes, being sure to keep it
         * in the planner_cxt even if we are working in GEQO.
         *
         * Note: one might consider trying to isolate the minimal subset of
         * the outerrels that proved the innerrel unique.  But it's not worth
         * the trouble, because the planner builds up joinrels incrementally
         * and so we'll see the minimally sufficient outerrels before any
         * supersets of them anyway.
         */
        old_context = MemoryContextSwitchTo(root->planner_cxt);
        innerrel->unique_for_rels = lappend(innerrel->unique_for_rels,
                                            bms_copy(outerrelids));
        MemoryContextSwitchTo(old_context);

        return true;            /* Success! */
    }
    else
    {
        /*
         * None of the join conditions for outerrel proved innerrel unique, so
         * we can safely reject this outerrel or any subset of it in future
         * checks.
         *
         * However, in normal planning mode, caching this knowledge is totally
         * pointless; it won't be queried again, because we build up joinrels
         * from smaller to larger.  It is useful in GEQO mode, where the
         * knowledge can be carried across successive planning attempts; and
         * it's likely to be useful when using join-search plugins, too. Hence
         * cache when join_search_private is non-NULL.  (Yeah, that's a hack,
         * but it seems reasonable.)
         *
         * Also, allow callers to override that heuristic and force caching;
         * that's useful for reduce_unique_semijoins, which calls here before
         * the normal join search starts.
         */
        if (force_cache || root->join_search_private)
        {
            old_context = MemoryContextSwitchTo(root->planner_cxt);
            innerrel->non_unique_for_rels =
                lappend(innerrel->non_unique_for_rels,
                        bms_copy(outerrelids));
            MemoryContextSwitchTo(old_context);
        }

        return false;
    }
}

/*
 * is_innerrel_unique_for
 *    Check if the innerrel provably contains at most one tuple matching any
 *    tuple from the outerrel, based on join clauses in the 'restrictlist'.
 */
static bool
is_innerrel_unique_for(PlannerInfo *root,
                       Relids joinrelids,
                       Relids outerrelids,
                       RelOptInfo *innerrel,
                       JoinType jointype,
                       List *restrictlist)
{
    List       *clause_list = NIL;
    ListCell   *lc;

    /*
     * Search for mergejoinable clauses that constrain the inner rel against
     * the outer rel.  If an operator is mergejoinable then it behaves like
     * equality for some btree opclass, so it's what we want.  The
     * mergejoinability test also eliminates clauses containing volatile
     * functions, which we couldn't depend on.
     */
    foreach(lc, restrictlist)
    {
        RestrictInfo *restrictinfo = (RestrictInfo *) lfirst(lc);

        /*
         * As noted above, if it's a pushed-down clause and we're at an outer
         * join, we can't use it.
         */
        if (IS_OUTER_JOIN(jointype) &&
            RINFO_IS_PUSHED_DOWN(restrictinfo, joinrelids))
            continue;

        /* Ignore if it's not a mergejoinable clause */
        if (!restrictinfo->can_join ||
            restrictinfo->mergeopfamilies == NIL)
            continue;           /* not mergejoinable */

        /*
         * Check if clause has the form "outer op inner" or "inner op outer",
         * and if so mark which side is inner.
         */
        if (!clause_sides_match_join(restrictinfo, outerrelids,
                                     innerrel->relids))
            continue;           /* no good for these input relations */

        /* OK, add to list */
        clause_list = lappend(clause_list, restrictinfo);
    }

    /* Let rel_is_distinct_for() do the hard work */
    return rel_is_distinct_for(root, innerrel, clause_list);
}