prepunion.c

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/*-------------------------------------------------------------------------
 *
 * prepunion.c
 *    Routines to plan set-operation queries.  The filename is a leftover
 *    from a time when only UNIONs were implemented.
 *
 * There are two code paths in the planner for set-operation queries.
 * If a subquery consists entirely of simple UNION ALL operations, it
 * is converted into an "append relation".  Otherwise, it is handled
 * by the general code in this module (plan_set_operations and its
 * subroutines).  There is some support code here for the append-relation
 * case, but most of the heavy lifting for that is done elsewhere,
 * notably in prepjointree.c and allpaths.c.
 *
 * Portions Copyright (c) 1996-2022, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *    src/backend/optimizer/prep/prepunion.c
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"
 
#include "access/htup_details.h"
#include "access/sysattr.h"
#include "catalog/partition.h"
#include "catalog/pg_inherits.h"
#include "catalog/pg_type.h"
#include "miscadmin.h"
#include "nodes/makefuncs.h"
#include "nodes/nodeFuncs.h"
#include "optimizer/cost.h"
#include "optimizer/pathnode.h"
#include "optimizer/paths.h"
#include "optimizer/planmain.h"
#include "optimizer/planner.h"
#include "optimizer/prep.h"
#include "optimizer/tlist.h"
#include "parser/parse_coerce.h"
#include "parser/parsetree.h"
#include "utils/lsyscache.h"
#include "utils/rel.h"
#include "utils/selfuncs.h"
#include "utils/syscache.h"
 
 
static RelOptInfo *recurse_set_operations(Node *setOp, PlannerInfo *root,
                                          List *colTypes, List *colCollations,
                                          bool junkOK,
                                          int flag, List *refnames_tlist,
                                          List **pTargetList,
                                          double *pNumGroups);
static RelOptInfo *generate_recursion_path(SetOperationStmt *setOp,
                                           PlannerInfo *root,
                                           List *refnames_tlist,
                                           List **pTargetList);
static RelOptInfo *generate_union_paths(SetOperationStmt *op, PlannerInfo *root,
                                        List *refnames_tlist,
                                        List **pTargetList);
static RelOptInfo *generate_nonunion_paths(SetOperationStmt *op, PlannerInfo *root,
                                           List *refnames_tlist,
                                           List **pTargetList);
static List *plan_union_children(PlannerInfo *root,
                                 SetOperationStmt *top_union,
                                 List *refnames_tlist,
                                 List **tlist_list);
static Path *make_union_unique(SetOperationStmt *op, Path *path, List *tlist,
                               PlannerInfo *root);
static void postprocess_setop_rel(PlannerInfo *root, RelOptInfo *rel);
static bool choose_hashed_setop(PlannerInfo *root, List *groupClauses,
                                Path *input_path,
                                double dNumGroups, double dNumOutputRows,
                                const char *construct);
static List *generate_setop_tlist(List *colTypes, List *colCollations,
                                  int flag,
                                  Index varno,
                                  bool hack_constants,
                                  List *input_tlist,
                                  List *refnames_tlist);
static List *generate_append_tlist(List *colTypes, List *colCollations,
                                   bool flag,
                                   List *input_tlists,
                                   List *refnames_tlist);
static List *generate_setop_grouplist(SetOperationStmt *op, List *targetlist);
 
 
/*
 * plan_set_operations
 *
 *    Plans the queries for a tree of set operations (UNION/INTERSECT/EXCEPT)
 *
 * This routine only deals with the setOperations tree of the given query.
 * Any top-level ORDER BY requested in root->parse->sortClause will be handled
 * when we return to grouping_planner; likewise for LIMIT.
 *
 * What we return is an "upperrel" RelOptInfo containing at least one Path
 * that implements the set-operation tree.  In addition, root->processed_tlist
 * receives a targetlist representing the output of the topmost setop node.
 */
RelOptInfo *
plan_set_operations(PlannerInfo *root)
{
    Query      *parse = root->parse;
    SetOperationStmt *topop = castNode(SetOperationStmt, parse->setOperations);
    Node       *node;
    RangeTblEntry *leftmostRTE;
    Query      *leftmostQuery;
    RelOptInfo *setop_rel;
    List       *top_tlist;
 
    Assert(topop);
 
    /* check for unsupported stuff */
    Assert(parse->jointree->fromlist == NIL);
    Assert(parse->jointree->quals == NULL);
    Assert(parse->groupClause == NIL);
    Assert(parse->havingQual == NULL);
    Assert(parse->windowClause == NIL);
    Assert(parse->distinctClause == NIL);
 
    /*
     * In the outer query level, we won't have any true equivalences to deal
     * with; but we do want to be able to make pathkeys, which will require
     * single-member EquivalenceClasses.  Indicate that EC merging is complete
     * so that pathkeys.c won't complain.
     */
    Assert(root->eq_classes == NIL);
    root->ec_merging_done = true;
 
    /*
     * We'll need to build RelOptInfos for each of the leaf subqueries, which
     * are RTE_SUBQUERY rangetable entries in this Query.  Prepare the index
     * arrays for those, and for AppendRelInfos in case they're needed.
     */
    setup_simple_rel_arrays(root);
 
    /*
     * Find the leftmost component Query.  We need to use its column names for
     * all generated tlists (else SELECT INTO won't work right).
     */
    node = topop->larg;
    while (node && IsA(node, SetOperationStmt))
        node = ((SetOperationStmt *) node)->larg;
    Assert(node && IsA(node, RangeTblRef));
    leftmostRTE = root->simple_rte_array[((RangeTblRef *) node)->rtindex];
    leftmostQuery = leftmostRTE->subquery;
    Assert(leftmostQuery != NULL);
 
    /*
     * If the topmost node is a recursive union, it needs special processing.
     */
    if (root->hasRecursion)
    {
        setop_rel = generate_recursion_path(topop, root,
                                            leftmostQuery->targetList,
                                            &top_tlist);
    }
    else
    {
        /*
         * Recurse on setOperations tree to generate paths for set ops. The
         * final output paths should have just the column types shown as the
         * output from the top-level node, plus possibly resjunk working
         * columns (we can rely on upper-level nodes to deal with that).
         */
        setop_rel = recurse_set_operations((Node *) topop, root,
                                           topop->colTypes, topop->colCollations,
                                           true, -1,
                                           leftmostQuery->targetList,
                                           &top_tlist,
                                           NULL);
    }
 
    /* Must return the built tlist into root->processed_tlist. */
    root->processed_tlist = top_tlist;
 
    return setop_rel;
}
 
/*
 * recurse_set_operations
 *    Recursively handle one step in a tree of set operations
 *
 * colTypes: OID list of set-op's result column datatypes
 * colCollations: OID list of set-op's result column collations
 * junkOK: if true, child resjunk columns may be left in the result
 * flag: if >= 0, add a resjunk output column indicating value of flag
 * refnames_tlist: targetlist to take column names from
 *
 * Returns a RelOptInfo for the subtree, as well as these output parameters:
 * *pTargetList: receives the fully-fledged tlist for the subtree's top plan
 * *pNumGroups: if not NULL, we estimate the number of distinct groups
 *      in the result, and store it there
 *
 * The pTargetList output parameter is mostly redundant with the pathtarget
 * of the returned RelOptInfo, but for the moment we need it because much of
 * the logic in this file depends on flag columns being marked resjunk.
 * Pending a redesign of how that works, this is the easy way out.
 *
 * We don't have to care about typmods here: the only allowed difference
 * between set-op input and output typmods is input is a specific typmod
 * and output is -1, and that does not require a coercion.
 */
static RelOptInfo *
recurse_set_operations(Node *setOp, PlannerInfo *root,
                       List *colTypes, List *colCollations,
                       bool junkOK,
                       int flag, List *refnames_tlist,
                       List **pTargetList,
                       double *pNumGroups)
{
    RelOptInfo *rel = NULL;     /* keep compiler quiet */
 
    /* Guard against stack overflow due to overly complex setop nests */
    check_stack_depth();
 
    if (IsA(setOp, RangeTblRef))
    {
        RangeTblRef *rtr = (RangeTblRef *) setOp;
        RangeTblEntry *rte = root->simple_rte_array[rtr->rtindex];
        Query      *subquery = rte->subquery;
        PlannerInfo *subroot;
        RelOptInfo *final_rel;
        Path       *subpath;
        Path       *path;
        List       *tlist;
 
        Assert(subquery != NULL);
 
        /* Build a RelOptInfo for this leaf subquery. */
        rel = build_simple_rel(root, rtr->rtindex, NULL);
 
        /* plan_params should not be in use in current query level */
        Assert(root->plan_params == NIL);
 
        /* Generate a subroot and Paths for the subquery */
        subroot = rel->subroot = subquery_planner(root->glob, subquery,
                                                  root,
                                                  false,
                                                  root->tuple_fraction);
 
        /*
         * It should not be possible for the primitive query to contain any
         * cross-references to other primitive queries in the setop tree.
         */
        if (root->plan_params)
            elog(ERROR, "unexpected outer reference in set operation subquery");
 
        /* Figure out the appropriate target list for this subquery. */
        tlist = generate_setop_tlist(colTypes, colCollations,
                                     flag,
                                     rtr->rtindex,
                                     true,
                                     subroot->processed_tlist,
                                     refnames_tlist);
        rel->reltarget = create_pathtarget(root, tlist);
 
        /* Return the fully-fledged tlist to caller, too */
        *pTargetList = tlist;
 
        /*
         * Mark rel with estimated output rows, width, etc.  Note that we have
         * to do this before generating outer-query paths, else
         * cost_subqueryscan is not happy.
         */
        set_subquery_size_estimates(root, rel);
 
        /*
         * Since we may want to add a partial path to this relation, we must
         * set its consider_parallel flag correctly.
         */
        final_rel = fetch_upper_rel(subroot, UPPERREL_FINAL, NULL);
        rel->consider_parallel = final_rel->consider_parallel;
 
        /*
         * For the moment, we consider only a single Path for the subquery.
         * This should change soon (make it look more like
         * set_subquery_pathlist).
         */
        subpath = get_cheapest_fractional_path(final_rel,
                                               root->tuple_fraction);
 
        /*
         * Stick a SubqueryScanPath atop that.
         *
         * We don't bother to determine the subquery's output ordering since
         * it won't be reflected in the set-op result anyhow; so just label
         * the SubqueryScanPath with nil pathkeys.  (XXX that should change
         * soon too, likely.)
         */
        path = (Path *) create_subqueryscan_path(root, rel, subpath,
                                                 NIL, NULL);
 
        add_path(rel, path);
 
        /*
         * If we have a partial path for the child relation, we can use that
         * to build a partial path for this relation.  But there's no point in
         * considering any path but the cheapest.
         */
        if (rel->consider_parallel && bms_is_empty(rel->lateral_relids) &&
            final_rel->partial_pathlist != NIL)
        {
            Path       *partial_subpath;
            Path       *partial_path;
 
            partial_subpath = linitial(final_rel->partial_pathlist);
            partial_path = (Path *)
                create_subqueryscan_path(root, rel, partial_subpath,
                                         NIL, NULL);
            add_partial_path(rel, partial_path);
        }
 
        /*
         * Estimate number of groups if caller wants it.  If the subquery used
         * grouping or aggregation, its output is probably mostly unique
         * anyway; otherwise do statistical estimation.
         *
         * XXX you don't really want to know about this: we do the estimation
         * using the subquery's original targetlist expressions, not the
         * subroot->processed_tlist which might seem more appropriate.  The
         * reason is that if the subquery is itself a setop, it may return a
         * processed_tlist containing "varno 0" Vars generated by
         * generate_append_tlist, and those would confuse estimate_num_groups
         * mightily.  We ought to get rid of the "varno 0" hack, but that
         * requires a redesign of the parsetree representation of setops, so
         * that there can be an RTE corresponding to each setop's output.
         */
        if (pNumGroups)
        {
            if (subquery->groupClause || subquery->groupingSets ||
                subquery->distinctClause ||
                subroot->hasHavingQual || subquery->hasAggs)
                *pNumGroups = subpath->rows;
            else
                *pNumGroups = estimate_num_groups(subroot,
                                                  get_tlist_exprs(subquery->targetList, false),
                                                  subpath->rows,
                                                  NULL,
                                                  NULL);
        }
    }
    else if (IsA(setOp, SetOperationStmt))
    {
        SetOperationStmt *op = (SetOperationStmt *) setOp;
 
        /* UNIONs are much different from INTERSECT/EXCEPT */
        if (op->op == SETOP_UNION)
            rel = generate_union_paths(op, root,
                                       refnames_tlist,
                                       pTargetList);
        else
            rel = generate_nonunion_paths(op, root,
                                          refnames_tlist,
                                          pTargetList);
        if (pNumGroups)
            *pNumGroups = rel->rows;
 
        /*
         * If necessary, add a Result node to project the caller-requested
         * output columns.
         *
         * XXX you don't really want to know about this: setrefs.c will apply
         * fix_upper_expr() to the Result node's tlist. This would fail if the
         * Vars generated by generate_setop_tlist() were not exactly equal()
         * to the corresponding tlist entries of the subplan. However, since
         * the subplan was generated by generate_union_paths() or
         * generate_nonunion_paths(), and hence its tlist was generated by
         * generate_append_tlist(), this will work.  We just tell
         * generate_setop_tlist() to use varno 0.
         */
        if (flag >= 0 ||
            !tlist_same_datatypes(*pTargetList, colTypes, junkOK) ||
            !tlist_same_collations(*pTargetList, colCollations, junkOK))
        {
            PathTarget *target;
            ListCell   *lc;
 
            *pTargetList = generate_setop_tlist(colTypes, colCollations,
                                                flag,
                                                0,
                                                false,
                                                *pTargetList,
                                                refnames_tlist);
            target = create_pathtarget(root, *pTargetList);
 
            /* Apply projection to each path */
            foreach(lc, rel->pathlist)
            {
                Path       *subpath = (Path *) lfirst(lc);
                Path       *path;
 
                Assert(subpath->param_info == NULL);
                path = apply_projection_to_path(root, subpath->parent,
                                                subpath, target);
                /* If we had to add a Result, path is different from subpath */
                if (path != subpath)
                    lfirst(lc) = path;
            }
 
            /* Apply projection to each partial path */
            foreach(lc, rel->partial_pathlist)
            {
                Path       *subpath = (Path *) lfirst(lc);
                Path       *path;
 
                Assert(subpath->param_info == NULL);
 
                /* avoid apply_projection_to_path, in case of multiple refs */
                path = (Path *) create_projection_path(root, subpath->parent,
                                                       subpath, target);
                lfirst(lc) = path;
            }
        }
    }
    else
    {
        elog(ERROR, "unrecognized node type: %d",
             (int) nodeTag(setOp));
        *pTargetList = NIL;
    }
 
    postprocess_setop_rel(root, rel);
 
    return rel;
}
 
/*
 * Generate paths for a recursive UNION node
 */
static RelOptInfo *
generate_recursion_path(SetOperationStmt *setOp, PlannerInfo *root,
                        List *refnames_tlist,
                        List **pTargetList)
{
    RelOptInfo *result_rel;
    Path       *path;
    RelOptInfo *lrel,
               *rrel;
    Path       *lpath;
    Path       *rpath;
    List       *lpath_tlist;
    List       *rpath_tlist;
    List       *tlist;
    List       *groupList;
    double      dNumGroups;
 
    /* Parser should have rejected other cases */
    if (setOp->op != SETOP_UNION)
        elog(ERROR, "only UNION queries can be recursive");
    /* Worktable ID should be assigned */
    Assert(root->wt_param_id >= 0);
 
    /*
     * Unlike a regular UNION node, process the left and right inputs
     * separately without any intention of combining them into one Append.
     */
    lrel = recurse_set_operations(setOp->larg, root,
                                  setOp->colTypes, setOp->colCollations,
                                  false, -1,
                                  refnames_tlist,
                                  &lpath_tlist,
                                  NULL);
    lpath = lrel->cheapest_total_path;
    /* The right path will want to look at the left one ... */
    root->non_recursive_path = lpath;
    rrel = recurse_set_operations(setOp->rarg, root,
                                  setOp->colTypes, setOp->colCollations,
                                  false, -1,
                                  refnames_tlist,
                                  &rpath_tlist,
                                  NULL);
    rpath = rrel->cheapest_total_path;
    root->non_recursive_path = NULL;
 
    /*
     * Generate tlist for RecursiveUnion path node --- same as in Append cases
     */
    tlist = generate_append_tlist(setOp->colTypes, setOp->colCollations, false,
                                  list_make2(lpath_tlist, rpath_tlist),
                                  refnames_tlist);
 
    *pTargetList = tlist;
 
    /* Build result relation. */
    result_rel = fetch_upper_rel(root, UPPERREL_SETOP,
                                 bms_union(lrel->relids, rrel->relids));
    result_rel->reltarget = create_pathtarget(root, tlist);
 
    /*
     * If UNION, identify the grouping operators
     */
    if (setOp->all)
    {
        groupList = NIL;
        dNumGroups = 0;
    }
    else
    {
        /* Identify the grouping semantics */
        groupList = generate_setop_grouplist(setOp, tlist);
 
        /* We only support hashing here */
        if (!grouping_is_hashable(groupList))
            ereport(ERROR,
                    (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
                     errmsg("could not implement recursive UNION"),
                     errdetail("All column datatypes must be hashable.")));
 
        /*
         * For the moment, take the number of distinct groups as equal to the
         * total input size, ie, the worst case.
         */
        dNumGroups = lpath->rows + rpath->rows * 10;
    }
 
    /*
     * And make the path node.
     */
    path = (Path *) create_recursiveunion_path(root,
                                               result_rel,
                                               lpath,
                                               rpath,
                                               result_rel->reltarget,
                                               groupList,
                                               root->wt_param_id,
                                               dNumGroups);
 
    add_path(result_rel, path);
    postprocess_setop_rel(root, result_rel);
    return result_rel;
}
 
/*
 * Generate paths for a UNION or UNION ALL node
 */
static RelOptInfo *
generate_union_paths(SetOperationStmt *op, PlannerInfo *root,
                     List *refnames_tlist,
                     List **pTargetList)
{
    Relids      relids = NULL;
    RelOptInfo *result_rel;
    double      save_fraction = root->tuple_fraction;
    ListCell   *lc;
    List       *pathlist = NIL;
    List       *partial_pathlist = NIL;
    bool        partial_paths_valid = true;
    bool        consider_parallel = true;
    List       *rellist;
    List       *tlist_list;
    List       *tlist;
    Path       *path;
 
    /*
     * If plain UNION, tell children to fetch all tuples.
     *
     * Note: in UNION ALL, we pass the top-level tuple_fraction unmodified to
     * each arm of the UNION ALL.  One could make a case for reducing the
     * tuple fraction for later arms (discounting by the expected size of the
     * earlier arms' results) but it seems not worth the trouble. The normal
     * case where tuple_fraction isn't already zero is a LIMIT at top level,
     * and passing it down as-is is usually enough to get the desired result
     * of preferring fast-start plans.
     */
    if (!op->all)
        root->tuple_fraction = 0.0;
 
    /*
     * If any of my children are identical UNION nodes (same op, all-flag, and
     * colTypes) then they can be merged into this node so that we generate
     * only one Append and unique-ification for the lot.  Recurse to find such
     * nodes and compute their children's paths.
     */
    rellist = plan_union_children(root, op, refnames_tlist, &tlist_list);
 
    /*
     * Generate tlist for Append plan node.
     *
     * The tlist for an Append plan isn't important as far as the Append is
     * concerned, but we must make it look real anyway for the benefit of the
     * next plan level up.
     */
    tlist = generate_append_tlist(op->colTypes, op->colCollations, false,
                                  tlist_list, refnames_tlist);
 
    *pTargetList = tlist;
 
    /* Build path lists and relid set. */
    foreach(lc, rellist)
    {
        RelOptInfo *rel = lfirst(lc);
 
        pathlist = lappend(pathlist, rel->cheapest_total_path);
 
        if (consider_parallel)
        {
            if (!rel->consider_parallel)
            {
                consider_parallel = false;
                partial_paths_valid = false;
            }
            else if (rel->partial_pathlist == NIL)
                partial_paths_valid = false;
            else
                partial_pathlist = lappend(partial_pathlist,
                                           linitial(rel->partial_pathlist));
        }
 
        relids = bms_union(relids, rel->relids);
    }
 
    /* Build result relation. */
    result_rel = fetch_upper_rel(root, UPPERREL_SETOP, relids);
    result_rel->reltarget = create_pathtarget(root, tlist);
    result_rel->consider_parallel = consider_parallel;
 
    /*
     * Append the child results together.
     */
    path = (Path *) create_append_path(root, result_rel, pathlist, NIL,
                                       NIL, NULL, 0, false, -1);
 
    /*
     * For UNION ALL, we just need the Append path.  For UNION, need to add
     * node(s) to remove duplicates.
     */
    if (!op->all)
        path = make_union_unique(op, path, tlist, root);
 
    add_path(result_rel, path);
 
    /*
     * Estimate number of groups.  For now we just assume the output is unique
     * --- this is certainly true for the UNION case, and we want worst-case
     * estimates anyway.
     */
    result_rel->rows = path->rows;
 
    /*
     * Now consider doing the same thing using the partial paths plus Append
     * plus Gather.
     */
    if (partial_paths_valid)
    {
        Path       *ppath;
        ListCell   *lc;
        int         parallel_workers = 0;
 
        /* Find the highest number of workers requested for any subpath. */
        foreach(lc, partial_pathlist)
        {
            Path       *path = lfirst(lc);
 
            parallel_workers = Max(parallel_workers, path->parallel_workers);
        }
        Assert(parallel_workers > 0);
 
        /*
         * If the use of parallel append is permitted, always request at least
         * log2(# of children) paths.  We assume it can be useful to have
         * extra workers in this case because they will be spread out across
         * the children.  The precise formula is just a guess; see
         * add_paths_to_append_rel.
         */
        if (enable_parallel_append)
        {
            parallel_workers = Max(parallel_workers,
                                   fls(list_length(partial_pathlist)));
            parallel_workers = Min(parallel_workers,
                                   max_parallel_workers_per_gather);
        }
        Assert(parallel_workers > 0);
 
        ppath = (Path *)
            create_append_path(root, result_rel, NIL, partial_pathlist,
                               NIL, NULL,
                               parallel_workers, enable_parallel_append,
                               -1);
        ppath = (Path *)
            create_gather_path(root, result_rel, ppath,
                               result_rel->reltarget, NULL, NULL);
        if (!op->all)
            ppath = make_union_unique(op, ppath, tlist, root);
        add_path(result_rel, ppath);
    }
 
    /* Undo effects of possibly forcing tuple_fraction to 0 */
    root->tuple_fraction = save_fraction;
 
    return result_rel;
}
 
/*
 * Generate paths for an INTERSECT, INTERSECT ALL, EXCEPT, or EXCEPT ALL node
 */
static RelOptInfo *
generate_nonunion_paths(SetOperationStmt *op, PlannerInfo *root,
                        List *refnames_tlist,
                        List **pTargetList)
{
    RelOptInfo *result_rel;
    RelOptInfo *lrel,
               *rrel;
    double      save_fraction = root->tuple_fraction;
    Path       *lpath,
               *rpath,
               *path;
    List       *lpath_tlist,
               *rpath_tlist,
               *tlist_list,
               *tlist,
               *groupList,
               *pathlist;
    double      dLeftGroups,
                dRightGroups,
                dNumGroups,
                dNumOutputRows;
    bool        use_hash;
    SetOpCmd    cmd;
    int         firstFlag;
 
    /*
     * Tell children to fetch all tuples.
     */
    root->tuple_fraction = 0.0;
 
    /* Recurse on children, ensuring their outputs are marked */
    lrel = recurse_set_operations(op->larg, root,
                                  op->colTypes, op->colCollations,
                                  false, 0,
                                  refnames_tlist,
                                  &lpath_tlist,
                                  &dLeftGroups);
    lpath = lrel->cheapest_total_path;
    rrel = recurse_set_operations(op->rarg, root,
                                  op->colTypes, op->colCollations,
                                  false, 1,
                                  refnames_tlist,
                                  &rpath_tlist,
                                  &dRightGroups);
    rpath = rrel->cheapest_total_path;
 
    /* Undo effects of forcing tuple_fraction to 0 */
    root->tuple_fraction = save_fraction;
 
    /*
     * For EXCEPT, we must put the left input first.  For INTERSECT, either
     * order should give the same results, and we prefer to put the smaller
     * input first in order to minimize the size of the hash table in the
     * hashing case.  "Smaller" means the one with the fewer groups.
     */
    if (op->op == SETOP_EXCEPT || dLeftGroups <= dRightGroups)
    {
        pathlist = list_make2(lpath, rpath);
        tlist_list = list_make2(lpath_tlist, rpath_tlist);
        firstFlag = 0;
    }
    else
    {
        pathlist = list_make2(rpath, lpath);
        tlist_list = list_make2(rpath_tlist, lpath_tlist);
        firstFlag = 1;
    }
 
    /*
     * Generate tlist for Append plan node.
     *
     * The tlist for an Append plan isn't important as far as the Append is
     * concerned, but we must make it look real anyway for the benefit of the
     * next plan level up.  In fact, it has to be real enough that the flag
     * column is shown as a variable not a constant, else setrefs.c will get
     * confused.
     */
    tlist = generate_append_tlist(op->colTypes, op->colCollations, true,
                                  tlist_list, refnames_tlist);
 
    *pTargetList = tlist;
 
    /* Build result relation. */
    result_rel = fetch_upper_rel(root, UPPERREL_SETOP,
                                 bms_union(lrel->relids, rrel->relids));
    result_rel->reltarget = create_pathtarget(root, tlist);
 
    /*
     * Append the child results together.
     */
    path = (Path *) create_append_path(root, result_rel, pathlist, NIL,
                                       NIL, NULL, 0, false, -1);
 
    /* Identify the grouping semantics */
    groupList = generate_setop_grouplist(op, tlist);
 
    /*
     * Estimate number of distinct groups that we'll need hashtable entries
     * for; this is the size of the left-hand input for EXCEPT, or the smaller
     * input for INTERSECT.  Also estimate the number of eventual output rows.
     * In non-ALL cases, we estimate each group produces one output row; in
     * ALL cases use the relevant relation size.  These are worst-case
     * estimates, of course, but we need to be conservative.
     */
    if (op->op == SETOP_EXCEPT)
    {
        dNumGroups = dLeftGroups;
        dNumOutputRows = op->all ? lpath->rows : dNumGroups;
    }
    else
    {
        dNumGroups = Min(dLeftGroups, dRightGroups);
        dNumOutputRows = op->all ? Min(lpath->rows, rpath->rows) : dNumGroups;
    }
 
    /*
     * Decide whether to hash or sort, and add a sort node if needed.
     */
    use_hash = choose_hashed_setop(root, groupList, path,
                                   dNumGroups, dNumOutputRows,
                                   (op->op == SETOP_INTERSECT) ? "INTERSECT" : "EXCEPT");
 
    if (groupList && !use_hash)
        path = (Path *) create_sort_path(root,
                                         result_rel,
                                         path,
                                         make_pathkeys_for_sortclauses(root,
                                                                       groupList,
                                                                       tlist),
                                         -1.0);
 
    /*
     * Finally, add a SetOp path node to generate the correct output.
     */
    switch (op->op)
    {
        case SETOP_INTERSECT:
            cmd = op->all ? SETOPCMD_INTERSECT_ALL : SETOPCMD_INTERSECT;
            break;
        case SETOP_EXCEPT:
            cmd = op->all ? SETOPCMD_EXCEPT_ALL : SETOPCMD_EXCEPT;
            break;
        default:
            elog(ERROR, "unrecognized set op: %d", (int) op->op);
            cmd = SETOPCMD_INTERSECT;   /* keep compiler quiet */
            break;
    }
    path = (Path *) create_setop_path(root,
                                      result_rel,
                                      path,
                                      cmd,
                                      use_hash ? SETOP_HASHED : SETOP_SORTED,
                                      groupList,
                                      list_length(op->colTypes) + 1,
                                      use_hash ? firstFlag : -1,
                                      dNumGroups,
                                      dNumOutputRows);
 
    result_rel->rows = path->rows;
    add_path(result_rel, path);
    return result_rel;
}
 
/*
 * Pull up children of a UNION node that are identically-propertied UNIONs.
 *
 * NOTE: we can also pull a UNION ALL up into a UNION, since the distinct
 * output rows will be lost anyway.
 *
 * NOTE: currently, we ignore collations while determining if a child has
 * the same properties.  This is semantically sound only so long as all
 * collations have the same notion of equality.  It is valid from an
 * implementation standpoint because we don't care about the ordering of
 * a UNION child's result: UNION ALL results are always unordered, and
 * generate_union_paths will force a fresh sort if the top level is a UNION.
 */
static List *
plan_union_children(PlannerInfo *root,
                    SetOperationStmt *top_union,
                    List *refnames_tlist,
                    List **tlist_list)
{
    List       *pending_rels = list_make1(top_union);
    List       *result = NIL;
    List       *child_tlist;
 
    *tlist_list = NIL;
 
    while (pending_rels != NIL)
    {
        Node       *setOp = linitial(pending_rels);
 
        pending_rels = list_delete_first(pending_rels);
 
        if (IsA(setOp, SetOperationStmt))
        {
            SetOperationStmt *op = (SetOperationStmt *) setOp;
 
            if (op->op == top_union->op &&
                (op->all == top_union->all || op->all) &&
                equal(op->colTypes, top_union->colTypes))
            {
                /* Same UNION, so fold children into parent */
                pending_rels = lcons(op->rarg, pending_rels);
                pending_rels = lcons(op->larg, pending_rels);
                continue;
            }
        }
 
        /*
         * Not same, so plan this child separately.
         *
         * Note we disallow any resjunk columns in child results.  This is
         * necessary since the Append node that implements the union won't do
         * any projection, and upper levels will get confused if some of our
         * output tuples have junk and some don't.  This case only arises when
         * we have an EXCEPT or INTERSECT as child, else there won't be
         * resjunk anyway.
         */
        result = lappend(result, recurse_set_operations(setOp, root,
                                                        top_union->colTypes,
                                                        top_union->colCollations,
                                                        false, -1,
                                                        refnames_tlist,
                                                        &child_tlist,
                                                        NULL));
        *tlist_list = lappend(*tlist_list, child_tlist);
    }
 
    return result;
}
 
/*
 * Add nodes to the given path tree to unique-ify the result of a UNION.
 */
static Path *
make_union_unique(SetOperationStmt *op, Path *path, List *tlist,
                  PlannerInfo *root)
{
    RelOptInfo *result_rel = fetch_upper_rel(root, UPPERREL_SETOP, NULL);
    List       *groupList;
    double      dNumGroups;
 
    /* Identify the grouping semantics */
    groupList = generate_setop_grouplist(op, tlist);
 
    /*
     * XXX for the moment, take the number of distinct groups as equal to the
     * total input size, ie, the worst case.  This is too conservative, but
     * it's not clear how to get a decent estimate of the true size.  One
     * should note as well the propensity of novices to write UNION rather
     * than UNION ALL even when they don't expect any duplicates...
     */
    dNumGroups = path->rows;
 
    /* Decide whether to hash or sort */
    if (choose_hashed_setop(root, groupList, path,
                            dNumGroups, dNumGroups,
                            "UNION"))
    {
        /* Hashed aggregate plan --- no sort needed */
        path = (Path *) create_agg_path(root,
                                        result_rel,
                                        path,
                                        create_pathtarget(root, tlist),
                                        AGG_HASHED,
                                        AGGSPLIT_SIMPLE,
                                        groupList,
                                        NIL,
                                        NULL,
                                        dNumGroups);
    }
    else
    {
        /* Sort and Unique */
        if (groupList)
            path = (Path *)
                create_sort_path(root,
                                 result_rel,
                                 path,
                                 make_pathkeys_for_sortclauses(root,
                                                               groupList,
                                                               tlist),
                                 -1.0);
        path = (Path *) create_upper_unique_path(root,
                                                 result_rel,
                                                 path,
                                                 list_length(path->pathkeys),
                                                 dNumGroups);
    }
 
    return path;
}
 
/*
 * postprocess_setop_rel - perform steps required after adding paths
 */
static void
postprocess_setop_rel(PlannerInfo *root, RelOptInfo *rel)
{
    /*
     * We don't currently worry about allowing FDWs to contribute paths to
     * this relation, but give extensions a chance.
     */
    if (create_upper_paths_hook)
        (*create_upper_paths_hook) (root, UPPERREL_SETOP,
                                    NULL, rel, NULL);
 
    /* Select cheapest path */
    set_cheapest(rel);
}
 
/*
 * choose_hashed_setop - should we use hashing for a set operation?
 */
static bool
choose_hashed_setop(PlannerInfo *root, List *groupClauses,
                    Path *input_path,
                    double dNumGroups, double dNumOutputRows,
                    const char *construct)
{
    int         numGroupCols = list_length(groupClauses);
    Size        hash_mem_limit = get_hash_memory_limit();
    bool        can_sort;
    bool        can_hash;
    Size        hashentrysize;
    Path        hashed_p;
    Path        sorted_p;
    double      tuple_fraction;
 
    /* Check whether the operators support sorting or hashing */
    can_sort = grouping_is_sortable(groupClauses);
    can_hash = grouping_is_hashable(groupClauses);
    if (can_hash && can_sort)
    {
        /* we have a meaningful choice to make, continue ... */
    }
    else if (can_hash)
        return true;
    else if (can_sort)
        return false;
    else
        ereport(ERROR,
                (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
        /* translator: %s is UNION, INTERSECT, or EXCEPT */
                 errmsg("could not implement %s", construct),
                 errdetail("Some of the datatypes only support hashing, while others only support sorting.")));
 
    /* Prefer sorting when enable_hashagg is off */
    if (!enable_hashagg)
        return false;
 
    /*
     * Don't do it if it doesn't look like the hashtable will fit into
     * hash_mem.
     */
    hashentrysize = MAXALIGN(input_path->pathtarget->width) + MAXALIGN(SizeofMinimalTupleHeader);
 
    if (hashentrysize * dNumGroups > hash_mem_limit)
        return false;
 
    /*
     * See if the estimated cost is no more than doing it the other way.
     *
     * We need to consider input_plan + hashagg versus input_plan + sort +
     * group.  Note that the actual result plan might involve a SetOp or
     * Unique node, not Agg or Group, but the cost estimates for Agg and Group
     * should be close enough for our purposes here.
     *
     * These path variables are dummies that just hold cost fields; we don't
     * make actual Paths for these steps.
     */
    cost_agg(&hashed_p, root, AGG_HASHED, NULL,
             numGroupCols, dNumGroups,
             NIL,
             input_path->startup_cost, input_path->total_cost,
             input_path->rows, input_path->pathtarget->width);
 
    /*
     * Now for the sorted case.  Note that the input is *always* unsorted,
     * since it was made by appending unrelated sub-relations together.
     */
    sorted_p.startup_cost = input_path->startup_cost;
    sorted_p.total_cost = input_path->total_cost;
    /* XXX cost_sort doesn't actually look at pathkeys, so just pass NIL */
    cost_sort(&sorted_p, root, NIL, sorted_p.total_cost,
              input_path->rows, input_path->pathtarget->width,
              0.0, work_mem, -1.0);
    cost_group(&sorted_p, root, numGroupCols, dNumGroups,
               NIL,
               sorted_p.startup_cost, sorted_p.total_cost,
               input_path->rows);
 
    /*
     * Now make the decision using the top-level tuple fraction.  First we
     * have to convert an absolute count (LIMIT) into fractional form.
     */
    tuple_fraction = root->tuple_fraction;
    if (tuple_fraction >= 1.0)
        tuple_fraction /= dNumOutputRows;
 
    if (compare_fractional_path_costs(&hashed_p, &sorted_p,
                                      tuple_fraction) < 0)
    {
        /* Hashed is cheaper, so use it */
        return true;
    }
    return false;
}
 
/*
 * Generate targetlist for a set-operation plan node
 *
 * colTypes: OID list of set-op's result column datatypes
 * colCollations: OID list of set-op's result column collations
 * flag: -1 if no flag column needed, 0 or 1 to create a const flag column
 * varno: varno to use in generated Vars
 * hack_constants: true to copy up constants (see comments in code)
 * input_tlist: targetlist of this node's input node
 * refnames_tlist: targetlist to take column names from
 */
static List *
generate_setop_tlist(List *colTypes, List *colCollations,
                     int flag,
                     Index varno,
                     bool hack_constants,
                     List *input_tlist,
                     List *refnames_tlist)
{
    List       *tlist = NIL;
    int         resno = 1;
    ListCell   *ctlc,
               *cclc,
               *itlc,
               *rtlc;
    TargetEntry *tle;
    Node       *expr;
 
    forfour(ctlc, colTypes, cclc, colCollations,
            itlc, input_tlist, rtlc, refnames_tlist)
    {
        Oid         colType = lfirst_oid(ctlc);
        Oid         colColl = lfirst_oid(cclc);
        TargetEntry *inputtle = (TargetEntry *) lfirst(itlc);
        TargetEntry *reftle = (TargetEntry *) lfirst(rtlc);
 
        Assert(inputtle->resno == resno);
        Assert(reftle->resno == resno);
        Assert(!inputtle->resjunk);
        Assert(!reftle->resjunk);
 
        /*
         * Generate columns referencing input columns and having appropriate
         * data types and column names.  Insert datatype coercions where
         * necessary.
         *
         * HACK: constants in the input's targetlist are copied up as-is
         * rather than being referenced as subquery outputs.  This is mainly
         * to ensure that when we try to coerce them to the output column's
         * datatype, the right things happen for UNKNOWN constants.  But do
         * this only at the first level of subquery-scan plans; we don't want
         * phony constants appearing in the output tlists of upper-level
         * nodes!
         */
        if (hack_constants && inputtle->expr && IsA(inputtle->expr, Const))
            expr = (Node *) inputtle->expr;
        else
            expr = (Node *) makeVar(varno,
                                    inputtle->resno,
                                    exprType((Node *) inputtle->expr),
                                    exprTypmod((Node *) inputtle->expr),
                                    exprCollation((Node *) inputtle->expr),
                                    0);
 
        if (exprType(expr) != colType)
        {
            /*
             * Note: it's not really cool to be applying coerce_to_common_type
             * here; one notable point is that assign_expr_collations never
             * gets run on any generated nodes.  For the moment that's not a
             * problem because we force the correct exposed collation below.
             * It would likely be best to make the parser generate the correct
             * output tlist for every set-op to begin with, though.
             */
            expr = coerce_to_common_type(NULL,  /* no UNKNOWNs here */
                                         expr,
                                         colType,
                                         "UNION/INTERSECT/EXCEPT");
        }
 
        /*
         * Ensure the tlist entry's exposed collation matches the set-op. This
         * is necessary because plan_set_operations() reports the result
         * ordering as a list of SortGroupClauses, which don't carry collation
         * themselves but just refer to tlist entries.  If we don't show the
         * right collation then planner.c might do the wrong thing in
         * higher-level queries.
         *
         * Note we use RelabelType, not CollateExpr, since this expression
         * will reach the executor without any further processing.
         */
        if (exprCollation(expr) != colColl)
            expr = applyRelabelType(expr,
                                    exprType(expr), exprTypmod(expr), colColl,
                                    COERCE_IMPLICIT_CAST, -1, false);
 
        tle = makeTargetEntry((Expr *) expr,
                              (AttrNumber) resno++,
                              pstrdup(reftle->resname),
                              false);
 
        /*
         * By convention, all non-resjunk columns in a setop tree have
         * ressortgroupref equal to their resno.  In some cases the ref isn't
         * needed, but this is a cleaner way than modifying the tlist later.
         */
        tle->ressortgroupref = tle->resno;
 
        tlist = lappend(tlist, tle);
    }
 
    if (flag >= 0)
    {
        /* Add a resjunk flag column */
        /* flag value is the given constant */
        expr = (Node *) makeConst(INT4OID,
                                  -1,
                                  InvalidOid,
                                  sizeof(int32),
                                  Int32GetDatum(flag),
                                  false,
                                  true);
        tle = makeTargetEntry((Expr *) expr,
                              (AttrNumber) resno++,
                              pstrdup("flag"),
                              true);
        tlist = lappend(tlist, tle);
    }
 
    return tlist;
}
 
/*
 * Generate targetlist for a set-operation Append node
 *
 * colTypes: OID list of set-op's result column datatypes
 * colCollations: OID list of set-op's result column collations
 * flag: true to create a flag column copied up from subplans
 * input_tlists: list of tlists for sub-plans of the Append
 * refnames_tlist: targetlist to take column names from
 *
 * The entries in the Append's targetlist should always be simple Vars;
 * we just have to make sure they have the right datatypes/typmods/collations.
 * The Vars are always generated with varno 0.
 *
 * XXX a problem with the varno-zero approach is that set_pathtarget_cost_width
 * cannot figure out a realistic width for the tlist we make here.  But we
 * ought to refactor this code to produce a PathTarget directly, anyway.
 */
static List *
generate_append_tlist(List *colTypes, List *colCollations,
                      bool flag,
                      List *input_tlists,
                      List *refnames_tlist)
{
    List       *tlist = NIL;
    int         resno = 1;
    ListCell   *curColType;
    ListCell   *curColCollation;
    ListCell   *ref_tl_item;
    int         colindex;
    TargetEntry *tle;
    Node       *expr;
    ListCell   *tlistl;
    int32      *colTypmods;
 
    /*
     * First extract typmods to use.
     *
     * If the inputs all agree on type and typmod of a particular column, use
     * that typmod; else use -1.
     */
    colTypmods = (int32 *) palloc(list_length(colTypes) * sizeof(int32));
 
    foreach(tlistl, input_tlists)
    {
        List       *subtlist = (List *) lfirst(tlistl);
        ListCell   *subtlistl;
 
        curColType = list_head(colTypes);
        colindex = 0;
        foreach(subtlistl, subtlist)
        {
            TargetEntry *subtle = (TargetEntry *) lfirst(subtlistl);
 
            if (subtle->resjunk)
                continue;
            Assert(curColType != NULL);
            if (exprType((Node *) subtle->expr) == lfirst_oid(curColType))
            {
                /* If first subplan, copy the typmod; else compare */
                int32       subtypmod = exprTypmod((Node *) subtle->expr);
 
                if (tlistl == list_head(input_tlists))
                    colTypmods[colindex] = subtypmod;
                else if (subtypmod != colTypmods[colindex])
                    colTypmods[colindex] = -1;
            }
            else
            {
                /* types disagree, so force typmod to -1 */
                colTypmods[colindex] = -1;
            }
            curColType = lnext(colTypes, curColType);
            colindex++;
        }
        Assert(curColType == NULL);
    }
 
    /*
     * Now we can build the tlist for the Append.
     */
    colindex = 0;
    forthree(curColType, colTypes, curColCollation, colCollations,
             ref_tl_item, refnames_tlist)
    {
        Oid         colType = lfirst_oid(curColType);
        int32       colTypmod = colTypmods[colindex++];
        Oid         colColl = lfirst_oid(curColCollation);
        TargetEntry *reftle = (TargetEntry *) lfirst(ref_tl_item);
 
        Assert(reftle->resno == resno);
        Assert(!reftle->resjunk);
        expr = (Node *) makeVar(0,
                                resno,
                                colType,
                                colTypmod,
                                colColl,
                                0);
        tle = makeTargetEntry((Expr *) expr,
                              (AttrNumber) resno++,
                              pstrdup(reftle->resname),
                              false);
 
        /*
         * By convention, all non-resjunk columns in a setop tree have
         * ressortgroupref equal to their resno.  In some cases the ref isn't
         * needed, but this is a cleaner way than modifying the tlist later.
         */
        tle->ressortgroupref = tle->resno;
 
        tlist = lappend(tlist, tle);
    }
 
    if (flag)
    {
        /* Add a resjunk flag column */
        /* flag value is shown as copied up from subplan */
        expr = (Node *) makeVar(0,
                                resno,
                                INT4OID,
                                -1,
                                InvalidOid,
                                0);
        tle = makeTargetEntry((Expr *) expr,
                              (AttrNumber) resno++,
                              pstrdup("flag"),
                              true);
        tlist = lappend(tlist, tle);
    }
 
    pfree(colTypmods);
 
    return tlist;
}
 
/*
 * generate_setop_grouplist
 *      Build a SortGroupClause list defining the sort/grouping properties
 *      of the setop's output columns.
 *
 * Parse analysis already determined the properties and built a suitable
 * list, except that the entries do not have sortgrouprefs set because
 * the parser output representation doesn't include a tlist for each
 * setop.  So what we need to do here is copy that list and install
 * proper sortgrouprefs into it (copying those from the targetlist).
 */
static List *
generate_setop_grouplist(SetOperationStmt *op, List *targetlist)
{
    List       *grouplist = copyObject(op->groupClauses);
    ListCell   *lg;
    ListCell   *lt;
 
    lg = list_head(grouplist);
    foreach(lt, targetlist)
    {
        TargetEntry *tle = (TargetEntry *) lfirst(lt);
        SortGroupClause *sgc;
 
        if (tle->resjunk)
        {
            /* resjunk columns should not have sortgrouprefs */
            Assert(tle->ressortgroupref == 0);
            continue;           /* ignore resjunk columns */
        }
 
        /* non-resjunk columns should have sortgroupref = resno */
        Assert(tle->ressortgroupref == tle->resno);
 
        /* non-resjunk columns should have grouping clauses */
        Assert(lg != NULL);
        sgc = (SortGroupClause *) lfirst(lg);
        lg = lnext(grouplist, lg);
        Assert(sgc->tleSortGroupRef == 0);
 
        sgc->tleSortGroupRef = tle->ressortgroupref;
    }
    Assert(lg == NULL);
    return grouplist;
}