prepunion.c

Go to the documentation of this file.

/*-------------------------------------------------------------------------
 *
 * 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) 1996-2026, 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 <math.h>
 
#include "access/htup_details.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/planner.h"
#include "optimizer/prep.h"
#include "optimizer/tlist.h"
#include "parser/parse_coerce.h"
#include "utils/selfuncs.h"
 
 
static RelOptInfo *recurse_set_operations(Node *setOp, PlannerInfo *root,
                                          SetOperationStmt *parentOp,
                                          List *colTypes, List *colCollations,
                                          List *refnames_tlist,
                                          List **pTargetList,
                                          bool *istrivial_tlist);
static RelOptInfo *generate_recursion_path(SetOperationStmt *setOp,
                                           PlannerInfo *root,
                                           List *refnames_tlist,
                                           List **pTargetList);
static void build_setop_child_paths(PlannerInfo *root, RelOptInfo *rel,
                                    bool trivial_tlist, List *child_tlist,
                                    List *interesting_pathkeys,
                                    double *pNumGroups);
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,
                                 List **istrivial_tlist);
static void postprocess_setop_rel(PlannerInfo *root, RelOptInfo *rel);
static List *generate_setop_tlist(List *colTypes, List *colCollations,
                                  Index varno,
                                  bool hack_constants,
                                  List *input_tlist,
                                  List *refnames_tlist,
                                  bool *trivial_tlist);
static List *generate_append_tlist(List *colTypes, List *colCollations,
                                   List *input_tlists,
                                   List *refnames_tlist);
static List *generate_setop_grouplist(SetOperationStmt *op, List *targetlist);
static PathTarget *create_setop_pathtarget(PlannerInfo *root, List *tlist,
                                           List *child_pathlist);
 
 
/*
 * 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, equivalence classes are limited to classes
     * which define that the top-level target entry is equivalent to the
     * corresponding child target entry.  There won't be any equivalence class
     * merging.  Mark that merging is complete to allow us to make pathkeys.
     */
    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
    {
        bool        trivial_tlist;
 
        /*
         * 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.
         */
        setop_rel = recurse_set_operations((Node *) topop, root,
                                           NULL,    /* no parent */
                                           topop->colTypes, topop->colCollations,
                                           leftmostQuery->targetList,
                                           &top_tlist,
                                           &trivial_tlist);
    }
 
    /* 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
 *
 * setOp: current step (could be a SetOperationStmt or a leaf RangeTblRef)
 * parentOp: parent step, or NULL if none (but see below)
 * colTypes: OID list of set-op's result column datatypes
 * colCollations: OID list of set-op's result column collations
 * refnames_tlist: targetlist to take column names from
 *
 * parentOp should be passed as NULL unless that step is interested in
 * getting sorted output from this step.  ("Sorted" means "sorted according
 * to the default btree opclasses of the result column datatypes".)
 *
 * Returns a RelOptInfo for the subtree, as well as these output parameters:
 * *pTargetList: receives the fully-fledged tlist for the subtree's top plan
 * *istrivial_tlist: true if, and only if, datatypes between parent and child
 * match.
 *
 * If setOp is a leaf node, this function plans the sub-query but does
 * not populate the pathlist of the returned RelOptInfo.  The caller will
 * generate SubqueryScan paths using useful path(s) of the subquery (see
 * build_setop_child_paths).  But this function does build the paths for
 * set-operation nodes.
 *
 * 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.
 * XXX Now that there are no flag columns and hence no resjunk columns, we
 * could probably refactor this file to deal only in pathtargets.
 *
 * 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,
                       SetOperationStmt *parentOp,
                       List *colTypes, List *colCollations,
                       List *refnames_tlist,
                       List **pTargetList,
                       bool *istrivial_tlist)
{
    RelOptInfo *rel;
 
    *istrivial_tlist = true;    /* for now */
 
    /* 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;
        List       *tlist;
        bool        trivial_tlist;
        char       *plan_name;
 
        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.  If we have a
         * parentOp, pass that down to encourage subquery_planner to consider
         * suitably-sorted Paths.
         */
        plan_name = choose_plan_name(root->glob, "setop", true);
        subroot = rel->subroot = subquery_planner(root->glob, subquery,
                                                  plan_name, root,
                                                  false, root->tuple_fraction,
                                                  parentOp);
 
        /*
         * 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,
                                     rtr->rtindex,
                                     true,
                                     subroot->processed_tlist,
                                     refnames_tlist,
                                     &trivial_tlist);
        rel->reltarget = create_pathtarget(root, tlist);
 
        /* Return the fully-fledged tlist to caller, too */
        *pTargetList = tlist;
        *istrivial_tlist = trivial_tlist;
    }
    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 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() or generate_setop_tlist(), this will work.
         * We just tell generate_setop_tlist() to use varno 0.
         */
        if (!tlist_same_datatypes(*pTargetList, colTypes, false) ||
            !tlist_same_collations(*pTargetList, colCollations, false))
        {
            PathTarget *target;
            bool        trivial_tlist;
            ListCell   *lc;
 
            *pTargetList = generate_setop_tlist(colTypes, colCollations,
                                                0,
                                                false,
                                                *pTargetList,
                                                refnames_tlist,
                                                &trivial_tlist);
            *istrivial_tlist = trivial_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;
            }
        }
        postprocess_setop_rel(root, rel);
    }
    else
    {
        elog(ERROR, "unrecognized node type: %d",
             (int) nodeTag(setOp));
        *pTargetList = NIL;
        rel = NULL;             /* keep compiler quiet */
    }
 
    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;
    bool        lpath_trivial_tlist;
    List       *rpath_tlist;
    bool        rpath_trivial_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,
                                  NULL, /* no value in sorted results */
                                  setOp->colTypes, setOp->colCollations,
                                  refnames_tlist,
                                  &lpath_tlist,
                                  &lpath_trivial_tlist);
    if (lrel->rtekind == RTE_SUBQUERY)
        build_setop_child_paths(root, lrel, lpath_trivial_tlist, lpath_tlist,
                                NIL, 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,
                                  NULL, /* no value in sorted results */
                                  setOp->colTypes, setOp->colCollations,
                                  refnames_tlist,
                                  &rpath_tlist,
                                  &rpath_trivial_tlist);
    if (rrel->rtekind == RTE_SUBQUERY)
        build_setop_child_paths(root, rrel, rpath_trivial_tlist, rpath_tlist,
                                NIL, 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,
                                  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;
}
 
/*
 * build_setop_child_paths
 *      Build paths for the set op child relation denoted by 'rel'.
 *
 * 'rel' is an RTE_SUBQUERY relation.  We have already generated paths within
 * the subquery's subroot; the task here is to create SubqueryScan paths for
 * 'rel', representing scans of the useful subquery paths.
 *
 * interesting_pathkeys: if not NIL, also include paths that suit these
 * pathkeys, sorting any unsorted paths as required.
 * *pNumGroups: if not NULL, we estimate the number of distinct groups
 * in the result, and store it there.
 */
static void
build_setop_child_paths(PlannerInfo *root, RelOptInfo *rel,
                        bool trivial_tlist, List *child_tlist,
                        List *interesting_pathkeys, double *pNumGroups)
{
    RelOptInfo *final_rel;
    List       *setop_pathkeys = rel->subroot->setop_pathkeys;
    ListCell   *lc;
 
    /* it can't be a set op child rel if it's not a subquery */
    Assert(rel->rtekind == RTE_SUBQUERY);
 
    /* when sorting is needed, add child rel equivalences */
    if (interesting_pathkeys != NIL)
        add_setop_child_rel_equivalences(root,
                                         rel,
                                         child_tlist,
                                         interesting_pathkeys);
 
    /*
     * 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(rel->subroot, UPPERREL_FINAL, NULL);
    rel->consider_parallel = final_rel->consider_parallel;
 
    /* Generate subquery scan paths for any interesting path in final_rel */
    foreach(lc, final_rel->pathlist)
    {
        Path       *subpath = (Path *) lfirst(lc);
        List       *pathkeys;
        Path       *cheapest_input_path = final_rel->cheapest_total_path;
        bool        is_sorted;
        int         presorted_keys;
 
        /* If the input rel is dummy, propagate that to this query level */
        if (is_dummy_rel(final_rel))
        {
            mark_dummy_rel(rel);
            continue;
        }
 
        /*
         * Include the cheapest path as-is so that the set operation can be
         * cheaply implemented using a method which does not require the input
         * to be sorted.
         */
        if (subpath == cheapest_input_path)
        {
            /* Convert subpath's pathkeys to outer representation */
            pathkeys = convert_subquery_pathkeys(root, rel, subpath->pathkeys,
                                                 make_tlist_from_pathtarget(subpath->pathtarget));
 
            /* Generate outer path using this subpath */
            add_path(rel, (Path *) create_subqueryscan_path(root,
                                                            rel,
                                                            subpath,
                                                            trivial_tlist,
                                                            pathkeys,
                                                            NULL));
        }
 
        /* skip dealing with sorted paths if the setop doesn't need them */
        if (interesting_pathkeys == NIL)
            continue;
 
        /*
         * Create paths to suit final sort order required for setop_pathkeys.
         * Here we'll sort the cheapest input path (if not sorted already) and
         * incremental sort any paths which are partially sorted.
         */
        is_sorted = pathkeys_count_contained_in(setop_pathkeys,
                                                subpath->pathkeys,
                                                &presorted_keys);
 
        if (!is_sorted)
        {
            double      limittuples = rel->subroot->limit_tuples;
 
            /*
             * Try at least sorting the cheapest path and also try
             * incrementally sorting any path which is partially sorted
             * already (no need to deal with paths which have presorted keys
             * when incremental sort is disabled unless it's the cheapest
             * input path).
             */
            if (subpath != cheapest_input_path &&
                (presorted_keys == 0 || !enable_incremental_sort))
                continue;
 
            /*
             * We've no need to consider both a sort and incremental sort.
             * We'll just do a sort if there are no presorted keys and an
             * incremental sort when there are presorted keys.
             */
            if (presorted_keys == 0 || !enable_incremental_sort)
                subpath = (Path *) create_sort_path(rel->subroot,
                                                    final_rel,
                                                    subpath,
                                                    setop_pathkeys,
                                                    limittuples);
            else
                subpath = (Path *) create_incremental_sort_path(rel->subroot,
                                                                final_rel,
                                                                subpath,
                                                                setop_pathkeys,
                                                                presorted_keys,
                                                                limittuples);
        }
 
        /*
         * subpath is now sorted, so add it to the pathlist.  We already added
         * the cheapest_input_path above, so don't add it again unless we just
         * sorted it.
         */
        if (subpath != cheapest_input_path)
        {
            /* Convert subpath's pathkeys to outer representation */
            pathkeys = convert_subquery_pathkeys(root, rel, subpath->pathkeys,
                                                 make_tlist_from_pathtarget(subpath->pathtarget));
 
            /* Generate outer path using this subpath */
            add_path(rel, (Path *) create_subqueryscan_path(root,
                                                            rel,
                                                            subpath,
                                                            trivial_tlist,
                                                            pathkeys,
                                                            NULL));
        }
    }
 
    /* if consider_parallel is false, there should be no partial paths */
    Assert(final_rel->consider_parallel ||
           final_rel->partial_pathlist == NIL);
 
    /*
     * 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,
                                     trivial_tlist,
                                     NIL, NULL);
        add_partial_path(rel, partial_path);
    }
 
    postprocess_setop_rel(root, rel);
 
    /*
     * 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 subroot->parse'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. Note, we use this not
     * subquery's targetlist but subroot->parse's targetlist, because it was
     * revised by self-join removal.  subquery's targetlist might contain the
     * references to the removed relids.
     */
    if (pNumGroups)
    {
        PlannerInfo *subroot = rel->subroot;
        Query      *subquery = subroot->parse;
 
        if (subquery->groupClause || subquery->groupingSets ||
            subquery->distinctClause || subroot->hasHavingQual ||
            subquery->hasAggs)
            *pNumGroups = rel->cheapest_total_path->rows;
        else
            *pNumGroups = estimate_num_groups(subroot,
                                              get_tlist_exprs(subroot->parse->targetList, false),
                                              rel->cheapest_total_path->rows,
                                              NULL,
                                              NULL);
    }
}
 
/*
 * 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;
    ListCell   *lc;
    ListCell   *lc2;
    ListCell   *lc3;
    AppendPathInput cheapest = {0};
    AppendPathInput ordered = {0};
    AppendPathInput partial = {0};
    bool        partial_paths_valid = true;
    bool        consider_parallel = true;
    List       *rellist;
    List       *tlist_list;
    List       *trivial_tlist_list;
    List       *tlist;
    List       *groupList = NIL;
    Path       *apath;
    Path       *gpath = NULL;
    bool        try_sorted = false;
    List       *union_pathkeys = NIL;
 
    /*
     * If any of my children are identical UNION nodes (same op, all-flag, and
     * colTypes/colCollations) then they can be merged into this node so that
     * we generate only one Append/MergeAppend and unique-ification for the
     * lot.  Recurse to find such nodes.
     */
    rellist = plan_union_children(root,
                                  op,
                                  refnames_tlist,
                                  &tlist_list,
                                  &trivial_tlist_list);
 
    /*
     * Generate tlist for Append/MergeAppend 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,
                                  tlist_list, refnames_tlist);
    *pTargetList = tlist;
 
    /* For UNIONs (not UNION ALL), try sorting, if sorting is possible */
    if (!op->all)
    {
        /* Identify the grouping semantics */
        groupList = generate_setop_grouplist(op, tlist);
 
        if (grouping_is_sortable(op->groupClauses))
        {
            try_sorted = true;
            /* Determine the pathkeys for sorting by the whole target list */
            union_pathkeys = make_pathkeys_for_sortclauses(root, groupList,
                                                           tlist);
 
            root->query_pathkeys = union_pathkeys;
        }
    }
 
    /*
     * Now that we've got the append target list, we can build the union child
     * paths.
     */
    forthree(lc, rellist, lc2, trivial_tlist_list, lc3, tlist_list)
    {
        RelOptInfo *rel = lfirst(lc);
        bool        trivial_tlist = lfirst_int(lc2);
        List       *child_tlist = lfirst_node(List, lc3);
 
        /* only build paths for the union children */
        if (rel->rtekind == RTE_SUBQUERY)
            build_setop_child_paths(root, rel, trivial_tlist, child_tlist,
                                    union_pathkeys, NULL);
    }
 
    /* Build path lists and relid set. */
    foreach(lc, rellist)
    {
        RelOptInfo *rel = lfirst(lc);
        Path       *ordered_path;
 
        /*
         * Record the relids so that we can identify the correct
         * UPPERREL_SETOP RelOptInfo below.
         */
        relids = bms_add_members(relids, rel->relids);
 
        /* Skip any UNION children that are proven not to yield any rows */
        if (is_dummy_rel(rel))
            continue;
 
        cheapest.subpaths = lappend(cheapest.subpaths,
                                    rel->cheapest_total_path);
 
        if (try_sorted)
        {
            ordered_path = get_cheapest_path_for_pathkeys(rel->pathlist,
                                                          union_pathkeys,
                                                          NULL,
                                                          TOTAL_COST,
                                                          false);
 
            if (ordered_path != NULL)
                ordered.subpaths = lappend(ordered.subpaths, ordered_path);
            else
            {
                /*
                 * If we can't find a sorted path, just give up trying to
                 * generate a list of correctly sorted child paths.  This can
                 * happen when type coercion was added to the targetlist due
                 * to mismatching types from the union children.
                 */
                try_sorted = false;
            }
        }
 
        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.partial_subpaths = lappend(partial.partial_subpaths,
                                                   linitial(rel->partial_pathlist));
        }
    }
 
    /* Build result relation. */
    result_rel = fetch_upper_rel(root, UPPERREL_SETOP, relids);
    result_rel->reltarget = create_setop_pathtarget(root, tlist,
                                                    cheapest.subpaths);
    result_rel->consider_parallel = consider_parallel;
    result_rel->consider_startup = (root->tuple_fraction > 0);
 
    /* If all UNION children were dummy rels, make the resulting rel dummy */
    if (cheapest.subpaths == NIL)
    {
        mark_dummy_rel(result_rel);
 
        return result_rel;
    }
 
    /*
     * Append the child results together using the cheapest paths from each
     * union child.
     */
    apath = (Path *) create_append_path(root, result_rel, cheapest,
                                        NIL, NULL, 0, false, -1);
 
    /*
     * 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 = apath->rows;
 
    /*
     * Now consider doing the same thing using the partial paths plus Append
     * plus Gather.
     */
    if (partial_paths_valid)
    {
        Path       *papath;
        int         parallel_workers = 0;
 
        /* Find the highest number of workers requested for any subpath. */
        foreach(lc, partial.partial_subpaths)
        {
            Path       *subpath = lfirst(lc);
 
            parallel_workers = Max(parallel_workers,
                                   subpath->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,
                                   pg_leftmost_one_pos32(list_length(partial.partial_subpaths)) + 1);
            parallel_workers = Min(parallel_workers,
                                   max_parallel_workers_per_gather);
        }
        Assert(parallel_workers > 0);
 
        papath = (Path *)
            create_append_path(root, result_rel, partial,
                               NIL, NULL, parallel_workers,
                               enable_parallel_append, -1);
        gpath = (Path *)
            create_gather_path(root, result_rel, papath,
                               result_rel->reltarget, NULL, NULL);
    }
 
    if (!op->all)
    {
        double      dNumGroups;
        bool        can_sort = grouping_is_sortable(groupList);
        bool        can_hash = grouping_is_hashable(groupList);
        Path       *first_path = linitial(cheapest.subpaths);
 
        /*
         * Estimate the number of UNION output rows.  In the case when only a
         * single UNION child remains, we can use estimate_num_groups() on
         * that child.  We must be careful not to do this when that child is
         * the result of some other set operation as the targetlist will
         * contain Vars with varno==0, which estimate_num_groups() wouldn't
         * like.
         */
        if (list_length(cheapest.subpaths) == 1 &&
            first_path->parent->reloptkind != RELOPT_UPPER_REL)
        {
            dNumGroups = estimate_num_groups(root,
                                             first_path->pathtarget->exprs,
                                             first_path->rows,
                                             NULL,
                                             NULL);
        }
        else
        {
            /*
             * Otherwise, for the moment, take the number of distinct groups
             * as equal to the total input size, i.e., 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 = apath->rows;
        }
 
        if (can_hash)
        {
            Path       *path;
 
            /*
             * Try a hash aggregate plan on 'apath'.  This is the cheapest
             * available path containing each append child.
             */
            path = (Path *) create_agg_path(root,
                                            result_rel,
                                            apath,
                                            result_rel->reltarget,
                                            AGG_HASHED,
                                            AGGSPLIT_SIMPLE,
                                            groupList,
                                            NIL,
                                            NULL,
                                            dNumGroups);
            add_path(result_rel, path);
 
            /* Try hash aggregate on the Gather path, if valid */
            if (gpath != NULL)
            {
                /* Hashed aggregate plan --- no sort needed */
                path = (Path *) create_agg_path(root,
                                                result_rel,
                                                gpath,
                                                result_rel->reltarget,
                                                AGG_HASHED,
                                                AGGSPLIT_SIMPLE,
                                                groupList,
                                                NIL,
                                                NULL,
                                                dNumGroups);
                add_path(result_rel, path);
            }
        }
 
        if (can_sort)
        {
            Path       *path = apath;
 
            /* Try Sort -> Unique on the Append path */
            if (groupList != NIL)
                path = (Path *) create_sort_path(root, result_rel, path,
                                                 make_pathkeys_for_sortclauses(root, groupList, tlist),
                                                 -1.0);
 
            path = (Path *) create_unique_path(root,
                                               result_rel,
                                               path,
                                               list_length(path->pathkeys),
                                               dNumGroups);
 
            add_path(result_rel, path);
 
            /* Try Sort -> Unique on the Gather path, if set */
            if (gpath != NULL)
            {
                path = gpath;
 
                path = (Path *) create_sort_path(root, result_rel, path,
                                                 make_pathkeys_for_sortclauses(root, groupList, tlist),
                                                 -1.0);
 
                path = (Path *) create_unique_path(root,
                                                   result_rel,
                                                   path,
                                                   list_length(path->pathkeys),
                                                   dNumGroups);
                add_path(result_rel, path);
            }
        }
 
        /*
         * Try making a MergeAppend path if we managed to find a path with the
         * correct pathkeys in each union child query.
         */
        if (try_sorted && groupList != NIL)
        {
            Path       *path;
 
            path = (Path *) create_merge_append_path(root,
                                                     result_rel,
                                                     ordered.subpaths,
                                                     NIL,
                                                     union_pathkeys,
                                                     NULL);
 
            /* and make the MergeAppend unique */
            path = (Path *) create_unique_path(root,
                                               result_rel,
                                               path,
                                               list_length(tlist),
                                               dNumGroups);
 
            add_path(result_rel, path);
        }
    }
    else
    {
        /* UNION ALL */
        add_path(result_rel, apath);
 
        if (gpath != NULL)
            add_path(result_rel, gpath);
    }
 
    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,
               *groupList;
    bool        lpath_trivial_tlist,
                rpath_trivial_tlist,
                result_trivial_tlist;
    List       *nonunion_pathkeys = NIL;
    double      dLeftGroups,
                dRightGroups,
                dNumGroups,
                dNumOutputRows;
    bool        can_sort;
    bool        can_hash;
    SetOpCmd    cmd;
 
    /*
     * Tell children to fetch all tuples.
     */
    root->tuple_fraction = 0.0;
 
    /* Recurse on children */
    lrel = recurse_set_operations(op->larg, root,
                                  op,
                                  op->colTypes, op->colCollations,
                                  refnames_tlist,
                                  &lpath_tlist,
                                  &lpath_trivial_tlist);
 
    rrel = recurse_set_operations(op->rarg, root,
                                  op,
                                  op->colTypes, op->colCollations,
                                  refnames_tlist,
                                  &rpath_tlist,
                                  &rpath_trivial_tlist);
 
    /*
     * Generate tlist for SetOp plan node.
     *
     * The tlist for a SetOp plan isn't important so far as the SetOp is
     * concerned, but we must make it look real anyway for the benefit of the
     * next plan level up.
     */
    tlist = generate_setop_tlist(op->colTypes, op->colCollations,
                                 0, false, lpath_tlist, refnames_tlist,
                                 &result_trivial_tlist);
 
    /* We should not have needed any type coercions in the tlist */
    Assert(result_trivial_tlist);
 
    *pTargetList = tlist;
 
    /* Identify the grouping semantics */
    groupList = generate_setop_grouplist(op, tlist);
 
    /* Check whether the operators support sorting or hashing */
    can_sort = grouping_is_sortable(groupList);
    can_hash = grouping_is_hashable(groupList);
    if (!can_sort && !can_hash)
        ereport(ERROR,
                (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
        /* translator: %s is INTERSECT or EXCEPT */
                 errmsg("could not implement %s",
                        (op->op == SETOP_INTERSECT) ? "INTERSECT" : "EXCEPT"),
                 errdetail("Some of the datatypes only support hashing, while others only support sorting.")));
 
    if (can_sort)
    {
        /* Determine the pathkeys for sorting by the whole target list */
        nonunion_pathkeys = make_pathkeys_for_sortclauses(root, groupList,
                                                          tlist);
 
        root->query_pathkeys = nonunion_pathkeys;
    }
 
    /*
     * Now that we've got all that info, we can build the child paths.
     */
    if (lrel->rtekind == RTE_SUBQUERY)
        build_setop_child_paths(root, lrel, lpath_trivial_tlist, lpath_tlist,
                                nonunion_pathkeys, &dLeftGroups);
    else
        dLeftGroups = lrel->rows;
    if (rrel->rtekind == RTE_SUBQUERY)
        build_setop_child_paths(root, rrel, rpath_trivial_tlist, rpath_tlist,
                                nonunion_pathkeys, &dRightGroups);
    else
        dRightGroups = rrel->rows;
 
    /* 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 (a) minimize the size of the hash table in the
     * hashing case, and (b) improve our chances of exploiting the executor's
     * fast path for empty left-hand input.  "Smaller" means the one with the
     * fewer groups.
     */
    if (op->op != SETOP_EXCEPT && dLeftGroups > dRightGroups)
    {
        /* need to swap the two inputs */
        RelOptInfo *tmprel;
        List       *tmplist;
        double      tmpd;
 
        tmprel = lrel;
        lrel = rrel;
        rrel = tmprel;
        tmplist = lpath_tlist;
        lpath_tlist = rpath_tlist;
        rpath_tlist = tmplist;
        tmpd = dLeftGroups;
        dLeftGroups = dRightGroups;
        dRightGroups = tmpd;
    }
 
    lpath = lrel->cheapest_total_path;
    rpath = rrel->cheapest_total_path;
 
    /* Build result relation. */
    result_rel = fetch_upper_rel(root, UPPERREL_SETOP,
                                 bms_union(lrel->relids, rrel->relids));
 
    /*
     * Create the PathTarget and set the width accordingly.  For EXCEPT, since
     * the set op result won't contain rows from the rpath, we only account
     * for the width of the lpath.  For INTERSECT, use both input paths.
     */
    if (op->op == SETOP_EXCEPT)
        result_rel->reltarget = create_setop_pathtarget(root, tlist,
                                                        list_make1(lpath));
    else
        result_rel->reltarget = create_setop_pathtarget(root, tlist,
                                                        list_make2(lpath, rpath));
 
    /* Check for provably empty setop inputs and add short-circuit paths. */
    if (op->op == SETOP_EXCEPT)
    {
        /*
         * For EXCEPTs, if the left side is dummy then there's no need to
         * inspect the right-hand side as scanning the right to find tuples to
         * remove won't make the left-hand input any more empty.
         */
        if (is_dummy_rel(lrel))
        {
            mark_dummy_rel(result_rel);
 
            return result_rel;
        }
 
        /* Handle EXCEPTs with dummy right input */
        if (is_dummy_rel(rrel))
        {
            if (op->all)
            {
                Path       *apath;
                AppendPathInput append = {0};
 
                append.subpaths = list_make1(lpath);
 
                /*
                 * EXCEPT ALL: If the right-hand input is dummy then we can
                 * simply scan the left-hand input.  To keep createplan.c
                 * happy, use a single child Append to handle the translation
                 * between the set op targetlist and the targetlist of the
                 * left input.  The Append will be removed in setrefs.c.
                 */
                apath = (Path *) create_append_path(root, result_rel,
                                                    append, NIL, NULL, 0,
                                                    false, -1);
 
                add_path(result_rel, apath);
 
                return result_rel;
            }
            else
            {
                /*
                 * To make EXCEPT with a dummy RHS work means having to
                 * deduplicate the left input.  That could be done with
                 * AggPaths, but it doesn't seem worth the effort.  Let the
                 * normal path generation code below handle this one.
                 */
            }
        }
    }
    else
    {
        /*
         * For INTERSECT, if either input is a dummy rel then we can mark the
         * result_rel as dummy since intersecting with an empty relation can
         * never yield any results.  This is true regardless of INTERSECT or
         * INTERSECT ALL.
         */
        if (is_dummy_rel(lrel) || is_dummy_rel(rrel))
        {
            mark_dummy_rel(result_rel);
 
            return result_rel;
        }
    }
 
    /*
     * 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 = dLeftGroups;
        dNumOutputRows = op->all ? Min(lpath->rows, rpath->rows) : dNumGroups;
    }
    result_rel->rows = dNumOutputRows;
 
    /* Select the SetOpCmd type */
    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;
    }
 
    /*
     * If we can hash, that just requires a SetOp atop the cheapest inputs.
     */
    if (can_hash)
    {
        path = (Path *) create_setop_path(root,
                                          result_rel,
                                          lpath,
                                          rpath,
                                          cmd,
                                          SETOP_HASHED,
                                          groupList,
                                          dNumGroups,
                                          dNumOutputRows);
        add_path(result_rel, path);
    }
 
    /*
     * If we can sort, generate the cheapest sorted input paths, and add a
     * SetOp atop those.
     */
    if (can_sort)
    {
        List       *pathkeys;
        Path       *slpath,
                   *srpath;
 
        /* First the left input ... */
        pathkeys = make_pathkeys_for_sortclauses(root,
                                                 groupList,
                                                 lpath_tlist);
        if (pathkeys_contained_in(pathkeys, lpath->pathkeys))
            slpath = lpath;     /* cheapest path is already sorted */
        else
        {
            slpath = get_cheapest_path_for_pathkeys(lrel->pathlist,
                                                    nonunion_pathkeys,
                                                    NULL,
                                                    TOTAL_COST,
                                                    false);
            /* Subquery failed to produce any presorted paths? */
            if (slpath == NULL)
                slpath = (Path *) create_sort_path(root,
                                                   lpath->parent,
                                                   lpath,
                                                   pathkeys,
                                                   -1.0);
        }
 
        /* and now the same for the right. */
        pathkeys = make_pathkeys_for_sortclauses(root,
                                                 groupList,
                                                 rpath_tlist);
        if (pathkeys_contained_in(pathkeys, rpath->pathkeys))
            srpath = rpath;     /* cheapest path is already sorted */
        else
        {
            srpath = get_cheapest_path_for_pathkeys(rrel->pathlist,
                                                    nonunion_pathkeys,
                                                    NULL,
                                                    TOTAL_COST,
                                                    false);
            /* Subquery failed to produce any presorted paths? */
            if (srpath == NULL)
                srpath = (Path *) create_sort_path(root,
                                                   rpath->parent,
                                                   rpath,
                                                   pathkeys,
                                                   -1.0);
        }
 
        path = (Path *) create_setop_path(root,
                                          result_rel,
                                          slpath,
                                          srpath,
                                          cmd,
                                          SETOP_SORTED,
                                          groupList,
                                          dNumGroups,
                                          dNumOutputRows);
        add_path(result_rel, path);
    }
 
    return result_rel;
}
 
/*
 * Pull up children of a UNION node that are identically-propertied UNIONs,
 * and perform planning of the queries underneath the N-way UNION.
 *
 * The result is a list of RelOptInfos containing Paths for sub-nodes, with
 * one entry for each descendant that is a leaf query or non-identical setop.
 * We also return parallel lists of the childrens' targetlists and
 * is-trivial-tlist flags.
 *
 * NOTE: we can also pull a UNION ALL up into a UNION, since the distinct
 * output rows will be lost anyway.
 */
static List *
plan_union_children(PlannerInfo *root,
                    SetOperationStmt *top_union,
                    List *refnames_tlist,
                    List **tlist_list,
                    List **istrivial_tlist)
{
    List       *pending_rels = list_make1(top_union);
    List       *result = NIL;
    List       *child_tlist;
    bool        trivial_tlist;
 
    *tlist_list = NIL;
    *istrivial_tlist = 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) &&
                equal(op->colCollations, top_union->colCollations))
            {
                /* 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.
         *
         * If top_union isn't a UNION ALL, then we are interested in sorted
         * output from the child, so pass top_union as parentOp.  Note that
         * this isn't necessarily the child node's immediate SetOperationStmt
         * parent, but that's fine: it's the effective parent.
         */
        result = lappend(result, recurse_set_operations(setOp, root,
                                                        top_union->all ? NULL : top_union,
                                                        top_union->colTypes,
                                                        top_union->colCollations,
                                                        refnames_tlist,
                                                        &child_tlist,
                                                        &trivial_tlist));
        *tlist_list = lappend(*tlist_list, child_tlist);
        *istrivial_tlist = lappend_int(*istrivial_tlist, trivial_tlist);
    }
 
    return result;
}
 
/*
 * 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);
}
 
/*
 * 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
 * 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
 * trivial_tlist: output parameter, set to true if targetlist is trivial
 */
static List *
generate_setop_tlist(List *colTypes, List *colCollations,
                     Index varno,
                     bool hack_constants,
                     List *input_tlist,
                     List *refnames_tlist,
                     bool *trivial_tlist)
{
    List       *tlist = NIL;
    int         resno = 1;
    ListCell   *ctlc,
               *cclc,
               *itlc,
               *rtlc;
    TargetEntry *tle;
    Node       *expr;
 
    *trivial_tlist = true;      /* until proven differently */
 
    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!
         *
         * Note that copying a constant doesn't in itself require us to mark
         * the tlist nontrivial; see trivial_subqueryscan() in setrefs.c.
         */
        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");
            *trivial_tlist = false; /* the coercion makes it not trivial */
        }
 
        /*
         * 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);
            *trivial_tlist = false; /* the relabel makes it not trivial */
        }
 
        tle = makeTargetEntry((Expr *) expr,
                              (AttrNumber) resno++,
                              pstrdup(reftle->resname),
                              false);
 
        /*
         * By convention, all output 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);
    }
 
    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
 * 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,
                      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 = palloc_array(int32, list_length(colTypes));
 
    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);
 
            Assert(!subtle->resjunk);
            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 output 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);
    }
 
    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;
 
        Assert(!tle->resjunk);
 
        /* 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;
}
 
/*
 * create_setop_pathtarget
 *      Do the normal create_pathtarget() work, plus set the resulting
 *      PathTarget's width to the average width of the Paths in child_pathlist
 *      weighted using the estimated row count of each path.
 *
 * Note: This is required because set op target lists use varno==0, which
 * results in a type default width estimate rather than one that's based on
 * statistics of the columns from the set op children.
 */
static PathTarget *
create_setop_pathtarget(PlannerInfo *root, List *tlist, List *child_pathlist)
{
    PathTarget *reltarget;
    ListCell   *lc;
    double      parent_rows = 0;
    double      parent_size = 0;
 
    reltarget = create_pathtarget(root, tlist);
 
    /* Calculate the total rows and total size. */
    foreach(lc, child_pathlist)
    {
        Path       *path = (Path *) lfirst(lc);
 
        parent_rows += path->rows;
        parent_size += path->parent->reltarget->width * path->rows;
    }
 
    if (parent_rows > 0)
        reltarget->width = rint(parent_size / parent_rows);
 
    return reltarget;
}