Teach planner how to rearrange join order for some classes of OUTER JOIN.

[postgresql] / src / backend / optimizer / plan / planner.c

/*-------------------------------------------------------------------------
 *
 * planner.c
 *        The query optimizer external interface.
 *
 * Portions Copyright (c) 1996-2005, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *        $PostgreSQL: pgsql/src/backend/optimizer/plan/planner.c,v 1.196 2005/12/20 02:30:36 tgl Exp $
 *
 *-------------------------------------------------------------------------
 */

#include "postgres.h"

#include <limits.h>

#include "catalog/pg_operator.h"
#include "catalog/pg_type.h"
#include "executor/executor.h"
#include "executor/nodeAgg.h"
#include "miscadmin.h"
#include "nodes/makefuncs.h"
#ifdef OPTIMIZER_DEBUG
#include "nodes/print.h"
#endif
#include "optimizer/clauses.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/subselect.h"
#include "optimizer/tlist.h"
#include "optimizer/var.h"
#include "parser/parsetree.h"
#include "parser/parse_expr.h"
#include "parser/parse_oper.h"
#include "utils/selfuncs.h"
#include "utils/syscache.h"


ParamListInfo PlannerBoundParamList = NULL;             /* current boundParams */


/* Expression kind codes for preprocess_expression */
#define EXPRKIND_QUAL   0
#define EXPRKIND_TARGET 1
#define EXPRKIND_RTFUNC 2
#define EXPRKIND_LIMIT  3
#define EXPRKIND_ININFO 4


static Node *preprocess_expression(PlannerInfo *root, Node *expr, int kind);
static void preprocess_qual_conditions(PlannerInfo *root, Node *jtnode);
static Plan *inheritance_planner(PlannerInfo *root, List *inheritlist);
static Plan *grouping_planner(PlannerInfo *root, double tuple_fraction);
static double preprocess_limit(PlannerInfo *root,
                                 double tuple_fraction,
                                 int *offset_est, int *count_est);
static bool choose_hashed_grouping(PlannerInfo *root, double tuple_fraction,
                                           Path *cheapest_path, Path *sorted_path,
                                           double dNumGroups, AggClauseCounts *agg_counts);
static bool hash_safe_grouping(PlannerInfo *root);
static List *make_subplanTargetList(PlannerInfo *root, List *tlist,
                                           AttrNumber **groupColIdx, bool *need_tlist_eval);
static void locate_grouping_columns(PlannerInfo *root,
                                                List *tlist,
                                                List *sub_tlist,
                                                AttrNumber *groupColIdx);
static List *postprocess_setop_tlist(List *new_tlist, List *orig_tlist);


/*****************************************************************************
 *
 *         Query optimizer entry point
 *
 *****************************************************************************/
Plan *
planner(Query *parse, bool isCursor, int cursorOptions,
                ParamListInfo boundParams)
{
        double          tuple_fraction;
        Plan       *result_plan;
        Index           save_PlannerQueryLevel;
        List       *save_PlannerParamList;
        ParamListInfo save_PlannerBoundParamList;

        /*
         * The planner can be called recursively (an example is when
         * eval_const_expressions tries to pre-evaluate an SQL function). So,
         * these global state variables must be saved and restored.
         *
         * Query level and the param list cannot be moved into the per-query
         * PlannerInfo structure since their whole purpose is communication across
         * multiple sub-queries. Also, boundParams is explicitly info from outside
         * the query, and so is likewise better handled as a global variable.
         *
         * Note we do NOT save and restore PlannerPlanId: it exists to assign
         * unique IDs to SubPlan nodes, and we want those IDs to be unique for the
         * life of a backend.  Also, PlannerInitPlan is saved/restored in
         * subquery_planner, not here.
         */
        save_PlannerQueryLevel = PlannerQueryLevel;
        save_PlannerParamList = PlannerParamList;
        save_PlannerBoundParamList = PlannerBoundParamList;

        /* Initialize state for handling outer-level references and params */
        PlannerQueryLevel = 0;          /* will be 1 in top-level subquery_planner */
        PlannerParamList = NIL;
        PlannerBoundParamList = boundParams;

        /* Determine what fraction of the plan is likely to be scanned */
        if (isCursor)
        {
                /*
                 * We have no real idea how many tuples the user will ultimately FETCH
                 * from a cursor, but it seems a good bet that he doesn't want 'em
                 * all.  Optimize for 10% retrieval (you gotta better number?  Should
                 * this be a SETtable parameter?)
                 */
                tuple_fraction = 0.10;
        }
        else
        {
                /* Default assumption is we need all the tuples */
                tuple_fraction = 0.0;
        }

        /* primary planning entry point (may recurse for subqueries) */
        result_plan = subquery_planner(parse, tuple_fraction, NULL);

        /* check we popped out the right number of levels */
        Assert(PlannerQueryLevel == 0);

        /*
         * If creating a plan for a scrollable cursor, make sure it can run
         * backwards on demand.  Add a Material node at the top at need.
         */
        if (isCursor && (cursorOptions & CURSOR_OPT_SCROLL))
        {
                if (!ExecSupportsBackwardScan(result_plan))
                        result_plan = materialize_finished_plan(result_plan);
        }

        /* final cleanup of the plan */
        result_plan = set_plan_references(result_plan, parse->rtable);

        /* executor wants to know total number of Params used overall */
        result_plan->nParamExec = list_length(PlannerParamList);

        /* restore state for outer planner, if any */
        PlannerQueryLevel = save_PlannerQueryLevel;
        PlannerParamList = save_PlannerParamList;
        PlannerBoundParamList = save_PlannerBoundParamList;

        return result_plan;
}


/*--------------------
 * subquery_planner
 *        Invokes the planner on a subquery.  We recurse to here for each
 *        sub-SELECT found in the query tree.
 *
 * parse is the querytree produced by the parser & rewriter.
 * tuple_fraction is the fraction of tuples we expect will be retrieved.
 * tuple_fraction is interpreted as explained for grouping_planner, below.
 *
 * If subquery_pathkeys isn't NULL, it receives a list of pathkeys indicating
 * the output sort ordering of the completed plan.
 *
 * Basically, this routine does the stuff that should only be done once
 * per Query object.  It then calls grouping_planner.  At one time,
 * grouping_planner could be invoked recursively on the same Query object;
 * that's not currently true, but we keep the separation between the two
 * routines anyway, in case we need it again someday.
 *
 * subquery_planner will be called recursively to handle sub-Query nodes
 * found within the query's expressions and rangetable.
 *
 * Returns a query plan.
 *--------------------
 */
Plan *
subquery_planner(Query *parse, double tuple_fraction,
                                 List **subquery_pathkeys)
{
        List       *saved_initplan = PlannerInitPlan;
        int                     saved_planid = PlannerPlanId;
        PlannerInfo *root;
        Plan       *plan;
        List       *newHaving;
        List       *lst;
        ListCell   *l;

        /* Set up for a new level of subquery */
        PlannerQueryLevel++;
        PlannerInitPlan = NIL;

        /* Create a PlannerInfo data structure for this subquery */
        root = makeNode(PlannerInfo);
        root->parse = parse;

        /*
         * Look for IN clauses at the top level of WHERE, and transform them into
         * joins.  Note that this step only handles IN clauses originally at top
         * level of WHERE; if we pull up any subqueries in the next step, their
         * INs are processed just before pulling them up.
         */
        root->in_info_list = NIL;
        if (parse->hasSubLinks)
                parse->jointree->quals = pull_up_IN_clauses(root,
                                                                                                        parse->jointree->quals);

        /*
         * Check to see if any subqueries in the rangetable can be merged into
         * this query.
         */
        parse->jointree = (FromExpr *)
                pull_up_subqueries(root, (Node *) parse->jointree, false);

        /*
         * Detect whether any rangetable entries are RTE_JOIN kind; if not, we can
         * avoid the expense of doing flatten_join_alias_vars().  Also check for
         * outer joins --- if none, we can skip reduce_outer_joins() and some
         * other processing.  This must be done after we have done
         * pull_up_subqueries, of course.
         *
         * Note: if reduce_outer_joins manages to eliminate all outer joins,
         * root->hasOuterJoins is not reset currently.  This is OK since its
         * purpose is merely to suppress unnecessary processing in simple cases.
         */
        root->hasJoinRTEs = false;
        root->hasOuterJoins = false;
        foreach(l, parse->rtable)
        {
                RangeTblEntry *rte = (RangeTblEntry *) lfirst(l);

                if (rte->rtekind == RTE_JOIN)
                {
                        root->hasJoinRTEs = true;
                        if (IS_OUTER_JOIN(rte->jointype))
                        {
                                root->hasOuterJoins = true;
                                /* Can quit scanning once we find an outer join */
                                break;
                        }
                }
        }

        /*
         * Set hasHavingQual to remember if HAVING clause is present.  Needed
         * because preprocess_expression will reduce a constant-true condition to
         * an empty qual list ... but "HAVING TRUE" is not a semantic no-op.
         */
        root->hasHavingQual = (parse->havingQual != NULL);

        /*
         * Do expression preprocessing on targetlist and quals.
         */
        parse->targetList = (List *)
                preprocess_expression(root, (Node *) parse->targetList,
                                                          EXPRKIND_TARGET);

        preprocess_qual_conditions(root, (Node *) parse->jointree);

        parse->havingQual = preprocess_expression(root, parse->havingQual,
                                                                                          EXPRKIND_QUAL);

        parse->limitOffset = preprocess_expression(root, parse->limitOffset,
                                                                                           EXPRKIND_LIMIT);
        parse->limitCount = preprocess_expression(root, parse->limitCount,
                                                                                          EXPRKIND_LIMIT);

        root->in_info_list = (List *)
                preprocess_expression(root, (Node *) root->in_info_list,
                                                          EXPRKIND_ININFO);

        /* Also need to preprocess expressions for function RTEs */
        foreach(l, parse->rtable)
        {
                RangeTblEntry *rte = (RangeTblEntry *) lfirst(l);

                if (rte->rtekind == RTE_FUNCTION)
                        rte->funcexpr = preprocess_expression(root, rte->funcexpr,
                                                                                                  EXPRKIND_RTFUNC);
        }

        /*
         * In some cases we may want to transfer a HAVING clause into WHERE. We
         * cannot do so if the HAVING clause contains aggregates (obviously) or
         * volatile functions (since a HAVING clause is supposed to be executed
         * only once per group).  Also, it may be that the clause is so expensive
         * to execute that we're better off doing it only once per group, despite
         * the loss of selectivity.  This is hard to estimate short of doing the
         * entire planning process twice, so we use a heuristic: clauses
         * containing subplans are left in HAVING.      Otherwise, we move or copy the
         * HAVING clause into WHERE, in hopes of eliminating tuples before
         * aggregation instead of after.
         *
         * If the query has explicit grouping then we can simply move such a
         * clause into WHERE; any group that fails the clause will not be in the
         * output because none of its tuples will reach the grouping or
         * aggregation stage.  Otherwise we must have a degenerate (variable-free)
         * HAVING clause, which we put in WHERE so that query_planner() can use it
         * in a gating Result node, but also keep in HAVING to ensure that we
         * don't emit a bogus aggregated row. (This could be done better, but it
         * seems not worth optimizing.)
         *
         * Note that both havingQual and parse->jointree->quals are in
         * implicitly-ANDed-list form at this point, even though they are declared
         * as Node *.
         */
        newHaving = NIL;
        foreach(l, (List *) parse->havingQual)
        {
                Node       *havingclause = (Node *) lfirst(l);

                if (contain_agg_clause(havingclause) ||
                        contain_volatile_functions(havingclause) ||
                        contain_subplans(havingclause))
                {
                        /* keep it in HAVING */
                        newHaving = lappend(newHaving, havingclause);
                }
                else if (parse->groupClause)
                {
                        /* move it to WHERE */
                        parse->jointree->quals = (Node *)
                                lappend((List *) parse->jointree->quals, havingclause);
                }
                else
                {
                        /* put a copy in WHERE, keep it in HAVING */
                        parse->jointree->quals = (Node *)
                                lappend((List *) parse->jointree->quals,
                                                copyObject(havingclause));
                        newHaving = lappend(newHaving, havingclause);
                }
        }
        parse->havingQual = (Node *) newHaving;

        /*
         * If we have any outer joins, try to reduce them to plain inner joins.
         * This step is most easily done after we've done expression
         * preprocessing.
         */
        if (root->hasOuterJoins)
                reduce_outer_joins(root);

        /*
         * Do the main planning.  If we have an inherited target relation, that
         * needs special processing, else go straight to grouping_planner.
         */
        if (parse->resultRelation &&
                (lst = expand_inherited_rtentry(root, parse->resultRelation)) != NIL)
                plan = inheritance_planner(root, lst);
        else
                plan = grouping_planner(root, tuple_fraction);

        /*
         * If any subplans were generated, or if we're inside a subplan, build
         * initPlan list and extParam/allParam sets for plan nodes, and attach the
         * initPlans to the top plan node.
         */
        if (PlannerPlanId != saved_planid || PlannerQueryLevel > 1)
                SS_finalize_plan(plan, parse->rtable);

        /* Return sort ordering info if caller wants it */
        if (subquery_pathkeys)
                *subquery_pathkeys = root->query_pathkeys;

        /* Return to outer subquery context */
        PlannerQueryLevel--;
        PlannerInitPlan = saved_initplan;
        /* we do NOT restore PlannerPlanId; that's not an oversight! */

        return plan;
}

/*
 * preprocess_expression
 *              Do subquery_planner's preprocessing work for an expression,
 *              which can be a targetlist, a WHERE clause (including JOIN/ON
 *              conditions), or a HAVING clause.
 */
static Node *
preprocess_expression(PlannerInfo *root, Node *expr, int kind)
{
        /*
         * Fall out quickly if expression is empty.  This occurs often enough to
         * be worth checking.  Note that null->null is the correct conversion for
         * implicit-AND result format, too.
         */
        if (expr == NULL)
                return NULL;

        /*
         * If the query has any join RTEs, replace join alias variables with
         * base-relation variables. We must do this before sublink processing,
         * else sublinks expanded out from join aliases wouldn't get processed.
         */
        if (root->hasJoinRTEs)
                expr = flatten_join_alias_vars(root, expr);

        /*
         * Simplify constant expressions.
         *
         * Note: this also flattens nested AND and OR expressions into N-argument
         * form.  All processing of a qual expression after this point must be
         * careful to maintain AND/OR flatness --- that is, do not generate a tree
         * with AND directly under AND, nor OR directly under OR.
         *
         * Because this is a relatively expensive process, we skip it when the
         * query is trivial, such as "SELECT 2+2;" or "INSERT ... VALUES()". The
         * expression will only be evaluated once anyway, so no point in
         * pre-simplifying; we can't execute it any faster than the executor can,
         * and we will waste cycles copying the tree.  Notice however that we
         * still must do it for quals (to get AND/OR flatness); and if we are in a
         * subquery we should not assume it will be done only once.
         */
        if (root->parse->jointree->fromlist != NIL ||
                kind == EXPRKIND_QUAL ||
                PlannerQueryLevel > 1)
                expr = eval_const_expressions(expr);

        /*
         * If it's a qual or havingQual, canonicalize it.
         */
        if (kind == EXPRKIND_QUAL)
        {
                expr = (Node *) canonicalize_qual((Expr *) expr);

#ifdef OPTIMIZER_DEBUG
                printf("After canonicalize_qual()\n");
                pprint(expr);
#endif
        }

        /* Expand SubLinks to SubPlans */
        if (root->parse->hasSubLinks)
                expr = SS_process_sublinks(expr, (kind == EXPRKIND_QUAL));

        /*
         * XXX do not insert anything here unless you have grokked the comments in
         * SS_replace_correlation_vars ...
         */

        /* Replace uplevel vars with Param nodes */
        if (PlannerQueryLevel > 1)
                expr = SS_replace_correlation_vars(expr);

        /*
         * If it's a qual or havingQual, convert it to implicit-AND format. (We
         * don't want to do this before eval_const_expressions, since the latter
         * would be unable to simplify a top-level AND correctly. Also,
         * SS_process_sublinks expects explicit-AND format.)
         */
        if (kind == EXPRKIND_QUAL)
                expr = (Node *) make_ands_implicit((Expr *) expr);

        return expr;
}

/*
 * preprocess_qual_conditions
 *              Recursively scan the query's jointree and do subquery_planner's
 *              preprocessing work on each qual condition found therein.
 */
static void
preprocess_qual_conditions(PlannerInfo *root, Node *jtnode)
{
        if (jtnode == NULL)
                return;
        if (IsA(jtnode, RangeTblRef))
        {
                /* nothing to do here */
        }
        else if (IsA(jtnode, FromExpr))
        {
                FromExpr   *f = (FromExpr *) jtnode;
                ListCell   *l;

                foreach(l, f->fromlist)
                        preprocess_qual_conditions(root, lfirst(l));

                f->quals = preprocess_expression(root, f->quals, EXPRKIND_QUAL);
        }
        else if (IsA(jtnode, JoinExpr))
        {
                JoinExpr   *j = (JoinExpr *) jtnode;

                preprocess_qual_conditions(root, j->larg);
                preprocess_qual_conditions(root, j->rarg);

                j->quals = preprocess_expression(root, j->quals, EXPRKIND_QUAL);
        }
        else
                elog(ERROR, "unrecognized node type: %d",
                         (int) nodeTag(jtnode));
}

/*--------------------
 * inheritance_planner
 *        Generate a plan in the case where the result relation is an
 *        inheritance set.
 *
 * We have to handle this case differently from cases where a source
 * relation is an inheritance set.      Source inheritance is expanded at
 * the bottom of the plan tree (see allpaths.c), but target inheritance
 * has to be expanded at the top.  The reason is that for UPDATE, each
 * target relation needs a different targetlist matching its own column
 * set.  (This is not so critical for DELETE, but for simplicity we treat
 * inherited DELETE the same way.)      Fortunately, the UPDATE/DELETE target
 * can never be the nullable side of an outer join, so it's OK to generate
 * the plan this way.
 *
 * inheritlist is an integer list of RT indexes for the result relation set.
 *
 * Returns a query plan.
 *--------------------
 */
static Plan *
inheritance_planner(PlannerInfo *root, List *inheritlist)
{
        Query      *parse = root->parse;
        int                     parentRTindex = parse->resultRelation;
        Oid                     parentOID = getrelid(parentRTindex, parse->rtable);
        int                     mainrtlength = list_length(parse->rtable);
        List       *subplans = NIL;
        List       *tlist = NIL;
        ListCell   *l;

        foreach(l, inheritlist)
        {
                int                     childRTindex = lfirst_int(l);
                Oid                     childOID = getrelid(childRTindex, parse->rtable);
                PlannerInfo subroot;
                Plan       *subplan;

                /*
                 * Generate modified query with this rel as target.  We have to be
                 * prepared to translate varnos in in_info_list as well as in the
                 * Query proper.
                 */
                memcpy(&subroot, root, sizeof(PlannerInfo));
                subroot.parse = (Query *)
                        adjust_inherited_attrs((Node *) parse,
                                                                   parentRTindex, parentOID,
                                                                   childRTindex, childOID);
                subroot.in_info_list = (List *)
                        adjust_inherited_attrs((Node *) root->in_info_list,
                                                                   parentRTindex, parentOID,
                                                                   childRTindex, childOID);
                /* There shouldn't be any OJ info to translate, though */
                Assert(subroot.oj_info_list == NIL);

                /* Generate plan */
                subplan = grouping_planner(&subroot, 0.0 /* retrieve all tuples */ );

                subplans = lappend(subplans, subplan);

                /*
                 * XXX my goodness this next bit is ugly.  Really need to think about
                 * ways to rein in planner's habit of scribbling on its input.
                 *
                 * Planning of the subquery might have modified the rangetable, either
                 * by addition of RTEs due to expansion of inherited source tables, or
                 * by changes of the Query structures inside subquery RTEs.  We have
                 * to ensure that this gets propagated back to the master copy.
                 * However, if we aren't done planning yet, we also need to ensure
                 * that subsequent calls to grouping_planner have virgin sub-Queries
                 * to work from.  So, if we are at the last list entry, just copy the
                 * subquery rangetable back to the master copy; if we are not, then
                 * extend the master copy by adding whatever the subquery added.  (We
                 * assume these added entries will go untouched by the future
                 * grouping_planner calls.      We are also effectively assuming that
                 * sub-Queries will get planned identically each time, or at least
                 * that the impacts on their rangetables will be the same each time.
                 * Did I say this is ugly?)
                 */
                if (lnext(l) == NULL)
                        parse->rtable = subroot.parse->rtable;
                else
                {
                        int                     subrtlength = list_length(subroot.parse->rtable);

                        if (subrtlength > mainrtlength)
                        {
                                List       *subrt;

                                subrt = list_copy_tail(subroot.parse->rtable, mainrtlength);
                                parse->rtable = list_concat(parse->rtable, subrt);
                                mainrtlength = subrtlength;
                        }
                }

                /* Save preprocessed tlist from first rel for use in Append */
                if (tlist == NIL)
                        tlist = subplan->targetlist;
        }

        /* Save the target-relations list for the executor, too */
        parse->resultRelations = inheritlist;

        /* Mark result as unordered (probably unnecessary) */
        root->query_pathkeys = NIL;

        return (Plan *) make_append(subplans, true, tlist);
}

/*--------------------
 * grouping_planner
 *        Perform planning steps related to grouping, aggregation, etc.
 *        This primarily means adding top-level processing to the basic
 *        query plan produced by query_planner.
 *
 * tuple_fraction is the fraction of tuples we expect will be retrieved
 *
 * tuple_fraction is interpreted as follows:
 *        0: expect all tuples to be retrieved (normal case)
 *        0 < tuple_fraction < 1: expect the given fraction of tuples available
 *              from the plan to be retrieved
 *        tuple_fraction >= 1: tuple_fraction is the absolute number of tuples
 *              expected to be retrieved (ie, a LIMIT specification)
 *
 * Returns a query plan.  Also, root->query_pathkeys is returned as the
 * actual output ordering of the plan (in pathkey format).
 *--------------------
 */
static Plan *
grouping_planner(PlannerInfo *root, double tuple_fraction)
{
        Query      *parse = root->parse;
        List       *tlist = parse->targetList;
        int                     offset_est = 0;
        int                     count_est = 0;
        Plan       *result_plan;
        List       *current_pathkeys;
        List       *sort_pathkeys;
        double          dNumGroups = 0;

        /* Tweak caller-supplied tuple_fraction if have LIMIT/OFFSET */
        if (parse->limitCount || parse->limitOffset)
                tuple_fraction = preprocess_limit(root, tuple_fraction,
                                                                                  &offset_est, &count_est);

        if (parse->setOperations)
        {
                List       *set_sortclauses;

                /*
                 * If there's a top-level ORDER BY, assume we have to fetch all the
                 * tuples.      This might seem too simplistic given all the hackery below
                 * to possibly avoid the sort ... but a nonzero tuple_fraction is only
                 * of use to plan_set_operations() when the setop is UNION ALL, and
                 * the result of UNION ALL is always unsorted.
                 */
                if (parse->sortClause)
                        tuple_fraction = 0.0;

                /*
                 * Construct the plan for set operations.  The result will not need
                 * any work except perhaps a top-level sort and/or LIMIT.
                 */
                result_plan = plan_set_operations(root, tuple_fraction,
                                                                                  &set_sortclauses);

                /*
                 * Calculate pathkeys representing the sort order (if any) of the set
                 * operation's result.  We have to do this before overwriting the sort
                 * key information...
                 */
                current_pathkeys = make_pathkeys_for_sortclauses(set_sortclauses,
                                                                                                        result_plan->targetlist);
                current_pathkeys = canonicalize_pathkeys(root, current_pathkeys);

                /*
                 * We should not need to call preprocess_targetlist, since we must be
                 * in a SELECT query node.      Instead, use the targetlist returned by
                 * plan_set_operations (since this tells whether it returned any
                 * resjunk columns!), and transfer any sort key information from the
                 * original tlist.
                 */
                Assert(parse->commandType == CMD_SELECT);

                tlist = postprocess_setop_tlist(result_plan->targetlist, tlist);

                /*
                 * Can't handle FOR UPDATE/SHARE here (parser should have checked
                 * already, but let's make sure).
                 */
                if (parse->rowMarks)
                        ereport(ERROR,
                                        (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
                                         errmsg("SELECT FOR UPDATE/SHARE is not allowed with UNION/INTERSECT/EXCEPT")));

                /*
                 * Calculate pathkeys that represent result ordering requirements
                 */
                sort_pathkeys = make_pathkeys_for_sortclauses(parse->sortClause,
                                                                                                          tlist);
                sort_pathkeys = canonicalize_pathkeys(root, sort_pathkeys);
        }
        else
        {
                /* No set operations, do regular planning */
                List       *sub_tlist;
                List       *group_pathkeys;
                AttrNumber *groupColIdx = NULL;
                bool            need_tlist_eval = true;
                QualCost        tlist_cost;
                Path       *cheapest_path;
                Path       *sorted_path;
                Path       *best_path;
                long            numGroups = 0;
                AggClauseCounts agg_counts;
                int                     numGroupCols = list_length(parse->groupClause);
                bool            use_hashed_grouping = false;

                MemSet(&agg_counts, 0, sizeof(AggClauseCounts));

                /* Preprocess targetlist */
                tlist = preprocess_targetlist(root, tlist);

                /*
                 * Generate appropriate target list for subplan; may be different from
                 * tlist if grouping or aggregation is needed.
                 */
                sub_tlist = make_subplanTargetList(root, tlist,
                                                                                   &groupColIdx, &need_tlist_eval);

                /*
                 * Calculate pathkeys that represent grouping/ordering requirements.
                 * Stash them in PlannerInfo so that query_planner can canonicalize
                 * them.
                 */
                root->group_pathkeys =
                        make_pathkeys_for_sortclauses(parse->groupClause, tlist);
                root->sort_pathkeys =
                        make_pathkeys_for_sortclauses(parse->sortClause, tlist);

                /*
                 * Will need actual number of aggregates for estimating costs.
                 *
                 * Note: we do not attempt to detect duplicate aggregates here; a
                 * somewhat-overestimated count is okay for our present purposes.
                 *
                 * Note: think not that we can turn off hasAggs if we find no aggs. It
                 * is possible for constant-expression simplification to remove all
                 * explicit references to aggs, but we still have to follow the
                 * aggregate semantics (eg, producing only one output row).
                 */
                if (parse->hasAggs)
                {
                        count_agg_clauses((Node *) tlist, &agg_counts);
                        count_agg_clauses(parse->havingQual, &agg_counts);
                }

                /*
                 * Figure out whether we need a sorted result from query_planner.
                 *
                 * If we have a GROUP BY clause, then we want a result sorted properly
                 * for grouping.  Otherwise, if there is an ORDER BY clause, we want
                 * to sort by the ORDER BY clause.      (Note: if we have both, and ORDER
                 * BY is a superset of GROUP BY, it would be tempting to request sort
                 * by ORDER BY --- but that might just leave us failing to exploit an
                 * available sort order at all. Needs more thought...)
                 */
                if (parse->groupClause)
                        root->query_pathkeys = root->group_pathkeys;
                else if (parse->sortClause)
                        root->query_pathkeys = root->sort_pathkeys;
                else
                        root->query_pathkeys = NIL;

                /*
                 * Generate the best unsorted and presorted paths for this Query (but
                 * note there may not be any presorted path).  query_planner will also
                 * estimate the number of groups in the query, and canonicalize all
                 * the pathkeys.
                 */
                query_planner(root, sub_tlist, tuple_fraction,
                                          &cheapest_path, &sorted_path, &dNumGroups);

                group_pathkeys = root->group_pathkeys;
                sort_pathkeys = root->sort_pathkeys;

                /*
                 * If grouping, decide whether we want to use hashed grouping.
                 */
                if (parse->groupClause)
                {
                        use_hashed_grouping =
                                choose_hashed_grouping(root, tuple_fraction,
                                                                           cheapest_path, sorted_path,
                                                                           dNumGroups, &agg_counts);

                        /* Also convert # groups to long int --- but 'ware overflow! */
                        numGroups = (long) Min(dNumGroups, (double) LONG_MAX);
                }

                /*
                 * Select the best path.  If we are doing hashed grouping, we will
                 * always read all the input tuples, so use the cheapest-total path.
                 * Otherwise, trust query_planner's decision about which to use.
                 */
                if (use_hashed_grouping || !sorted_path)
                        best_path = cheapest_path;
                else
                        best_path = sorted_path;

                /*
                 * Check to see if it's possible to optimize MIN/MAX aggregates. If
                 * so, we will forget all the work we did so far to choose a "regular"
                 * path ... but we had to do it anyway to be able to tell which way is
                 * cheaper.
                 */
                result_plan = optimize_minmax_aggregates(root,
                                                                                                 tlist,
                                                                                                 best_path);
                if (result_plan != NULL)
                {
                        /*
                         * optimize_minmax_aggregates generated the full plan, with the
                         * right tlist, and it has no sort order.
                         */
                        current_pathkeys = NIL;
                }
                else
                {
                        /*
                         * Normal case --- create a plan according to query_planner's
                         * results.
                         */
                        result_plan = create_plan(root, best_path);
                        current_pathkeys = best_path->pathkeys;

                        /*
                         * create_plan() returns a plan with just a "flat" tlist of
                         * required Vars.  Usually we need to insert the sub_tlist as the
                         * tlist of the top plan node.  However, we can skip that if we
                         * determined that whatever query_planner chose to return will be
                         * good enough.
                         */
                        if (need_tlist_eval)
                        {
                                /*
                                 * If the top-level plan node is one that cannot do expression
                                 * evaluation, we must insert a Result node to project the
                                 * desired tlist.
                                 */
                                if (!is_projection_capable_plan(result_plan))
                                {
                                        result_plan = (Plan *) make_result(sub_tlist, NULL,
                                                                                                           result_plan);
                                }
                                else
                                {
                                        /*
                                         * Otherwise, just replace the subplan's flat tlist with
                                         * the desired tlist.
                                         */
                                        result_plan->targetlist = sub_tlist;
                                }

                                /*
                                 * Also, account for the cost of evaluation of the sub_tlist.
                                 *
                                 * Up to now, we have only been dealing with "flat" tlists,
                                 * containing just Vars.  So their evaluation cost is zero
                                 * according to the model used by cost_qual_eval() (or if you
                                 * prefer, the cost is factored into cpu_tuple_cost).  Thus we
                                 * can avoid accounting for tlist cost throughout
                                 * query_planner() and subroutines.  But now we've inserted a
                                 * tlist that might contain actual operators, sub-selects, etc
                                 * --- so we'd better account for its cost.
                                 *
                                 * Below this point, any tlist eval cost for added-on nodes
                                 * should be accounted for as we create those nodes.
                                 * Presently, of the node types we can add on, only Agg and
                                 * Group project new tlists (the rest just copy their input
                                 * tuples) --- so make_agg() and make_group() are responsible
                                 * for computing the added cost.
                                 */
                                cost_qual_eval(&tlist_cost, sub_tlist);
                                result_plan->startup_cost += tlist_cost.startup;
                                result_plan->total_cost += tlist_cost.startup +
                                        tlist_cost.per_tuple * result_plan->plan_rows;
                        }
                        else
                        {
                                /*
                                 * Since we're using query_planner's tlist and not the one
                                 * make_subplanTargetList calculated, we have to refigure any
                                 * grouping-column indexes make_subplanTargetList computed.
                                 */
                                locate_grouping_columns(root, tlist, result_plan->targetlist,
                                                                                groupColIdx);
                        }

                        /*
                         * Insert AGG or GROUP node if needed, plus an explicit sort step
                         * if necessary.
                         *
                         * HAVING clause, if any, becomes qual of the Agg or Group node.
                         */
                        if (use_hashed_grouping)
                        {
                                /* Hashed aggregate plan --- no sort needed */
                                result_plan = (Plan *) make_agg(root,
                                                                                                tlist,
                                                                                                (List *) parse->havingQual,
                                                                                                AGG_HASHED,
                                                                                                numGroupCols,
                                                                                                groupColIdx,
                                                                                                numGroups,
                                                                                                agg_counts.numAggs,
                                                                                                result_plan);
                                /* Hashed aggregation produces randomly-ordered results */
                                current_pathkeys = NIL;
                        }
                        else if (parse->hasAggs)
                        {
                                /* Plain aggregate plan --- sort if needed */
                                AggStrategy aggstrategy;

                                if (parse->groupClause)
                                {
                                        if (!pathkeys_contained_in(group_pathkeys,
                                                                                           current_pathkeys))
                                        {
                                                result_plan = (Plan *)
                                                        make_sort_from_groupcols(root,
                                                                                                         parse->groupClause,
                                                                                                         groupColIdx,
                                                                                                         result_plan);
                                                current_pathkeys = group_pathkeys;
                                        }
                                        aggstrategy = AGG_SORTED;

                                        /*
                                         * The AGG node will not change the sort ordering of its
                                         * groups, so current_pathkeys describes the result too.
                                         */
                                }
                                else
                                {
                                        aggstrategy = AGG_PLAIN;
                                        /* Result will be only one row anyway; no sort order */
                                        current_pathkeys = NIL;
                                }

                                result_plan = (Plan *) make_agg(root,
                                                                                                tlist,
                                                                                                (List *) parse->havingQual,
                                                                                                aggstrategy,
                                                                                                numGroupCols,
                                                                                                groupColIdx,
                                                                                                numGroups,
                                                                                                agg_counts.numAggs,
                                                                                                result_plan);
                        }
                        else if (parse->groupClause)
                        {
                                /*
                                 * GROUP BY without aggregation, so insert a group node (plus
                                 * the appropriate sort node, if necessary).
                                 *
                                 * Add an explicit sort if we couldn't make the path come out
                                 * the way the GROUP node needs it.
                                 */
                                if (!pathkeys_contained_in(group_pathkeys, current_pathkeys))
                                {
                                        result_plan = (Plan *)
                                                make_sort_from_groupcols(root,
                                                                                                 parse->groupClause,
                                                                                                 groupColIdx,
                                                                                                 result_plan);
                                        current_pathkeys = group_pathkeys;
                                }

                                result_plan = (Plan *) make_group(root,
                                                                                                  tlist,
                                                                                                  (List *) parse->havingQual,
                                                                                                  numGroupCols,
                                                                                                  groupColIdx,
                                                                                                  dNumGroups,
                                                                                                  result_plan);
                                /* The Group node won't change sort ordering */
                        }
                        else if (root->hasHavingQual)
                        {
                                /*
                                 * No aggregates, and no GROUP BY, but we have a HAVING qual.
                                 * This is a degenerate case in which we are supposed to emit
                                 * either 0 or 1 row depending on whether HAVING succeeds.
                                 * Furthermore, there cannot be any variables in either HAVING
                                 * or the targetlist, so we actually do not need the FROM
                                 * table at all!  We can just throw away the plan-so-far and
                                 * generate a Result node.      This is a sufficiently unusual
                                 * corner case that it's not worth contorting the structure of
                                 * this routine to avoid having to generate the plan in the
                                 * first place.
                                 */
                                result_plan = (Plan *) make_result(tlist,
                                                                                                   parse->havingQual,
                                                                                                   NULL);
                        }
                }                                               /* end of non-minmax-aggregate case */
        }                                                       /* end of if (setOperations) */

        /*
         * If we were not able to make the plan come out in the right order, add
         * an explicit sort step.
         */
        if (parse->sortClause)
        {
                if (!pathkeys_contained_in(sort_pathkeys, current_pathkeys))
                {
                        result_plan = (Plan *)
                                make_sort_from_sortclauses(root,
                                                                                   parse->sortClause,
                                                                                   result_plan);
                        current_pathkeys = sort_pathkeys;
                }
        }

        /*
         * If there is a DISTINCT clause, add the UNIQUE node.
         */
        if (parse->distinctClause)
        {
                result_plan = (Plan *) make_unique(result_plan, parse->distinctClause);

                /*
                 * If there was grouping or aggregation, leave plan_rows as-is (ie,
                 * assume the result was already mostly unique).  If not, use the
                 * number of distinct-groups calculated by query_planner.
                 */
                if (!parse->groupClause && !root->hasHavingQual && !parse->hasAggs)
                        result_plan->plan_rows = dNumGroups;
        }

        /*
         * Finally, if there is a LIMIT/OFFSET clause, add the LIMIT node.
         */
        if (parse->limitCount || parse->limitOffset)
        {
                result_plan = (Plan *) make_limit(result_plan,
                                                                                  parse->limitOffset,
                                                                                  parse->limitCount,
                                                                                  offset_est,
                                                                                  count_est);
        }

        /*
         * Return the actual output ordering in query_pathkeys for possible use by
         * an outer query level.
         */
        root->query_pathkeys = current_pathkeys;

        return result_plan;
}

/*
 * preprocess_limit - do pre-estimation for LIMIT and/or OFFSET clauses
 *
 * We try to estimate the values of the LIMIT/OFFSET clauses, and pass the
 * results back in *count_est and *offset_est.  These variables are set to
 * 0 if the corresponding clause is not present, and -1 if it's present
 * but we couldn't estimate the value for it.  (The "0" convention is OK
 * for OFFSET but a little bit bogus for LIMIT: effectively we estimate
 * LIMIT 0 as though it were LIMIT 1.  But this is in line with the planner's
 * usual practice of never estimating less than one row.)  These values will
 * be passed to make_limit, which see if you change this code.
 *
 * The return value is the suitably adjusted tuple_fraction to use for
 * planning the query.  This adjustment is not overridable, since it reflects
 * plan actions that grouping_planner() will certainly take, not assumptions
 * about context.
 */
static double
preprocess_limit(PlannerInfo *root, double tuple_fraction,
                                 int *offset_est, int *count_est)
{
        Query      *parse = root->parse;
        Node       *est;
        double          limit_fraction;

        /* Should not be called unless LIMIT or OFFSET */
        Assert(parse->limitCount || parse->limitOffset);

        /*
         * Try to obtain the clause values.  We use estimate_expression_value
         * primarily because it can sometimes do something useful with Params.
         */
        if (parse->limitCount)
        {
                est = estimate_expression_value(parse->limitCount);
                if (est && IsA(est, Const))
                {
                        if (((Const *) est)->constisnull)
                        {
                                /* NULL indicates LIMIT ALL, ie, no limit */
                                *count_est = 0; /* treat as not present */
                        }
                        else
                        {
                                *count_est = DatumGetInt32(((Const *) est)->constvalue);
                                if (*count_est <= 0)
                                        *count_est = 1;         /* force to at least 1 */
                        }
                }
                else
                        *count_est = -1;        /* can't estimate */
        }
        else
                *count_est = 0;                 /* not present */

        if (parse->limitOffset)
        {
                est = estimate_expression_value(parse->limitOffset);
                if (est && IsA(est, Const))
                {
                        if (((Const *) est)->constisnull)
                        {
                                /* Treat NULL as no offset; the executor will too */
                                *offset_est = 0;        /* treat as not present */
                        }
                        else
                        {
                                *offset_est = DatumGetInt32(((Const *) est)->constvalue);
                                if (*offset_est < 0)
                                        *offset_est = 0;        /* less than 0 is same as 0 */
                        }
                }
                else
                        *offset_est = -1;       /* can't estimate */
        }
        else
                *offset_est = 0;                /* not present */

        if (*count_est != 0)
        {
                /*
                 * A LIMIT clause limits the absolute number of tuples returned.
                 * However, if it's not a constant LIMIT then we have to guess; for
                 * lack of a better idea, assume 10% of the plan's result is wanted.
                 */
                if (*count_est < 0 || *offset_est < 0)
                {
                        /* LIMIT or OFFSET is an expression ... punt ... */
                        limit_fraction = 0.10;
                }
                else
                {
                        /* LIMIT (plus OFFSET, if any) is max number of tuples needed */
                        limit_fraction = (double) *count_est + (double) *offset_est;
                }

                /*
                 * If we have absolute limits from both caller and LIMIT, use the
                 * smaller value; likewise if they are both fractional.  If one is
                 * fractional and the other absolute, we can't easily determine which
                 * is smaller, but we use the heuristic that the absolute will usually
                 * be smaller.
                 */
                if (tuple_fraction >= 1.0)
                {
                        if (limit_fraction >= 1.0)
                        {
                                /* both absolute */
                                tuple_fraction = Min(tuple_fraction, limit_fraction);
                        }
                        else
                        {
                                /* caller absolute, limit fractional; use caller's value */
                        }
                }
                else if (tuple_fraction > 0.0)
                {
                        if (limit_fraction >= 1.0)
                        {
                                /* caller fractional, limit absolute; use limit */
                                tuple_fraction = limit_fraction;
                        }
                        else
                        {
                                /* both fractional */
                                tuple_fraction = Min(tuple_fraction, limit_fraction);
                        }
                }
                else
                {
                        /* no info from caller, just use limit */
                        tuple_fraction = limit_fraction;
                }
        }
        else if (*offset_est != 0 && tuple_fraction > 0.0)
        {
                /*
                 * We have an OFFSET but no LIMIT.      This acts entirely differently
                 * from the LIMIT case: here, we need to increase rather than decrease
                 * the caller's tuple_fraction, because the OFFSET acts to cause more
                 * tuples to be fetched instead of fewer.  This only matters if we got
                 * a tuple_fraction > 0, however.
                 *
                 * As above, use 10% if OFFSET is present but unestimatable.
                 */
                if (*offset_est < 0)
                        limit_fraction = 0.10;
                else
                        limit_fraction = (double) *offset_est;

                /*
                 * If we have absolute counts from both caller and OFFSET, add them
                 * together; likewise if they are both fractional.      If one is
                 * fractional and the other absolute, we want to take the larger, and
                 * we heuristically assume that's the fractional one.
                 */
                if (tuple_fraction >= 1.0)
                {
                        if (limit_fraction >= 1.0)
                        {
                                /* both absolute, so add them together */
                                tuple_fraction += limit_fraction;
                        }
                        else
                        {
                                /* caller absolute, limit fractional; use limit */
                                tuple_fraction = limit_fraction;
                        }
                }
                else
                {
                        if (limit_fraction >= 1.0)
                        {
                                /* caller fractional, limit absolute; use caller's value */
                        }
                        else
                        {
                                /* both fractional, so add them together */
                                tuple_fraction += limit_fraction;
                                if (tuple_fraction >= 1.0)
                                        tuple_fraction = 0.0;           /* assume fetch all */
                        }
                }
        }

        return tuple_fraction;
}

/*
 * choose_hashed_grouping - should we use hashed grouping?
 */
static bool
choose_hashed_grouping(PlannerInfo *root, double tuple_fraction,
                                           Path *cheapest_path, Path *sorted_path,
                                           double dNumGroups, AggClauseCounts *agg_counts)
{
        int                     numGroupCols = list_length(root->parse->groupClause);
        double          cheapest_path_rows;
        int                     cheapest_path_width;
        Size            hashentrysize;
        List       *current_pathkeys;
        Path            hashed_p;
        Path            sorted_p;

        /*
         * Check can't-do-it conditions, including whether the grouping operators
         * are hashjoinable.
         *
         * Executor doesn't support hashed aggregation with DISTINCT aggregates.
         * (Doing so would imply storing *all* the input values in the hash table,
         * which seems like a certain loser.)
         */
        if (!enable_hashagg)
                return false;
        if (agg_counts->numDistinctAggs != 0)
                return false;
        if (!hash_safe_grouping(root))
                return false;

        /*
         * Don't do it if it doesn't look like the hashtable will fit into
         * work_mem.
         *
         * Beware here of the possibility that cheapest_path->parent is NULL. This
         * could happen if user does something silly like SELECT 'foo' GROUP BY 1;
         */
        if (cheapest_path->parent)
        {
                cheapest_path_rows = cheapest_path->parent->rows;
                cheapest_path_width = cheapest_path->parent->width;
        }
        else
        {
                cheapest_path_rows = 1; /* assume non-set result */
                cheapest_path_width = 100;              /* arbitrary */
        }

        /* Estimate per-hash-entry space at tuple width... */
        hashentrysize = cheapest_path_width;
        /* plus space for pass-by-ref transition values... */
        hashentrysize += agg_counts->transitionSpace;
        /* plus the per-hash-entry overhead */
        hashentrysize += hash_agg_entry_size(agg_counts->numAggs);

        if (hashentrysize * dNumGroups > work_mem * 1024L)
                return false;

        /*
         * See if the estimated cost is no more than doing it the other way. While
         * avoiding the need for sorted input is usually a win, the fact that the
         * output won't be sorted may be a loss; so we need to do an actual cost
         * comparison.
         *
         * We need to consider cheapest_path + hashagg [+ final sort] versus
         * either cheapest_path [+ sort] + group or agg [+ final sort] or
         * presorted_path + group or agg [+ final sort] where brackets indicate a
         * step that may not be needed. We assume query_planner() will have
         * returned a presorted path only if it's a winner compared to
         * cheapest_path for this purpose.
         *
         * 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, agg_counts->numAggs,
                         numGroupCols, dNumGroups,
                         cheapest_path->startup_cost, cheapest_path->total_cost,
                         cheapest_path_rows);
        /* Result of hashed agg is always unsorted */
        if (root->sort_pathkeys)
                cost_sort(&hashed_p, root, root->sort_pathkeys, hashed_p.total_cost,
                                  dNumGroups, cheapest_path_width);

        if (sorted_path)
        {
                sorted_p.startup_cost = sorted_path->startup_cost;
                sorted_p.total_cost = sorted_path->total_cost;
                current_pathkeys = sorted_path->pathkeys;
        }
        else
        {
                sorted_p.startup_cost = cheapest_path->startup_cost;
                sorted_p.total_cost = cheapest_path->total_cost;
                current_pathkeys = cheapest_path->pathkeys;
        }
        if (!pathkeys_contained_in(root->group_pathkeys, current_pathkeys))
        {
                cost_sort(&sorted_p, root, root->group_pathkeys, sorted_p.total_cost,
                                  cheapest_path_rows, cheapest_path_width);
                current_pathkeys = root->group_pathkeys;
        }

        if (root->parse->hasAggs)
                cost_agg(&sorted_p, root, AGG_SORTED, agg_counts->numAggs,
                                 numGroupCols, dNumGroups,
                                 sorted_p.startup_cost, sorted_p.total_cost,
                                 cheapest_path_rows);
        else
                cost_group(&sorted_p, root, numGroupCols, dNumGroups,
                                   sorted_p.startup_cost, sorted_p.total_cost,
                                   cheapest_path_rows);
        /* The Agg or Group node will preserve ordering */
        if (root->sort_pathkeys &&
                !pathkeys_contained_in(root->sort_pathkeys, current_pathkeys))
                cost_sort(&sorted_p, root, root->sort_pathkeys, sorted_p.total_cost,
                                  dNumGroups, cheapest_path_width);

        /*
         * Now make the decision using the top-level tuple fraction.  First we
         * have to convert an absolute count (LIMIT) into fractional form.
         */
        if (tuple_fraction >= 1.0)
                tuple_fraction /= dNumGroups;

        if (compare_fractional_path_costs(&hashed_p, &sorted_p,
                                                                          tuple_fraction) < 0)
        {
                /* Hashed is cheaper, so use it */
                return true;
        }
        return false;
}

/*
 * hash_safe_grouping - are grouping operators hashable?
 *
 * We assume hashed aggregation will work if the datatype's equality operator
 * is marked hashjoinable.
 */
static bool
hash_safe_grouping(PlannerInfo *root)
{
        ListCell   *gl;

        foreach(gl, root->parse->groupClause)
        {
                GroupClause *grpcl = (GroupClause *) lfirst(gl);
                TargetEntry *tle = get_sortgroupclause_tle(grpcl,
                                                                                                   root->parse->targetList);
                Operator        optup;
                bool            oprcanhash;

                optup = equality_oper(exprType((Node *) tle->expr), true);
                if (!optup)
                        return false;
                oprcanhash = ((Form_pg_operator) GETSTRUCT(optup))->oprcanhash;
                ReleaseSysCache(optup);
                if (!oprcanhash)
                        return false;
        }
        return true;
}

/*---------------
 * make_subplanTargetList
 *        Generate appropriate target list when grouping is required.
 *
 * When grouping_planner inserts Aggregate, Group, or Result plan nodes
 * above the result of query_planner, we typically want to pass a different
 * target list to query_planner than the outer plan nodes should have.
 * This routine generates the correct target list for the subplan.
 *
 * The initial target list passed from the parser already contains entries
 * for all ORDER BY and GROUP BY expressions, but it will not have entries
 * for variables used only in HAVING clauses; so we need to add those
 * variables to the subplan target list.  Also, we flatten all expressions
 * except GROUP BY items into their component variables; the other expressions
 * will be computed by the inserted nodes rather than by the subplan.
 * For example, given a query like
 *              SELECT a+b,SUM(c+d) FROM table GROUP BY a+b;
 * we want to pass this targetlist to the subplan:
 *              a,b,c,d,a+b
 * where the a+b target will be used by the Sort/Group steps, and the
 * other targets will be used for computing the final results.  (In the
 * above example we could theoretically suppress the a and b targets and
 * pass down only c,d,a+b, but it's not really worth the trouble to
 * eliminate simple var references from the subplan.  We will avoid doing
 * the extra computation to recompute a+b at the outer level; see
 * replace_vars_with_subplan_refs() in setrefs.c.)
 *
 * If we are grouping or aggregating, *and* there are no non-Var grouping
 * expressions, then the returned tlist is effectively dummy; we do not
 * need to force it to be evaluated, because all the Vars it contains
 * should be present in the output of query_planner anyway.
 *
 * 'tlist' is the query's target list.
 * 'groupColIdx' receives an array of column numbers for the GROUP BY
 *                      expressions (if there are any) in the subplan's target list.
 * 'need_tlist_eval' is set true if we really need to evaluate the
 *                      result tlist.
 *
 * The result is the targetlist to be passed to the subplan.
 *---------------
 */
static List *
make_subplanTargetList(PlannerInfo *root,
                                           List *tlist,
                                           AttrNumber **groupColIdx,
                                           bool *need_tlist_eval)
{
        Query      *parse = root->parse;
        List       *sub_tlist;
        List       *extravars;
        int                     numCols;

        *groupColIdx = NULL;

        /*
         * If we're not grouping or aggregating, there's nothing to do here;
         * query_planner should receive the unmodified target list.
         */
        if (!parse->hasAggs && !parse->groupClause && !root->hasHavingQual)
        {
                *need_tlist_eval = true;
                return tlist;
        }

        /*
         * Otherwise, start with a "flattened" tlist (having just the vars
         * mentioned in the targetlist and HAVING qual --- but not upper- level
         * Vars; they will be replaced by Params later on).
         */
        sub_tlist = flatten_tlist(tlist);
        extravars = pull_var_clause(parse->havingQual, false);
        sub_tlist = add_to_flat_tlist(sub_tlist, extravars);
        list_free(extravars);
        *need_tlist_eval = false;       /* only eval if not flat tlist */

        /*
         * If grouping, create sub_tlist entries for all GROUP BY expressions
         * (GROUP BY items that are simple Vars should be in the list already),
         * and make an array showing where the group columns are in the sub_tlist.
         */
        numCols = list_length(parse->groupClause);
        if (numCols > 0)
        {
                int                     keyno = 0;
                AttrNumber *grpColIdx;
                ListCell   *gl;

                grpColIdx = (AttrNumber *) palloc(sizeof(AttrNumber) * numCols);
                *groupColIdx = grpColIdx;

                foreach(gl, parse->groupClause)
                {
                        GroupClause *grpcl = (GroupClause *) lfirst(gl);
                        Node       *groupexpr = get_sortgroupclause_expr(grpcl, tlist);
                        TargetEntry *te = NULL;
                        ListCell   *sl;

                        /* Find or make a matching sub_tlist entry */
                        foreach(sl, sub_tlist)
                        {
                                te = (TargetEntry *) lfirst(sl);
                                if (equal(groupexpr, te->expr))
                                        break;
                        }
                        if (!sl)
                        {
                                te = makeTargetEntry((Expr *) groupexpr,
                                                                         list_length(sub_tlist) + 1,
                                                                         NULL,
                                                                         false);
                                sub_tlist = lappend(sub_tlist, te);
                                *need_tlist_eval = true;                /* it's not flat anymore */
                        }

                        /* and save its resno */
                        grpColIdx[keyno++] = te->resno;
                }
        }

        return sub_tlist;
}

/*
 * locate_grouping_columns
 *              Locate grouping columns in the tlist chosen by query_planner.
 *
 * This is only needed if we don't use the sub_tlist chosen by
 * make_subplanTargetList.      We have to forget the column indexes found
 * by that routine and re-locate the grouping vars in the real sub_tlist.
 */
static void
locate_grouping_columns(PlannerInfo *root,
                                                List *tlist,
                                                List *sub_tlist,
                                                AttrNumber *groupColIdx)
{
        int                     keyno = 0;
        ListCell   *gl;

        /*
         * No work unless grouping.
         */
        if (!root->parse->groupClause)
        {
                Assert(groupColIdx == NULL);
                return;
        }
        Assert(groupColIdx != NULL);

        foreach(gl, root->parse->groupClause)
        {
                GroupClause *grpcl = (GroupClause *) lfirst(gl);
                Node       *groupexpr = get_sortgroupclause_expr(grpcl, tlist);
                TargetEntry *te = NULL;
                ListCell   *sl;

                foreach(sl, sub_tlist)
                {
                        te = (TargetEntry *) lfirst(sl);
                        if (equal(groupexpr, te->expr))
                                break;
                }
                if (!sl)
                        elog(ERROR, "failed to locate grouping columns");

                groupColIdx[keyno++] = te->resno;
        }
}

/*
 * postprocess_setop_tlist
 *        Fix up targetlist returned by plan_set_operations().
 *
 * We need to transpose sort key info from the orig_tlist into new_tlist.
 * NOTE: this would not be good enough if we supported resjunk sort keys
 * for results of set operations --- then, we'd need to project a whole
 * new tlist to evaluate the resjunk columns.  For now, just ereport if we
 * find any resjunk columns in orig_tlist.
 */
static List *
postprocess_setop_tlist(List *new_tlist, List *orig_tlist)
{
        ListCell   *l;
        ListCell   *orig_tlist_item = list_head(orig_tlist);

        foreach(l, new_tlist)
        {
                TargetEntry *new_tle = (TargetEntry *) lfirst(l);
                TargetEntry *orig_tle;

                /* ignore resjunk columns in setop result */
                if (new_tle->resjunk)
                        continue;

                Assert(orig_tlist_item != NULL);
                orig_tle = (TargetEntry *) lfirst(orig_tlist_item);
                orig_tlist_item = lnext(orig_tlist_item);
                if (orig_tle->resjunk)  /* should not happen */
                        elog(ERROR, "resjunk output columns are not implemented");
                Assert(new_tle->resno == orig_tle->resno);
                new_tle->ressortgroupref = orig_tle->ressortgroupref;
        }
        if (orig_tlist_item != NULL)
                elog(ERROR, "resjunk output columns are not implemented");
        return new_tlist;
}