[postgresql] / src / backend / optimizer / path / allpaths.c

/*-------------------------------------------------------------------------
 *
 * allpaths.c
 *        Routines to find possible search paths for processing a query
 *
 * Portions Copyright (c) 1996-2008, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *        $PostgreSQL: pgsql/src/backend/optimizer/path/allpaths.c,v 1.171 2008/06/27 03:56:55 tgl Exp $
 *
 *-------------------------------------------------------------------------
 */

#include "postgres.h"

#include <math.h>

#ifdef OPTIMIZER_DEBUG
#include "nodes/print.h"
#endif
#include "optimizer/clauses.h"
#include "optimizer/cost.h"
#include "optimizer/geqo.h"
#include "optimizer/pathnode.h"
#include "optimizer/paths.h"
#include "optimizer/plancat.h"
#include "optimizer/planner.h"
#include "optimizer/prep.h"
#include "optimizer/var.h"
#include "parser/parse_clause.h"
#include "parser/parse_expr.h"
#include "parser/parsetree.h"
#include "rewrite/rewriteManip.h"


/* These parameters are set by GUC */
bool            enable_geqo = false;    /* just in case GUC doesn't set it */
int                     geqo_threshold;

/* Hook for plugins to replace standard_join_search() */
join_search_hook_type join_search_hook = NULL;


static void set_base_rel_pathlists(PlannerInfo *root);
static void set_rel_pathlist(PlannerInfo *root, RelOptInfo *rel,
                                 Index rti, RangeTblEntry *rte);
static void set_plain_rel_pathlist(PlannerInfo *root, RelOptInfo *rel,
                                           RangeTblEntry *rte);
static void set_append_rel_pathlist(PlannerInfo *root, RelOptInfo *rel,
                                                Index rti, RangeTblEntry *rte);
static void set_dummy_rel_pathlist(RelOptInfo *rel);
static void set_subquery_pathlist(PlannerInfo *root, RelOptInfo *rel,
                                          Index rti, RangeTblEntry *rte);
static void set_function_pathlist(PlannerInfo *root, RelOptInfo *rel,
                                          RangeTblEntry *rte);
static void set_values_pathlist(PlannerInfo *root, RelOptInfo *rel,
                                        RangeTblEntry *rte);
static RelOptInfo *make_rel_from_joinlist(PlannerInfo *root, List *joinlist);
static bool subquery_is_pushdown_safe(Query *subquery, Query *topquery,
                                                  bool *differentTypes);
static bool recurse_pushdown_safe(Node *setOp, Query *topquery,
                                          bool *differentTypes);
static void compare_tlist_datatypes(List *tlist, List *colTypes,
                                                bool *differentTypes);
static bool qual_is_pushdown_safe(Query *subquery, Index rti, Node *qual,
                                          bool *differentTypes);
static void subquery_push_qual(Query *subquery,
                                   RangeTblEntry *rte, Index rti, Node *qual);
static void recurse_push_qual(Node *setOp, Query *topquery,
                                  RangeTblEntry *rte, Index rti, Node *qual);


/*
 * make_one_rel
 *        Finds all possible access paths for executing a query, returning a
 *        single rel that represents the join of all base rels in the query.
 */
RelOptInfo *
make_one_rel(PlannerInfo *root, List *joinlist)
{
        RelOptInfo *rel;

        /*
         * Generate access paths for the base rels.
         */
        set_base_rel_pathlists(root);

        /*
         * Generate access paths for the entire join tree.
         */
        rel = make_rel_from_joinlist(root, joinlist);

        /*
         * The result should join all and only the query's base rels.
         */
#ifdef USE_ASSERT_CHECKING
        {
                int                     num_base_rels = 0;
                Index           rti;

                for (rti = 1; rti < root->simple_rel_array_size; rti++)
                {
                        RelOptInfo *brel = root->simple_rel_array[rti];

                        if (brel == NULL)
                                continue;

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

                        /* ignore RTEs that are "other rels" */
                        if (brel->reloptkind != RELOPT_BASEREL)
                                continue;

                        Assert(bms_is_member(rti, rel->relids));
                        num_base_rels++;
                }

                Assert(bms_num_members(rel->relids) == num_base_rels);
        }
#endif

        return rel;
}

/*
 * set_base_rel_pathlists
 *        Finds all paths available for scanning each base-relation entry.
 *        Sequential scan and any available indices are considered.
 *        Each useful path is attached to its relation's 'pathlist' field.
 */
static void
set_base_rel_pathlists(PlannerInfo *root)
{
        Index           rti;

        for (rti = 1; rti < root->simple_rel_array_size; rti++)
        {
                RelOptInfo *rel = root->simple_rel_array[rti];

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

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

                /* ignore RTEs that are "other rels" */
                if (rel->reloptkind != RELOPT_BASEREL)
                        continue;

                set_rel_pathlist(root, rel, rti, root->simple_rte_array[rti]);
        }
}

/*
 * set_rel_pathlist
 *        Build access paths for a base relation
 */
static void
set_rel_pathlist(PlannerInfo *root, RelOptInfo *rel,
                                 Index rti, RangeTblEntry *rte)
{
        if (rte->inh)
        {
                /* It's an "append relation", process accordingly */
                set_append_rel_pathlist(root, rel, rti, rte);
        }
        else if (rel->rtekind == RTE_SUBQUERY)
        {
                /* Subquery --- generate a separate plan for it */
                set_subquery_pathlist(root, rel, rti, rte);
        }
        else if (rel->rtekind == RTE_FUNCTION)
        {
                /* RangeFunction --- generate a separate plan for it */
                set_function_pathlist(root, rel, rte);
        }
        else if (rel->rtekind == RTE_VALUES)
        {
                /* Values list --- generate a separate plan for it */
                set_values_pathlist(root, rel, rte);
        }
        else
        {
                /* Plain relation */
                Assert(rel->rtekind == RTE_RELATION);
                set_plain_rel_pathlist(root, rel, rte);
        }

#ifdef OPTIMIZER_DEBUG
        debug_print_rel(root, rel);
#endif
}

/*
 * set_plain_rel_pathlist
 *        Build access paths for a plain relation (no subquery, no inheritance)
 */
static void
set_plain_rel_pathlist(PlannerInfo *root, RelOptInfo *rel, RangeTblEntry *rte)
{
        /*
         * If we can prove we don't need to scan the rel via constraint exclusion,
         * set up a single dummy path for it.  We only need to check for regular
         * baserels; if it's an otherrel, CE was already checked in
         * set_append_rel_pathlist().
         */
        if (rel->reloptkind == RELOPT_BASEREL &&
                relation_excluded_by_constraints(root, rel, rte))
        {
                set_dummy_rel_pathlist(rel);
                return;
        }

        /* Mark rel with estimated output rows, width, etc */
        set_baserel_size_estimates(root, rel);

        /* Test any partial indexes of rel for applicability */
        check_partial_indexes(root, rel);

        /*
         * Check to see if we can extract any restriction conditions from join
         * quals that are OR-of-AND structures.  If so, add them to the rel's
         * restriction list, and recompute the size estimates.
         */
        if (create_or_index_quals(root, rel))
                set_baserel_size_estimates(root, rel);

        /*
         * Generate paths and add them to the rel's pathlist.
         *
         * Note: add_path() will discard any paths that are dominated by another
         * available path, keeping only those paths that are superior along at
         * least one dimension of cost or sortedness.
         */

        /* Consider sequential scan */
        add_path(rel, create_seqscan_path(root, rel));

        /* Consider index scans */
        create_index_paths(root, rel);

        /* Consider TID scans */
        create_tidscan_paths(root, rel);

        /* Now find the cheapest of the paths for this rel */
        set_cheapest(rel);
}

/*
 * set_append_rel_pathlist
 *        Build access paths for an "append relation"
 *
 * The passed-in rel and RTE represent the entire append relation.      The
 * relation's contents are computed by appending together the output of
 * the individual member relations.  Note that in the inheritance case,
 * the first member relation is actually the same table as is mentioned in
 * the parent RTE ... but it has a different RTE and RelOptInfo.  This is
 * a good thing because their outputs are not the same size.
 */
static void
set_append_rel_pathlist(PlannerInfo *root, RelOptInfo *rel,
                                                Index rti, RangeTblEntry *rte)
{
        int                     parentRTindex = rti;
        List       *subpaths = NIL;
        double          parent_rows;
        double          parent_size;
        double     *parent_attrsizes;
        int                     nattrs;
        ListCell   *l;

        /*
         * XXX for now, can't handle inherited expansion of FOR UPDATE/SHARE; can
         * we do better?  (This will take some redesign because the executor
         * currently supposes that every rowMark relation is involved in every row
         * returned by the query.)
         */
        if (get_rowmark(root->parse, parentRTindex))
                ereport(ERROR,
                                (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
                                 errmsg("SELECT FOR UPDATE/SHARE is not supported for inheritance queries")));

        /*
         * Initialize to compute size estimates for whole append relation.
         *
         * We handle width estimates by weighting the widths of different
         * child rels proportionally to their number of rows.  This is sensible
         * because the use of width estimates is mainly to compute the total
         * relation "footprint" if we have to sort or hash it.  To do this,
         * we sum the total equivalent size (in "double" arithmetic) and then
         * divide by the total rowcount estimate.  This is done separately for
         * the total rel width and each attribute.
         *
         * Note: if you consider changing this logic, beware that child rels could
         * have zero rows and/or width, if they were excluded by constraints.
         */
        parent_rows = 0;
        parent_size = 0;
        nattrs = rel->max_attr - rel->min_attr + 1;
        parent_attrsizes = (double *) palloc0(nattrs * sizeof(double));

        /*
         * Generate access paths for each member relation, and pick the cheapest
         * path for each one.
         */
        foreach(l, root->append_rel_list)
        {
                AppendRelInfo *appinfo = (AppendRelInfo *) lfirst(l);
                int                     childRTindex;
                RangeTblEntry *childRTE;
                RelOptInfo *childrel;
                Path       *childpath;
                ListCell   *parentvars;
                ListCell   *childvars;

                /* append_rel_list contains all append rels; ignore others */
                if (appinfo->parent_relid != parentRTindex)
                        continue;

                childRTindex = appinfo->child_relid;
                childRTE = root->simple_rte_array[childRTindex];

                /*
                 * The child rel's RelOptInfo was already created during
                 * add_base_rels_to_query.
                 */
                childrel = find_base_rel(root, childRTindex);
                Assert(childrel->reloptkind == RELOPT_OTHER_MEMBER_REL);

                /*
                 * We have to copy the parent's targetlist and quals to the child,
                 * with appropriate substitution of variables.  However, only the
                 * baserestrictinfo quals are needed before we can check for
                 * constraint exclusion; so do that first and then check to see if we
                 * can disregard this child.
                 */
                childrel->baserestrictinfo = (List *)
                        adjust_appendrel_attrs((Node *) rel->baserestrictinfo,
                                                                   appinfo);

                if (relation_excluded_by_constraints(root, childrel, childRTE))
                {
                        /*
                         * This child need not be scanned, so we can omit it from the
                         * appendrel.  Mark it with a dummy cheapest-path though, in case
                         * best_appendrel_indexscan() looks at it later.
                         */
                        set_dummy_rel_pathlist(childrel);
                        continue;
                }

                /* CE failed, so finish copying targetlist and join quals */
                childrel->joininfo = (List *)
                        adjust_appendrel_attrs((Node *) rel->joininfo,
                                                                   appinfo);
                childrel->reltargetlist = (List *)
                        adjust_appendrel_attrs((Node *) rel->reltargetlist,
                                                                   appinfo);

                /*
                 * We have to make child entries in the EquivalenceClass data
                 * structures as well.
                 */
                if (rel->has_eclass_joins)
                {
                        add_child_rel_equivalences(root, appinfo, rel, childrel);
                        childrel->has_eclass_joins = true;
                }

                /*
                 * Copy the parent's attr_needed data as well, with appropriate
                 * adjustment of relids and attribute numbers.
                 */
                pfree(childrel->attr_needed);
                childrel->attr_needed =
                        adjust_appendrel_attr_needed(rel, appinfo,
                                                                                 childrel->min_attr,
                                                                                 childrel->max_attr);

                /*
                 * Compute the child's access paths, and add the cheapest one to the
                 * Append path we are constructing for the parent.
                 *
                 * It's possible that the child is itself an appendrel, in which case
                 * we can "cut out the middleman" and just add its child paths to our
                 * own list.  (We don't try to do this earlier because we need to
                 * apply both levels of transformation to the quals.)
                 */
                set_rel_pathlist(root, childrel, childRTindex, childRTE);

                childpath = childrel->cheapest_total_path;
                if (IsA(childpath, AppendPath))
                        subpaths = list_concat(subpaths,
                                                                   ((AppendPath *) childpath)->subpaths);
                else
                        subpaths = lappend(subpaths, childpath);

                /*
                 * Accumulate size information from each child.
                 */
                if (childrel->rows > 0)
                {
                        parent_rows += childrel->rows;
                        parent_size += childrel->width * childrel->rows;

                        forboth(parentvars, rel->reltargetlist,
                                        childvars, childrel->reltargetlist)
                        {
                                Var                *parentvar = (Var *) lfirst(parentvars);
                                Var                *childvar = (Var *) lfirst(childvars);

                                if (IsA(parentvar, Var) &&
                                        IsA(childvar, Var))
                                {
                                        int                     pndx = parentvar->varattno - rel->min_attr;
                                        int                     cndx = childvar->varattno - childrel->min_attr;

                                        parent_attrsizes[pndx] += childrel->attr_widths[cndx] * childrel->rows;
                                }
                        }
                }
        }

        /*
         * Save the finished size estimates.
         */
        rel->rows = parent_rows;
        if (parent_rows > 0)
        {
                int             i;

                rel->width = rint(parent_size / parent_rows);
                for (i = 0; i < nattrs; i++)
                        rel->attr_widths[i] = rint(parent_attrsizes[i] / parent_rows);
        }
        else
                rel->width = 0;                 /* attr_widths should be zero already */

        /*
         * Set "raw tuples" count equal to "rows" for the appendrel; needed
         * because some places assume rel->tuples is valid for any baserel.
         */
        rel->tuples = parent_rows;

        pfree(parent_attrsizes);

        /*
         * Finally, build Append path and install it as the only access path for
         * the parent rel.      (Note: this is correct even if we have zero or one
         * live subpath due to constraint exclusion.)
         */
        add_path(rel, (Path *) create_append_path(rel, subpaths));

        /* Select cheapest path (pretty easy in this case...) */
        set_cheapest(rel);
}

/*
 * set_dummy_rel_pathlist
 *        Build a dummy path for a relation that's been excluded by constraints
 *
 * Rather than inventing a special "dummy" path type, we represent this as an
 * AppendPath with no members (see also IS_DUMMY_PATH macro).
 */
static void
set_dummy_rel_pathlist(RelOptInfo *rel)
{
        /* Set dummy size estimates --- we leave attr_widths[] as zeroes */
        rel->rows = 0;
        rel->width = 0;

        add_path(rel, (Path *) create_append_path(rel, NIL));

        /* Select cheapest path (pretty easy in this case...) */
        set_cheapest(rel);
}

/* quick-and-dirty test to see if any joining is needed */
static bool
has_multiple_baserels(PlannerInfo *root)
{
        int                     num_base_rels = 0;
        Index           rti;

        for (rti = 1; rti < root->simple_rel_array_size; rti++)
        {
                RelOptInfo *brel = root->simple_rel_array[rti];

                if (brel == NULL)
                        continue;

                /* ignore RTEs that are "other rels" */
                if (brel->reloptkind == RELOPT_BASEREL)
                        if (++num_base_rels > 1)
                                return true;
        }
        return false;
}

/*
 * set_subquery_pathlist
 *              Build the (single) access path for a subquery RTE
 */
static void
set_subquery_pathlist(PlannerInfo *root, RelOptInfo *rel,
                                          Index rti, RangeTblEntry *rte)
{
        Query      *parse = root->parse;
        Query      *subquery = rte->subquery;
        bool       *differentTypes;
        double          tuple_fraction;
        PlannerInfo *subroot;
        List       *pathkeys;

        /* We need a workspace for keeping track of set-op type coercions */
        differentTypes = (bool *)
                palloc0((list_length(subquery->targetList) + 1) * sizeof(bool));

        /*
         * If there are any restriction clauses that have been attached to the
         * subquery relation, consider pushing them down to become WHERE or HAVING
         * quals of the subquery itself.  This transformation is useful because it
         * may allow us to generate a better plan for the subquery than evaluating
         * all the subquery output rows and then filtering them.
         *
         * There are several cases where we cannot push down clauses. Restrictions
         * involving the subquery are checked by subquery_is_pushdown_safe().
         * Restrictions on individual clauses are checked by
         * qual_is_pushdown_safe().  Also, we don't want to push down
         * pseudoconstant clauses; better to have the gating node above the
         * subquery.
         *
         * Non-pushed-down clauses will get evaluated as qpquals of the
         * SubqueryScan node.
         *
         * XXX Are there any cases where we want to make a policy decision not to
         * push down a pushable qual, because it'd result in a worse plan?
         */
        if (rel->baserestrictinfo != NIL &&
                subquery_is_pushdown_safe(subquery, subquery, differentTypes))
        {
                /* OK to consider pushing down individual quals */
                List       *upperrestrictlist = NIL;
                ListCell   *l;

                foreach(l, rel->baserestrictinfo)
                {
                        RestrictInfo *rinfo = (RestrictInfo *) lfirst(l);
                        Node       *clause = (Node *) rinfo->clause;

                        if (!rinfo->pseudoconstant &&
                                qual_is_pushdown_safe(subquery, rti, clause, differentTypes))
                        {
                                /* Push it down */
                                subquery_push_qual(subquery, rte, rti, clause);
                        }
                        else
                        {
                                /* Keep it in the upper query */
                                upperrestrictlist = lappend(upperrestrictlist, rinfo);
                        }
                }
                rel->baserestrictinfo = upperrestrictlist;
        }

        pfree(differentTypes);

        /*
         * We can safely pass the outer tuple_fraction down to the subquery if the
         * outer level has no joining, aggregation, or sorting to do. Otherwise
         * we'd better tell the subquery to plan for full retrieval. (XXX This
         * could probably be made more intelligent ...)
         */
        if (parse->hasAggs ||
                parse->groupClause ||
                parse->havingQual ||
                parse->distinctClause ||
                parse->sortClause ||
                has_multiple_baserels(root))
                tuple_fraction = 0.0;   /* default case */
        else
                tuple_fraction = root->tuple_fraction;

        /* Generate the plan for the subquery */
        rel->subplan = subquery_planner(root->glob, subquery,
                                                                        root->query_level + 1,
                                                                        tuple_fraction,
                                                                        &subroot);
        rel->subrtable = subroot->parse->rtable;

        /* Copy number of output rows from subplan */
        rel->tuples = rel->subplan->plan_rows;

        /* Mark rel with estimated output rows, width, etc */
        set_baserel_size_estimates(root, rel);

        /* Convert subquery pathkeys to outer representation */
        pathkeys = convert_subquery_pathkeys(root, rel, subroot->query_pathkeys);

        /* Generate appropriate path */
        add_path(rel, create_subqueryscan_path(rel, pathkeys));

        /* Select cheapest path (pretty easy in this case...) */
        set_cheapest(rel);
}

/*
 * set_function_pathlist
 *              Build the (single) access path for a function RTE
 */
static void
set_function_pathlist(PlannerInfo *root, RelOptInfo *rel, RangeTblEntry *rte)
{
        /* Mark rel with estimated output rows, width, etc */
        set_function_size_estimates(root, rel);

        /* Generate appropriate path */
        add_path(rel, create_functionscan_path(root, rel));

        /* Select cheapest path (pretty easy in this case...) */
        set_cheapest(rel);
}

/*
 * set_values_pathlist
 *              Build the (single) access path for a VALUES RTE
 */
static void
set_values_pathlist(PlannerInfo *root, RelOptInfo *rel, RangeTblEntry *rte)
{
        /* Mark rel with estimated output rows, width, etc */
        set_values_size_estimates(root, rel);

        /* Generate appropriate path */
        add_path(rel, create_valuesscan_path(root, rel));

        /* Select cheapest path (pretty easy in this case...) */
        set_cheapest(rel);
}

/*
 * make_rel_from_joinlist
 *        Build access paths using a "joinlist" to guide the join path search.
 *
 * See comments for deconstruct_jointree() for definition of the joinlist
 * data structure.
 */
static RelOptInfo *
make_rel_from_joinlist(PlannerInfo *root, List *joinlist)
{
        int                     levels_needed;
        List       *initial_rels;
        ListCell   *jl;

        /*
         * Count the number of child joinlist nodes.  This is the depth of the
         * dynamic-programming algorithm we must employ to consider all ways of
         * joining the child nodes.
         */
        levels_needed = list_length(joinlist);

        if (levels_needed <= 0)
                return NULL;                    /* nothing to do? */

        /*
         * Construct a list of rels corresponding to the child joinlist nodes.
         * This may contain both base rels and rels constructed according to
         * sub-joinlists.
         */
        initial_rels = NIL;
        foreach(jl, joinlist)
        {
                Node       *jlnode = (Node *) lfirst(jl);
                RelOptInfo *thisrel;

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

                        thisrel = find_base_rel(root, varno);
                }
                else if (IsA(jlnode, List))
                {
                        /* Recurse to handle subproblem */
                        thisrel = make_rel_from_joinlist(root, (List *) jlnode);
                }
                else
                {
                        elog(ERROR, "unrecognized joinlist node type: %d",
                                 (int) nodeTag(jlnode));
                        thisrel = NULL;         /* keep compiler quiet */
                }

                initial_rels = lappend(initial_rels, thisrel);
        }

        if (levels_needed == 1)
        {
                /*
                 * Single joinlist node, so we're done.
                 */
                return (RelOptInfo *) linitial(initial_rels);
        }
        else
        {
                /*
                 * Consider the different orders in which we could join the rels,
                 * using a plugin, GEQO, or the regular join search code.
                 *
                 * We put the initial_rels list into a PlannerInfo field because
                 * has_legal_joinclause() needs to look at it (ugly :-().
                 */
                root->initial_rels = initial_rels;

                if (join_search_hook)
                        return (*join_search_hook) (root, levels_needed, initial_rels);
                else if (enable_geqo && levels_needed >= geqo_threshold)
                        return geqo(root, levels_needed, initial_rels);
                else
                        return standard_join_search(root, levels_needed, initial_rels);
        }
}

/*
 * standard_join_search
 *        Find possible joinpaths for a query by successively finding ways
 *        to join component relations into join relations.
 *
 * 'levels_needed' is the number of iterations needed, ie, the number of
 *              independent jointree items in the query.  This is > 1.
 *
 * 'initial_rels' is a list of RelOptInfo nodes for each independent
 *              jointree item.  These are the components to be joined together.
 *              Note that levels_needed == list_length(initial_rels).
 *
 * Returns the final level of join relations, i.e., the relation that is
 * the result of joining all the original relations together.
 * At least one implementation path must be provided for this relation and
 * all required sub-relations.
 *
 * To support loadable plugins that modify planner behavior by changing the
 * join searching algorithm, we provide a hook variable that lets a plugin
 * replace or supplement this function.  Any such hook must return the same
 * final join relation as the standard code would, but it might have a
 * different set of implementation paths attached, and only the sub-joinrels
 * needed for these paths need have been instantiated.
 *
 * Note to plugin authors: the functions invoked during standard_join_search()
 * modify root->join_rel_list and root->join_rel_hash.  If you want to do more
 * than one join-order search, you'll probably need to save and restore the
 * original states of those data structures.  See geqo_eval() for an example.
 */
RelOptInfo *
standard_join_search(PlannerInfo *root, int levels_needed, List *initial_rels)
{
        List      **joinitems;
        int                     lev;
        RelOptInfo *rel;

        /*
         * We employ a simple "dynamic programming" algorithm: we first find all
         * ways to build joins of two jointree items, then all ways to build joins
         * of three items (from two-item joins and single items), then four-item
         * joins, and so on until we have considered all ways to join all the
         * items into one rel.
         *
         * joinitems[j] is a list of all the j-item rels.  Initially we set
         * joinitems[1] to represent all the single-jointree-item relations.
         */
        joinitems = (List **) palloc0((levels_needed + 1) * sizeof(List *));

        joinitems[1] = initial_rels;

        for (lev = 2; lev <= levels_needed; lev++)
        {
                ListCell   *x;

                /*
                 * Determine all possible pairs of relations to be joined at this
                 * level, and build paths for making each one from every available
                 * pair of lower-level relations.
                 */
                joinitems[lev] = join_search_one_level(root, lev, joinitems);

                /*
                 * Do cleanup work on each just-processed rel.
                 */
                foreach(x, joinitems[lev])
                {
                        rel = (RelOptInfo *) lfirst(x);

                        /* Find and save the cheapest paths for this rel */
                        set_cheapest(rel);

#ifdef OPTIMIZER_DEBUG
                        debug_print_rel(root, rel);
#endif
                }
        }

        /*
         * We should have a single rel at the final level.
         */
        if (joinitems[levels_needed] == NIL)
                elog(ERROR, "failed to build any %d-way joins", levels_needed);
        Assert(list_length(joinitems[levels_needed]) == 1);

        rel = (RelOptInfo *) linitial(joinitems[levels_needed]);

        return rel;
}

/*****************************************************************************
 *                      PUSHING QUALS DOWN INTO SUBQUERIES
 *****************************************************************************/

/*
 * subquery_is_pushdown_safe - is a subquery safe for pushing down quals?
 *
 * subquery is the particular component query being checked.  topquery
 * is the top component of a set-operations tree (the same Query if no
 * set-op is involved).
 *
 * Conditions checked here:
 *
 * 1. If the subquery has a LIMIT clause, we must not push down any quals,
 * since that could change the set of rows returned.
 *
 * 2. If the subquery contains EXCEPT or EXCEPT ALL set ops we cannot push
 * quals into it, because that would change the results.
 *
 * 3. For subqueries using UNION/UNION ALL/INTERSECT/INTERSECT ALL, we can
 * push quals into each component query, but the quals can only reference
 * subquery columns that suffer no type coercions in the set operation.
 * Otherwise there are possible semantic gotchas.  So, we check the
 * component queries to see if any of them have different output types;
 * differentTypes[k] is set true if column k has different type in any
 * component.
 */
static bool
subquery_is_pushdown_safe(Query *subquery, Query *topquery,
                                                  bool *differentTypes)
{
        SetOperationStmt *topop;

        /* Check point 1 */
        if (subquery->limitOffset != NULL || subquery->limitCount != NULL)
                return false;

        /* Are we at top level, or looking at a setop component? */
        if (subquery == topquery)
        {
                /* Top level, so check any component queries */
                if (subquery->setOperations != NULL)
                        if (!recurse_pushdown_safe(subquery->setOperations, topquery,
                                                                           differentTypes))
                                return false;
        }
        else
        {
                /* Setop component must not have more components (too weird) */
                if (subquery->setOperations != NULL)
                        return false;
                /* Check whether setop component output types match top level */
                topop = (SetOperationStmt *) topquery->setOperations;
                Assert(topop && IsA(topop, SetOperationStmt));
                compare_tlist_datatypes(subquery->targetList,
                                                                topop->colTypes,
                                                                differentTypes);
        }
        return true;
}

/*
 * Helper routine to recurse through setOperations tree
 */
static bool
recurse_pushdown_safe(Node *setOp, Query *topquery,
                                          bool *differentTypes)
{
        if (IsA(setOp, RangeTblRef))
        {
                RangeTblRef *rtr = (RangeTblRef *) setOp;
                RangeTblEntry *rte = rt_fetch(rtr->rtindex, topquery->rtable);
                Query      *subquery = rte->subquery;

                Assert(subquery != NULL);
                return subquery_is_pushdown_safe(subquery, topquery, differentTypes);
        }
        else if (IsA(setOp, SetOperationStmt))
        {
                SetOperationStmt *op = (SetOperationStmt *) setOp;

                /* EXCEPT is no good */
                if (op->op == SETOP_EXCEPT)
                        return false;
                /* Else recurse */
                if (!recurse_pushdown_safe(op->larg, topquery, differentTypes))
                        return false;
                if (!recurse_pushdown_safe(op->rarg, topquery, differentTypes))
                        return false;
        }
        else
        {
                elog(ERROR, "unrecognized node type: %d",
                         (int) nodeTag(setOp));
        }
        return true;
}

/*
 * Compare tlist's datatypes against the list of set-operation result types.
 * For any items that are different, mark the appropriate element of
 * differentTypes[] to show that this column will have type conversions.
 *
 * 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 void
compare_tlist_datatypes(List *tlist, List *colTypes,
                                                bool *differentTypes)
{
        ListCell   *l;
        ListCell   *colType = list_head(colTypes);

        foreach(l, tlist)
        {
                TargetEntry *tle = (TargetEntry *) lfirst(l);

                if (tle->resjunk)
                        continue;                       /* ignore resjunk columns */
                if (colType == NULL)
                        elog(ERROR, "wrong number of tlist entries");
                if (exprType((Node *) tle->expr) != lfirst_oid(colType))
                        differentTypes[tle->resno] = true;
                colType = lnext(colType);
        }
        if (colType != NULL)
                elog(ERROR, "wrong number of tlist entries");
}

/*
 * qual_is_pushdown_safe - is a particular qual safe to push down?
 *
 * qual is a restriction clause applying to the given subquery (whose RTE
 * has index rti in the parent query).
 *
 * Conditions checked here:
 *
 * 1. The qual must not contain any subselects (mainly because I'm not sure
 * it will work correctly: sublinks will already have been transformed into
 * subplans in the qual, but not in the subquery).
 *
 * 2. The qual must not refer to the whole-row output of the subquery
 * (since there is no easy way to name that within the subquery itself).
 *
 * 3. The qual must not refer to any subquery output columns that were
 * found to have inconsistent types across a set operation tree by
 * subquery_is_pushdown_safe().
 *
 * 4. If the subquery uses DISTINCT ON, we must not push down any quals that
 * refer to non-DISTINCT output columns, because that could change the set
 * of rows returned.  This condition is vacuous for DISTINCT, because then
 * there are no non-DISTINCT output columns, but unfortunately it's fairly
 * expensive to tell the difference between DISTINCT and DISTINCT ON in the
 * parsetree representation.  It's cheaper to just make sure all the Vars
 * in the qual refer to DISTINCT columns.
 *
 * 5. We must not push down any quals that refer to subselect outputs that
 * return sets, else we'd introduce functions-returning-sets into the
 * subquery's WHERE/HAVING quals.
 *
 * 6. We must not push down any quals that refer to subselect outputs that
 * contain volatile functions, for fear of introducing strange results due
 * to multiple evaluation of a volatile function.
 */
static bool
qual_is_pushdown_safe(Query *subquery, Index rti, Node *qual,
                                          bool *differentTypes)
{
        bool            safe = true;
        List       *vars;
        ListCell   *vl;
        Bitmapset  *tested = NULL;

        /* Refuse subselects (point 1) */
        if (contain_subplans(qual))
                return false;

        /*
         * Examine all Vars used in clause; since it's a restriction clause, all
         * such Vars must refer to subselect output columns.
         */
        vars = pull_var_clause(qual, false);
        foreach(vl, vars)
        {
                Var                *var = (Var *) lfirst(vl);
                TargetEntry *tle;

                Assert(var->varno == rti);

                /* Check point 2 */
                if (var->varattno == 0)
                {
                        safe = false;
                        break;
                }

                /*
                 * We use a bitmapset to avoid testing the same attno more than once.
                 * (NB: this only works because subquery outputs can't have negative
                 * attnos.)
                 */
                if (bms_is_member(var->varattno, tested))
                        continue;
                tested = bms_add_member(tested, var->varattno);

                /* Check point 3 */
                if (differentTypes[var->varattno])
                {
                        safe = false;
                        break;
                }

                /* Must find the tlist element referenced by the Var */
                tle = get_tle_by_resno(subquery->targetList, var->varattno);
                Assert(tle != NULL);
                Assert(!tle->resjunk);

                /* If subquery uses DISTINCT or DISTINCT ON, check point 4 */
                if (subquery->distinctClause != NIL &&
                        !targetIsInSortList(tle, InvalidOid, subquery->distinctClause))
                {
                        /* non-DISTINCT column, so fail */
                        safe = false;
                        break;
                }

                /* Refuse functions returning sets (point 5) */
                if (expression_returns_set((Node *) tle->expr))
                {
                        safe = false;
                        break;
                }

                /* Refuse volatile functions (point 6) */
                if (contain_volatile_functions((Node *) tle->expr))
                {
                        safe = false;
                        break;
                }
        }

        list_free(vars);
        bms_free(tested);

        return safe;
}

/*
 * subquery_push_qual - push down a qual that we have determined is safe
 */
static void
subquery_push_qual(Query *subquery, RangeTblEntry *rte, Index rti, Node *qual)
{
        if (subquery->setOperations != NULL)
        {
                /* Recurse to push it separately to each component query */
                recurse_push_qual(subquery->setOperations, subquery,
                                                  rte, rti, qual);
        }
        else
        {
                /*
                 * We need to replace Vars in the qual (which must refer to outputs of
                 * the subquery) with copies of the subquery's targetlist expressions.
                 * Note that at this point, any uplevel Vars in the qual should have
                 * been replaced with Params, so they need no work.
                 *
                 * This step also ensures that when we are pushing into a setop tree,
                 * each component query gets its own copy of the qual.
                 */
                qual = ResolveNew(qual, rti, 0, rte,
                                                  subquery->targetList,
                                                  CMD_SELECT, 0);

                /*
                 * Now attach the qual to the proper place: normally WHERE, but if the
                 * subquery uses grouping or aggregation, put it in HAVING (since the
                 * qual really refers to the group-result rows).
                 */
                if (subquery->hasAggs || subquery->groupClause || subquery->havingQual)
                        subquery->havingQual = make_and_qual(subquery->havingQual, qual);
                else
                        subquery->jointree->quals =
                                make_and_qual(subquery->jointree->quals, qual);

                /*
                 * We need not change the subquery's hasAggs or hasSublinks flags,
                 * since we can't be pushing down any aggregates that weren't there
                 * before, and we don't push down subselects at all.
                 */
        }
}

/*
 * Helper routine to recurse through setOperations tree
 */
static void
recurse_push_qual(Node *setOp, Query *topquery,
                                  RangeTblEntry *rte, Index rti, Node *qual)
{
        if (IsA(setOp, RangeTblRef))
        {
                RangeTblRef *rtr = (RangeTblRef *) setOp;
                RangeTblEntry *subrte = rt_fetch(rtr->rtindex, topquery->rtable);
                Query      *subquery = subrte->subquery;

                Assert(subquery != NULL);
                subquery_push_qual(subquery, rte, rti, qual);
        }
        else if (IsA(setOp, SetOperationStmt))
        {
                SetOperationStmt *op = (SetOperationStmt *) setOp;

                recurse_push_qual(op->larg, topquery, rte, rti, qual);
                recurse_push_qual(op->rarg, topquery, rte, rti, qual);
        }
        else
        {
                elog(ERROR, "unrecognized node type: %d",
                         (int) nodeTag(setOp));
        }
}

/*****************************************************************************
 *                      DEBUG SUPPORT
 *****************************************************************************/

#ifdef OPTIMIZER_DEBUG

static void
print_relids(Relids relids)
{
        Relids          tmprelids;
        int                     x;
        bool            first = true;

        tmprelids = bms_copy(relids);
        while ((x = bms_first_member(tmprelids)) >= 0)
        {
                if (!first)
                        printf(" ");
                printf("%d", x);
                first = false;
        }
        bms_free(tmprelids);
}

static void
print_restrictclauses(PlannerInfo *root, List *clauses)
{
        ListCell   *l;

        foreach(l, clauses)
        {
                RestrictInfo *c = lfirst(l);

                print_expr((Node *) c->clause, root->parse->rtable);
                if (lnext(l))
                        printf(", ");
        }
}

static void
print_path(PlannerInfo *root, Path *path, int indent)
{
        const char *ptype;
        bool            join = false;
        Path       *subpath = NULL;
        int                     i;

        switch (nodeTag(path))
        {
                case T_Path:
                        ptype = "SeqScan";
                        break;
                case T_IndexPath:
                        ptype = "IdxScan";
                        break;
                case T_BitmapHeapPath:
                        ptype = "BitmapHeapScan";
                        break;
                case T_BitmapAndPath:
                        ptype = "BitmapAndPath";
                        break;
                case T_BitmapOrPath:
                        ptype = "BitmapOrPath";
                        break;
                case T_TidPath:
                        ptype = "TidScan";
                        break;
                case T_AppendPath:
                        ptype = "Append";
                        break;
                case T_ResultPath:
                        ptype = "Result";
                        break;
                case T_MaterialPath:
                        ptype = "Material";
                        subpath = ((MaterialPath *) path)->subpath;
                        break;
                case T_UniquePath:
                        ptype = "Unique";
                        subpath = ((UniquePath *) path)->subpath;
                        break;
                case T_NestPath:
                        ptype = "NestLoop";
                        join = true;
                        break;
                case T_MergePath:
                        ptype = "MergeJoin";
                        join = true;
                        break;
                case T_HashPath:
                        ptype = "HashJoin";
                        join = true;
                        break;
                default:
                        ptype = "???Path";
                        break;
        }

        for (i = 0; i < indent; i++)
                printf("\t");
        printf("%s", ptype);

        if (path->parent)
        {
                printf("(");
                print_relids(path->parent->relids);
                printf(") rows=%.0f", path->parent->rows);
        }
        printf(" cost=%.2f..%.2f\n", path->startup_cost, path->total_cost);

        if (path->pathkeys)
        {
                for (i = 0; i < indent; i++)
                        printf("\t");
                printf("  pathkeys: ");
                print_pathkeys(path->pathkeys, root->parse->rtable);
        }

        if (join)
        {
                JoinPath   *jp = (JoinPath *) path;

                for (i = 0; i < indent; i++)
                        printf("\t");
                printf("  clauses: ");
                print_restrictclauses(root, jp->joinrestrictinfo);
                printf("\n");

                if (IsA(path, MergePath))
                {
                        MergePath  *mp = (MergePath *) path;

                        if (mp->outersortkeys || mp->innersortkeys)
                        {
                                for (i = 0; i < indent; i++)
                                        printf("\t");
                                printf("  sortouter=%d sortinner=%d\n",
                                           ((mp->outersortkeys) ? 1 : 0),
                                           ((mp->innersortkeys) ? 1 : 0));
                        }
                }

                print_path(root, jp->outerjoinpath, indent + 1);
                print_path(root, jp->innerjoinpath, indent + 1);
        }

        if (subpath)
                print_path(root, subpath, indent + 1);
}

void
debug_print_rel(PlannerInfo *root, RelOptInfo *rel)
{
        ListCell   *l;

        printf("RELOPTINFO (");
        print_relids(rel->relids);
        printf("): rows=%.0f width=%d\n", rel->rows, rel->width);

        if (rel->baserestrictinfo)
        {
                printf("\tbaserestrictinfo: ");
                print_restrictclauses(root, rel->baserestrictinfo);
                printf("\n");
        }

        if (rel->joininfo)
        {
                printf("\tjoininfo: ");
                print_restrictclauses(root, rel->joininfo);
                printf("\n");
        }

        printf("\tpath list:\n");
        foreach(l, rel->pathlist)
                print_path(root, lfirst(l), 1);
        printf("\n\tcheapest startup path:\n");
        print_path(root, rel->cheapest_startup_path, 1);
        printf("\n\tcheapest total path:\n");
        print_path(root, rel->cheapest_total_path, 1);
        printf("\n");
        fflush(stdout);
}

#endif   /* OPTIMIZER_DEBUG */