对PostgreSQL源代码中的build_jion_rel的理解

/*
 * build_join_rel
 *      Returns relation entry corresponding to the union of two given rels,
 *      creating a new relation entry if none already exists.
 *
 * 'joinrelids' is the Relids set that uniquely identifies the join
 * 'outer_rel' and 'inner_rel' are relation nodes for the relations to be
 *        joined
 * 'sjinfo': join context info
 * 'restrictlist_ptr': result variable.  If not NULL, *restrictlist_ptr
 *        receives the list of RestrictInfo nodes that apply to this
 *        particular pair of joinable relations.
 *
 * restrictlist_ptr makes the routine's API a little grotty, but it saves
 * duplicated calculation of the restrictlist...
 */
RelOptInfo *
build_join_rel(PlannerInfo *root,
               Relids joinrelids,
               RelOptInfo *outer_rel,
               RelOptInfo *inner_rel,
               SpecialJoinInfo *sjinfo,
               List **restrictlist_ptr)
{
    RelOptInfo *joinrel;
    List       *restrictlist;

    /*
     * See if we already have a joinrel for this set of base rels.
     */
    joinrel = find_join_rel(root, joinrelids);

    if (joinrel)
    {
        /*
         * Yes, so we only need to figure the restrictlist for this particular
         * pair of component relations.
         */
        if (restrictlist_ptr)
            *restrictlist_ptr = build_joinrel_restrictlist(root,
                                                           joinrel,
                                                           outer_rel,
                                                           inner_rel);
        return joinrel;
    }

    /*
     * Nope, so make one.
     */
    joinrel = makeNode(RelOptInfo);
    joinrel->reloptkind = RELOPT_JOINREL;
    joinrel->relids = bms_copy(joinrelids);
    joinrel->rows = 0;
    joinrel->width = 0;
    joinrel->reltargetlist = NIL;
    joinrel->pathlist = NIL;
    joinrel->ppilist = NIL;
    joinrel->cheapest_startup_path = NULL;
    joinrel->cheapest_total_path = NULL;
    joinrel->cheapest_unique_path = NULL;
    joinrel->cheapest_parameterized_paths = NIL;
    joinrel->relid = 0;            /* indicates not a baserel */
    joinrel->rtekind = RTE_JOIN;
    joinrel->min_attr = 0;
    joinrel->max_attr = 0;
    joinrel->attr_needed = NULL;
    joinrel->attr_widths = NULL;
    joinrel->indexlist = NIL;
    joinrel->pages = 0;
    joinrel->tuples = 0;
    joinrel->allvisfrac = 0;
    joinrel->subplan = NULL;
    joinrel->subroot = NULL;
    joinrel->fdwroutine = NULL;
    joinrel->fdw_private = NULL;
    joinrel->baserestrictinfo = NIL;
    joinrel->baserestrictcost.startup = 0;
    joinrel->baserestrictcost.per_tuple = 0;
    joinrel->joininfo = NIL;
    joinrel->has_eclass_joins = false;

    /*
     * Create a new tlist containing just the vars that need to be output from
     * this join (ie, are needed for higher joinclauses or final output).
     *
     * NOTE: the tlist order for a join rel will depend on which pair of outer
     * and inner rels we first try to build it from.  But the contents should
     * be the same regardless.
     */
    build_joinrel_tlist(root, joinrel, outer_rel);
    build_joinrel_tlist(root, joinrel, inner_rel);
    add_placeholders_to_joinrel(root, joinrel);

    /*
     * Construct restrict and join clause lists for the new joinrel. (The
     * caller might or might not need the restrictlist, but I need it anyway
     * for set_joinrel_size_estimates().)
     */
    restrictlist = build_joinrel_restrictlist(root, joinrel,
                                              outer_rel, inner_rel);
    if (restrictlist_ptr)
        *restrictlist_ptr = restrictlist;
    build_joinrel_joinlist(joinrel, outer_rel, inner_rel);

    /*
     * This is also the right place to check whether the joinrel has any
     * pending EquivalenceClass joins.
     */
    joinrel->has_eclass_joins = has_relevant_eclass_joinclause(root, joinrel);

    /*
     * Set estimates of the joinrel's size.
     */
    set_joinrel_size_estimates(root, joinrel, outer_rel, inner_rel,
                               sjinfo, restrictlist);

    /*
     * Add the joinrel to the query's joinrel list, and store it into the
     * auxiliary hashtable if there is one.  NB: GEQO requires us to append
     * the new joinrel to the end of the list!
     */
    root->join_rel_list = lappend(root->join_rel_list, joinrel);

    if (root->join_rel_hash)
    {
        JoinHashEntry *hentry;
        bool        found;

        hentry = (JoinHashEntry *) hash_search(root->join_rel_hash,
                                               &(joinrel->relids),
                                               HASH_ENTER,
                                               &found);
        Assert(!found);
        hentry->join_rel = joinrel;
    }

    /*
     * Also, if dynamic-programming join search is active, add the new joinrel
     * to the appropriate sublist.    Note: you might think the Assert on number
     * of members should be for equality, but some of the level 1 rels might
     * have been joinrels already, so we can only assert <=.
     */
    if (root->join_rel_level)
    {
        Assert(root->join_cur_level > 0);
        Assert(root->join_cur_level <= bms_num_members(joinrel->relids));
        root->join_rel_level[root->join_cur_level] =
            lappend(root->join_rel_level[root->join_cur_level], joinrel);
    }

    return joinrel;
}