对

RelOptInfo *

make_join_rel(PlannerInfo *root, RelOptInfo *rel1, RelOptInfo *rel2) 函数进行研究：

看看 inner join 时候发生的事情：

RelOptInfo *make_join_rel(PlannerInfo *root, RelOptInfo *rel1, RelOptInfo *rel2){   ...   SpecialJoinInfo *sjinfo;   ...   /* Check validity and determine join type. */   if (!join_is_legal(root, rel1, rel2, joinrelids,                      &sjinfo, &reversed))   ...   /*    * If it's a plain inner join, then we won't have found anything in    * join_info_list.    Make up a SpecialJoinInfo so that selectivity    * estimation functions will know what's being joined.    */   if (sjinfo == NULL)   {        sjinfo = &sjinfo_data;        sjinfo->type = T_SpecialJoinInfo;        sjinfo->min_lefthand = rel1->relids;        sjinfo->min_righthand = rel2->relids;        sjinfo->syn_lefthand = rel1->relids;        sjinfo->syn_righthand = rel2->relids;        sjinfo->jointype = JOIN_INNER;        /* we don't bother trying to make the remaining fields valid */        sjinfo->lhs_strict = false;        sjinfo->delay_upper_joins = false;        sjinfo->join_quals = NIL;   }   ...   switch (sjinfo->jointype)   {        case JOIN_INNER:            fprintf(stderr,"JOIN_INNER \n");            if (is_dummy_rel(rel1) || is_dummy_rel(rel2) ||                restriction_is_constant_false(restrictlist, false))            {                mark_dummy_rel(joinrel);                break;            }            add_paths_to_joinrel(root, joinrel, rel1, rel2,                                 JOIN_INNER, sjinfo,                                 restrictlist);            add_paths_to_joinrel(root, joinrel, rel2, rel1,                                 JOIN_INNER, sjinfo,                                 restrictlist);            break;         ...   }   ...}