此处，分析 add_paths_to_joinrel：
/* * add_paths_to_joinrel * Given a join relation and two component rels from which it can be made, * consider all possible paths that use the two component rels as outer * and inner rel respectively. Add these paths to the join rel's pathlist * if they survive comparison with other paths (and remove any existing * paths that are dominated by these paths). * * Modifies the pathlist field of the joinrel node to contain the best * paths found so far. * * jointype is not necessarily the same as sjinfo->jointype; it might be * "flipped around" if we are considering joining the rels in the opposite * direction from what's indicated in sjinfo. * * Also, this routine and others in this module accept the special JoinTypes * JOIN_UNIQUE_OUTER and JOIN_UNIQUE_INNER to indicate that we should * unique-ify the outer or inner relation and then apply a regular inner * join. These values are not allowed to propagate outside this module, * however. Path cost estimation code may need to recognize that it's * dealing with such a case --- the combination of nominal jointype INNER * with sjinfo->jointype == JOIN_SEMI indicates that. */ void add_paths_to_joinrel(PlannerInfo *root, RelOptInfo *joinrel, RelOptInfo *outerrel, RelOptInfo *innerrel, JoinType jointype, SpecialJoinInfo *sjinfo, List *restrictlist) { List *mergeclause_list = NIL; bool mergejoin_allowed = true; SemiAntiJoinFactors semifactors; Relids param_source_rels = NULL; ListCell *lc; ///fprintf(stderr,"In add_paths_to_joinrel......\n"); /* * Find potential mergejoin clauses. We can skip this if we are not * interested in doing a mergejoin. However, mergejoin may be our only * way of implementing a full outer join, so override enable_mergejoin if * it's a full join. */ if (enable_mergejoin || jointype == JOIN_FULL) { fprintf(stderr,"----------------add_paths_to_joinrel---1\n"); mergeclause_list = select_mergejoin_clauses(root, joinrel, outerrel, innerrel, restrictlist, jointype, &mergejoin_allowed); } /* * If it's SEMI or ANTI join, compute correction factors for cost * estimation. These will be the same for all paths. */ if (jointype == JOIN_SEMI || jointype == JOIN_ANTI) { fprintf(stderr,"----------------add_paths_to_joinrel---2\n"); compute_semi_anti_join_factors(root, outerrel, innerrel, jointype, sjinfo, restrictlist, &semifactors); } /* * Decide whether it's sensible to generate parameterized paths for this * joinrel, and if so, which relations such paths should require. There * is no need to create a parameterized result path unless there is a join * order restriction that prevents joining one of our input rels directly * to the parameter source rel instead of joining to the other input rel. * This restriction reduces the number of parameterized paths we have to * deal with at higher join levels, without compromising the quality of * the resulting plan. We express the restriction as a Relids set that * must overlap the parameterization of any proposed join path. */ foreach(lc, root->join_info_list) { fprintf(stderr,"----------------add_paths_to_joinrel---3\n"); SpecialJoinInfo *sjinfo = (SpecialJoinInfo *) lfirst(lc); /* * SJ is relevant to this join if we have some part of its RHS * (possibly not all of it), and haven't yet joined to its LHS. (This * test is pretty simplistic, but should be sufficient considering the * join has already been proven legal.) If the SJ is relevant, it * presents constraints for joining to anything not in its RHS. */ if (bms_overlap(joinrel->relids, sjinfo->min_righthand) && !bms_overlap(joinrel->relids, sjinfo->min_lefthand)) { fprintf(stderr,"----------------add_paths_to_joinrel---4\n"); param_source_rels = bms_join(param_source_rels, bms_difference(root->all_baserels, sjinfo->min_righthand)); } /* full joins constrain both sides symmetrically */ if (sjinfo->jointype == JOIN_FULL && bms_overlap(joinrel->relids, sjinfo->min_lefthand) && !bms_overlap(joinrel->relids, sjinfo->min_righthand)) { fprintf(stderr,"----------------add_paths_to_joinrel---5\n"); param_source_rels = bms_join(param_source_rels, bms_difference(root->all_baserels, sjinfo->min_lefthand)); } } /* * 1. Consider mergejoin paths where both relations must be explicitly * sorted. Skip this if we can't mergejoin. */ if (mergejoin_allowed) { fprintf(stderr,"----------------add_paths_to_joinrel---6\n"); sort_inner_and_outer(root, joinrel, outerrel, innerrel, restrictlist, mergeclause_list, jointype, sjinfo, param_source_rels); } /* * 2. Consider paths where the outer relation need not be explicitly * sorted. This includes both nestloops and mergejoins where the outer * path is already ordered. Again, skip this if we can't mergejoin. * (That's okay because we know that nestloop can't handle right/full * joins at all, so it wouldn't work in the prohibited cases either.) */ if (mergejoin_allowed) { fprintf(stderr,"----------------add_paths_to_joinrel---7\n"); match_unsorted_outer(root, joinrel, outerrel, innerrel, restrictlist, mergeclause_list, jointype, sjinfo, &semifactors, param_source_rels); } #ifdef NOT_USED /* * 3. Consider paths where the inner relation need not be explicitly * sorted. This includes mergejoins only (nestloops were already built in * match_unsorted_outer). * * Diked out as redundant 2/13/2000 -- tgl. There isn't any really * significant difference between the inner and outer side of a mergejoin, * so match_unsorted_inner creates no paths that aren't equivalent to * those made by match_unsorted_outer when add_paths_to_joinrel() is * invoked with the two rels given in the other order. */ if (mergejoin_allowed) { fprintf(stderr,"----------------add_paths_to_joinrel---8\n"); match_unsorted_inner(root, joinrel, outerrel, innerrel, restrictlist, mergeclause_list, jointype, sjinfo, &semifactors, param_source_rels); } #endif /* * 4. Consider paths where both outer and inner relations must be hashed * before being joined. As above, disregard enable_hashjoin for full * joins, because there may be no other alternative. */ if (enable_hashjoin || jointype == JOIN_FULL) { fprintf(stderr,"----------------add_paths_to_joinrel---9\n"); hash_inner_and_outer(root, joinrel, outerrel, innerrel, restrictlist, jointype, sjinfo, &semifactors, param_source_rels); } }
加入调试代码后，可以看得比较清：
我的如下的查询，执行了： 1、6、7、9。
postgres=# select * from sales s inner join customers c on c.cust_id = s.cust_id; cust_id | item | cust_id | cust_name ---------+----------+---------+----------- 2 | camera | 2 | John Doe 3 | computer | 3 | Jane Doe 3 | monitor | 3 | Jane Doe (3 rows) postgres=#