在主函數subquery_planner完成外連接消除后,接下來調用grouping_planner函數,本節簡單介紹了此函數的主體邏輯。

一、源碼解讀

grouping_planner函數源碼：

/*--------------------
  * grouping_planner
  *    Perform planning steps related to grouping, aggregation, etc.
  *
  * 生成與分組/聚合相關的"規劃步驟"
  *
  * This function adds all required top-level processing to the scan/join
  * Path(s) produced by query_planner.
  *
  * 該函數還處理了所有需要在頂層處理的掃描/連接路徑(通過query_planner函數生成)
  *
  * If inheritance_update is true, we're being called from inheritance_planner
  * and should not include a ModifyTable step in the resulting Path(s).
  * (inheritance_planner will create a single ModifyTable node covering all the
  * target tables.)
  *
  * 如果標志inheritance_update為true,這個函數的調用者是inheritance_planner,在結果路徑中
  * 不應包含ModifyTable步驟(inheritance_planner會創建一個單獨的覆蓋所有目標表的ModifyTable節點).
  *
  * 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)
  *
  * tuple_fraction是我們希望搜索的元組比例:
  * 0:正常情況下,期望掃描所有的元組
  * 大于0小于1:按給定的比例掃描
  * 大于等于1:掃描的元組數量(比如通過LIMIT語句指定)
  *
  * Returns nothing; the useful output is in the Paths we attach to the
  * (UPPERREL_FINAL, NULL) upperrel in *root.  In addition,
  * root->processed_tlist contains the final processed targetlist.
  *
  * 該函數沒有返回值,有用的輸出是root->upperrel->Paths,另外,root->processed_tlist中存儲最終的投影列
  *
  * Note that we have not done set_cheapest() on the final rel; it's convenient
  * to leave this to the caller.
  *--------------------
  */
 static void
 grouping_planner(PlannerInfo *root, bool inheritance_update,
                  double tuple_fraction)
 {
     Query      *parse = root->parse;
     List       *tlist;
     int64       offset_est = 0;
     int64       count_est = 0;
     double      limit_tuples = -1.0;
     bool        have_postponed_srfs = false;
     PathTarget *final_target;
     List       *final_targets;
     List       *final_targets_contain_srfs;
     bool        final_target_parallel_safe;
     RelOptInfo *current_rel;
     RelOptInfo *final_rel;
     ListCell   *lc;
 
     /* Tweak caller-supplied tuple_fraction if have LIMIT/OFFSET */
     if (parse->limitCount || parse->limitOffset)//存在LIMIT/OFFSET語句
     {
         tuple_fraction = preprocess_limit(root, tuple_fraction,
                                           &offset_est, &count_est);//獲取元組數量
 
         /*
          * If we have a known LIMIT, and don't have an unknown OFFSET, we can
          * estimate the effects of using a bounded sort.
          */
         if (count_est > 0 && offset_est >= 0)
             limit_tuples = (double) count_est + (double) offset_est;//
     }
 
     /* Make tuple_fraction accessible to lower-level routines */
     root->tuple_fraction = tuple_fraction;//設置值
 
     if (parse->setOperations)//集合操作,如UNION等
     {
         /*
          * If there's a top-level ORDER BY, assume we have to fetch all the
          * tuples.  This might be too simplistic given all the hackery below
          * to possibly avoid the sort; but the odds of accurate estimates here
          * are pretty low anyway.  XXX try to get rid of this in favor of
          * letting plan_set_operations generate both fast-start and
          * cheapest-total paths.
          */
         if (parse->sortClause)
             root->tuple_fraction = 0.0;//存在排序操作,需掃描所有的元組
 
         /*
          * Construct Paths for set operations.  The results will not need any
          * work except perhaps a top-level sort and/or LIMIT.  Note that any
          * special work for recursive unions is the responsibility of
          * plan_set_operations.
          */
         current_rel = plan_set_operations(root);//調用集合操作的"規劃"函數
 
         /*
          * 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 = root->processed_tlist;  /* from plan_set_operations */
 
         /* for safety, copy processed_tlist instead of modifying in-place */
         tlist = postprocess_setop_tlist(copyObject(tlist), parse->targetList);
 
         /* Save aside the final decorated tlist */
         root->processed_tlist = tlist;
 
         /* Also extract the PathTarget form of the setop result tlist */
         final_target = current_rel->cheapest_total_path->pathtarget;
 
         /* And check whether it's parallel safe */
         final_target_parallel_safe =
             is_parallel_safe(root, (Node *) final_target->exprs);
 
         /* The setop result tlist couldn't contain any SRFs */
         Assert(!parse->hasTargetSRFs);
         final_targets = final_targets_contain_srfs = NIL;
 
         /*
          * Can't handle FOR [KEY] UPDATE/SHARE here (parser should have
          * checked already, but let's make sure).
          */
         if (parse->rowMarks)
             ereport(ERROR,
                     (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
             /*------
               translator: %s is a SQL row locking clause such as FOR UPDATE */
                      errmsg("%s is not allowed with UNION/INTERSECT/EXCEPT",
                             LCS_asString(linitial_node(RowMarkClause,
                                                        parse->rowMarks)->strength))));
 
         /*
          * Calculate pathkeys that represent result ordering requirements
          */
         Assert(parse->distinctClause == NIL);
         root->sort_pathkeys = make_pathkeys_for_sortclauses(root,
                                                             parse->sortClause,
                                                             tlist);
     }
     else//非集合操作
     {
         /* No set operations, do regular planning */
         PathTarget *sort_input_target;
         List       *sort_input_targets;
         List       *sort_input_targets_contain_srfs;
         bool        sort_input_target_parallel_safe;
         PathTarget *grouping_target;
         List       *grouping_targets;
         List       *grouping_targets_contain_srfs;
         bool        grouping_target_parallel_safe;
         PathTarget *scanjoin_target;
         List       *scanjoin_targets;
         List       *scanjoin_targets_contain_srfs;
         bool        scanjoin_target_parallel_safe;
         bool        scanjoin_target_same_exprs;
         bool        have_grouping;
         AggClauseCosts agg_costs;
         WindowFuncLists *wflists = NULL;
         List       *activeWindows = NIL;
         grouping_sets_data *gset_data = NULL;
         standard_qp_extra qp_extra;
 
         /* A recursive query should always have setOperations */
         Assert(!root->hasRecursion);//檢查
 
         /* Preprocess grouping sets and GROUP BY clause, if any */
         if (parse->groupingSets)//
         {
             gset_data = preprocess_grouping_sets(root);//預處理grouping sets語句
         }
         else
         {
             /* Preprocess regular GROUP BY clause, if any */
             if (parse->groupClause)
                 parse->groupClause = preprocess_groupclause(root, NIL);//處理普通的Group By語句
         }
 
         /* Preprocess targetlist */
         tlist = preprocess_targetlist(root);//處理投影列
 
         /*
          * We are now done hacking up the query's targetlist.  Most of the
          * remaining planning work will be done with the PathTarget
          * representation of tlists, but save aside the full representation so
          * that we can transfer its decoration (resnames etc) to the topmost
          * tlist of the finished Plan.
          */
         root->processed_tlist = tlist;//賦值
 
         /*
          * Collect statistics about aggregates for estimating costs, and mark
          * all the aggregates with resolved aggtranstypes.  We must do this
          * before slicing and dicing the tlist into various pathtargets, else
          * some copies of the Aggref nodes might escape being marked with the
          * correct transtypes.
          *
          * Note: currently, we do not detect duplicate aggregates here.  This
          * may result in somewhat-overestimated cost, which is fine for our
          * purposes since all Paths will get charged the same.  But at some
          * point we might wish to do that detection in the planner, rather
          * than during executor startup.
          */
         MemSet(&agg_costs, 0, sizeof(AggClauseCosts));
         if (parse->hasAggs)//存在聚合函數
         {
             get_agg_clause_costs(root, (Node *) tlist, AGGSPLIT_SIMPLE,
                                  &agg_costs);//收集用于估算成本的統計信息
             get_agg_clause_costs(root, parse->havingQual, AGGSPLIT_SIMPLE,
                                  &agg_costs);//收集用于估算成本的統計信息
         }
 
         /*
          * Locate any window functions in the tlist.  (We don't need to look
          * anywhere else, since expressions used in ORDER BY will be in there
          * too.)  Note that they could all have been eliminated by constant
          * folding, in which case we don't need to do any more work.
          */
         if (parse->hasWindowFuncs)//窗口函數
         {
             wflists = find_window_functions((Node *) tlist,
                                             list_length(parse->windowClause));
             if (wflists->numWindowFuncs > 0)
                 activeWindows = select_active_windows(root, wflists);
             else
                 parse->hasWindowFuncs = false;
         }
 
         /*
          * Preprocess MIN/MAX aggregates, if any.  Note: be careful about
          * adding logic between here and the query_planner() call.  Anything
          * that is needed in MIN/MAX-optimizable cases will have to be
          * duplicated in planagg.c.
          */
         if (parse->hasAggs)//預處理最大最小聚合
             preprocess_minmax_aggregates(root, tlist);
 
         /*
          * Figure out whether there's a hard limit on the number of rows that
          * query_planner's result subplan needs to return.  Even if we know a
          * hard limit overall, it doesn't apply if the query has any
          * grouping/aggregation operations, or SRFs in the tlist.
          */
         if (parse->groupClause ||
             parse->groupingSets ||
             parse->distinctClause ||
             parse->hasAggs ||
             parse->hasWindowFuncs ||
             parse->hasTargetSRFs ||
             root->hasHavingQual)//存在Group By/Grouping Set等語句,則limit_tuples設置為-1
             root->limit_tuples = -1.0;
         else
             root->limit_tuples = limit_tuples;//否則,正常賦值
 
         /* Set up data needed by standard_qp_callback */
         qp_extra.tlist = tlist;//賦值
         qp_extra.activeWindows = activeWindows;
         qp_extra.groupClause = (gset_data
                                 ? (gset_data->rollups ? linitial_node(RollupData, gset_data->rollups)->groupClause : NIL)
                                 : parse->groupClause);
 
         /*
          * Generate the best unsorted and presorted paths for the scan/join
          * portion of this Query, ie the processing represented by the
          * FROM/WHERE clauses.  (Note there may not be any presorted paths.)
          * We also generate (in standard_qp_callback) pathkey representations
          * of the query's sort clause, distinct clause, etc.
          */
     //為查詢中的掃描/連接部分生成最優的未排序/預排序路徑(如FROM/WHERE語句表示的處理過程)
         current_rel = query_planner(root, tlist,
                                     standard_qp_callback, &qp_extra);
 
         /*
          * Convert the query's result tlist into PathTarget format.
          * 轉換查詢結果為PathTarget格式
      *
          * Note: it's desirable to not do this till after query_planner(),
          * because the target width estimates can use per-Var width numbers
          * that were obtained within query_planner().
          */
         final_target = create_pathtarget(root, tlist);
         final_target_parallel_safe =
             is_parallel_safe(root, (Node *) final_target->exprs);
 
         /*
          * If ORDER BY was given, consider whether we should use a post-sort
          * projection, and compute the adjusted target for preceding steps if
          * so.
          */
         if (parse->sortClause)//存在sort語句?
         {
             sort_input_target = make_sort_input_target(root,
                                                        final_target,
                                                        &have_postponed_srfs);
             sort_input_target_parallel_safe =
                 is_parallel_safe(root, (Node *) sort_input_target->exprs);
         }
         else
         {
             sort_input_target = final_target;//不存在,則直接賦值
             sort_input_target_parallel_safe = final_target_parallel_safe;
         }
 
         /*
          * If we have window functions to deal with, the output from any
          * grouping step needs to be what the window functions want;
          * otherwise, it should be sort_input_target.
          */
         if (activeWindows)//存在窗口函數?
         {
             grouping_target = make_window_input_target(root,
                                                        final_target,
                                                        activeWindows);
             grouping_target_parallel_safe =
                 is_parallel_safe(root, (Node *) grouping_target->exprs);
         }
         else
         {
             grouping_target = sort_input_target;
             grouping_target_parallel_safe = sort_input_target_parallel_safe;
         }
 
         /*
          * If we have grouping or aggregation to do, the topmost scan/join
          * plan node must emit what the grouping step wants; otherwise, it
          * should emit grouping_target.
          */
         have_grouping = (parse->groupClause || parse->groupingSets ||
                          parse->hasAggs || root->hasHavingQual);
         if (have_grouping)
         {//存在group等分組語句
             scanjoin_target = make_group_input_target(root, final_target);
             scanjoin_target_parallel_safe =
                 is_parallel_safe(root, (Node *) grouping_target->exprs);
         }
         else
         {
             scanjoin_target = grouping_target;
             scanjoin_target_parallel_safe = grouping_target_parallel_safe;
         }
 
         /*
          * If there are any SRFs in the targetlist, we must separate each of
          * these PathTargets into SRF-computing and SRF-free targets.  Replace
          * each of the named targets with a SRF-free version, and remember the
          * list of additional projection steps we need to add afterwards.
          */
         if (parse->hasTargetSRFs)//存在SRFs
         {
             /* final_target doesn't recompute any SRFs in sort_input_target */
             split_pathtarget_at_srfs(root, final_target, sort_input_target,
                                      &final_targets,
                                      &final_targets_contain_srfs);
             final_target = linitial_node(PathTarget, final_targets);
             Assert(!linitial_int(final_targets_contain_srfs));
             /* likewise for sort_input_target vs. grouping_target */
             split_pathtarget_at_srfs(root, sort_input_target, grouping_target,
                                      &sort_input_targets,
                                      &sort_input_targets_contain_srfs);
             sort_input_target = linitial_node(PathTarget, sort_input_targets);
             Assert(!linitial_int(sort_input_targets_contain_srfs));
             /* likewise for grouping_target vs. scanjoin_target */
             split_pathtarget_at_srfs(root, grouping_target, scanjoin_target,
                                      &grouping_targets,
                                      &grouping_targets_contain_srfs);
             grouping_target = linitial_node(PathTarget, grouping_targets);
             Assert(!linitial_int(grouping_targets_contain_srfs));
             /* scanjoin_target will not have any SRFs precomputed for it */
             split_pathtarget_at_srfs(root, scanjoin_target, NULL,
                                      &scanjoin_targets,
                                      &scanjoin_targets_contain_srfs);
             scanjoin_target = linitial_node(PathTarget, scanjoin_targets);
             Assert(!linitial_int(scanjoin_targets_contain_srfs));
         }
         else
         {
             /* initialize lists; for most of these, dummy values are OK */
             final_targets = final_targets_contain_srfs = NIL;
             sort_input_targets = sort_input_targets_contain_srfs = NIL;
             grouping_targets = grouping_targets_contain_srfs = NIL;
             scanjoin_targets = list_make1(scanjoin_target);
             scanjoin_targets_contain_srfs = NIL;
         }
 
         /* Apply scan/join target. */
     //應用掃描/連接target
         scanjoin_target_same_exprs = list_length(scanjoin_targets) == 1
             && equal(scanjoin_target->exprs, current_rel->reltarget->exprs);
         apply_scanjoin_target_to_paths(root, current_rel, scanjoin_targets,
                                        scanjoin_targets_contain_srfs,
                                        scanjoin_target_parallel_safe,
                                        scanjoin_target_same_exprs);
 
         /*
          * Save the various upper-rel PathTargets we just computed into
          * root->upper_targets[].  The core code doesn't use this, but it
          * provides a convenient place for extensions to get at the info.  For
          * consistency, we save all the intermediate targets, even though some
          * of the corresponding upperrels might not be needed for this query.
          */
     //賦值
         root->upper_targets[UPPERREL_FINAL] = final_target;
         root->upper_targets[UPPERREL_WINDOW] = sort_input_target;
         root->upper_targets[UPPERREL_GROUP_AGG] = grouping_target;
 
         /*
          * If we have grouping and/or aggregation, consider ways to implement
          * that.  We build a new upperrel representing the output of this
          * phase.
          */
         if (have_grouping)//存在分組操作
         {
             current_rel = create_grouping_paths(root,
                                                 current_rel,
                                                 grouping_target,
                                                 grouping_target_parallel_safe,
                                                 &agg_costs,
                                                 gset_data);//創建分組訪問路徑
             /* Fix things up if grouping_target contains SRFs */
             if (parse->hasTargetSRFs)
                 adjust_paths_for_srfs(root, current_rel,
                                       grouping_targets,
                                       grouping_targets_contain_srfs);
         }
 
         /*
          * If we have window functions, consider ways to implement those.  We
          * build a new upperrel representing the output of this phase.
          */
         if (activeWindows)//存在窗口函數
         {
             current_rel = create_window_paths(root,
                                               current_rel,
                                               grouping_target,
                                               sort_input_target,
                                               sort_input_target_parallel_safe,
                                               tlist,
                                               wflists,
                                               activeWindows);
             /* Fix things up if sort_input_target contains SRFs */
             if (parse->hasTargetSRFs)
                 adjust_paths_for_srfs(root, current_rel,
                                       sort_input_targets,
                                       sort_input_targets_contain_srfs);
         }
 
         /*
          * If there is a DISTINCT clause, consider ways to implement that. We
          * build a new upperrel representing the output of this phase.
          */
         if (parse->distinctClause)//存在distinct?
         {
             current_rel = create_distinct_paths(root,
                                                 current_rel);
         }
     }                           /* end of if (setOperations) */
 
     /*
      * If ORDER BY was given, consider ways to implement that, and generate a
      * new upperrel containing only paths that emit the correct ordering and
      * project the correct final_target.  We can apply the original
      * limit_tuples limit in sort costing here, but only if there are no
      * postponed SRFs.
      */
     if (parse->sortClause)//存在sort語句?
     {
         current_rel = create_ordered_paths(root,
                                            current_rel,
                                            final_target,
                                            final_target_parallel_safe,
                                            have_postponed_srfs ? -1.0 :
                                            limit_tuples);
         /* Fix things up if final_target contains SRFs */
         if (parse->hasTargetSRFs)
             adjust_paths_for_srfs(root, current_rel,
                                   final_targets,
                                   final_targets_contain_srfs);
     }
 
     /*
      * Now we are prepared to build the final-output upperrel.
      */
     final_rel = fetch_upper_rel(root, UPPERREL_FINAL, NULL);//獲取最終的RelOptInfo(用于替換RTE)
 
     /*
      * If the input rel is marked consider_parallel and there's nothing that's
      * not parallel-safe in the LIMIT clause, then the final_rel can be marked
      * consider_parallel as well.  Note that if the query has rowMarks or is
      * not a SELECT, consider_parallel will be false for every relation in the
      * query.
      */
     if (current_rel->consider_parallel &&
         is_parallel_safe(root, parse->limitOffset) &&
         is_parallel_safe(root, parse->limitCount))
         final_rel->consider_parallel = true;//并行
 
     /*
      * If the current_rel belongs to a single FDW, so does the final_rel.
      */
     final_rel->serverid = current_rel->serverid;
     final_rel->userid = current_rel->userid;
     final_rel->useridiscurrent = current_rel->useridiscurrent;
     final_rel->fdwroutine = current_rel->fdwroutine;
 
     /*
      * Generate paths for the final_rel.  Insert all surviving paths, with
      * LockRows, Limit, and/or ModifyTable steps added if needed.
      */
     foreach(lc, current_rel->pathlist)//逐一遍歷訪問路徑
     {
         Path       *path = (Path *) lfirst(lc);
 
         /*
          * If there is a FOR [KEY] UPDATE/SHARE clause, add the LockRows node.
          * (Note: we intentionally test parse->rowMarks not root->rowMarks
          * here.  If there are only non-locking rowmarks, they should be
          * handled by the ModifyTable node instead.  However, root->rowMarks
          * is what goes into the LockRows node.)
          */
         if (parse->rowMarks)
         {
             path = (Path *) create_lockrows_path(root, final_rel, path,
                                                  root->rowMarks,
                                                  SS_assign_special_param(root));
         }
 
         /*
          * If there is a LIMIT/OFFSET clause, add the LIMIT node.
          */
         if (limit_needed(parse))
         {
             path = (Path *) create_limit_path(root, final_rel, path,
                                               parse->limitOffset,
                                               parse->limitCount,
                                               offset_est, count_est);
         }
 
         /*
          * If this is an INSERT/UPDATE/DELETE, and we're not being called from
          * inheritance_planner, add the ModifyTable node.
          */
         if (parse->commandType != CMD_SELECT && !inheritance_update)//非查詢語句
         {
             List       *withCheckOptionLists;
             List       *returningLists;
             List       *rowMarks;
 
             /*
              * Set up the WITH CHECK OPTION and RETURNING lists-of-lists, if
              * needed.
              */
             if (parse->withCheckOptions)
                 withCheckOptionLists = list_make1(parse->withCheckOptions);
             else
                 withCheckOptionLists = NIL;
 
             if (parse->returningList)
                 returningLists = list_make1(parse->returningList);
             else
                 returningLists = NIL;
 
             /*
              * If there was a FOR [KEY] UPDATE/SHARE clause, the LockRows node
              * will have dealt with fetching non-locked marked rows, else we
              * need to have ModifyTable do that.
              */
             if (parse->rowMarks)
                 rowMarks = NIL;
             else
                 rowMarks = root->rowMarks;
 
             path = (Path *)
                 create_modifytable_path(root, final_rel,
                                         parse->commandType,
                                         parse->canSetTag,
                                         parse->resultRelation,
                                         NIL,
                                         false,
                                         list_make1_int(parse->resultRelation),
                                         list_make1(path),
                                         list_make1(root),
                                         withCheckOptionLists,
                                         returningLists,
                                         rowMarks,
                                         parse->onConflict,
                                         SS_assign_special_param(root));
         }
 
         /* And shove it into final_rel */
         add_path(final_rel, path);
     }
 
     /*
      * Generate partial paths for final_rel, too,if outer query levels might
      * be able to make use of them.
      */
     if (final_rel->consider_parallel && root->query_level > 1 &&
         !limit_needed(parse))
     {
         Assert(!parse->rowMarks && parse->commandType == CMD_SELECT);
         foreach(lc, current_rel->partial_pathlist)
         {
             Path       *partial_path = (Path *) lfirst(lc);
 
             add_partial_path(final_rel, partial_path);
         }
     }
 
     /*
      * If there is an FDW that's responsible for all baserels of the query,
      * let it consider adding ForeignPaths.
      */
     if (final_rel->fdwroutine &&
         final_rel->fdwroutine->GetForeignUpperPaths)
         final_rel->fdwroutine->GetForeignUpperPaths(root, UPPERREL_FINAL,
                                                     current_rel, final_rel,
                                                     NULL);
 
     /* Let extensions possibly add some more paths */
     if (create_upper_paths_hook)
         (*create_upper_paths_hook) (root, UPPERREL_FINAL,
                                     current_rel, final_rel, NULL);
 
     /* Note: currently, we leave it to callers to do set_cheapest() */
 }

二、參考資料

planner.c