先前花了二十多个小节介绍query_planner及其子函数make_one_rel，已基本介绍完毕。本节回过头来Review query_planner函数的调用方-查询优化实现中的grouping_planner函数，该函数执行与与分组/聚集相关的"规划步骤"。

一、源码解读

分组/聚集等操作是在一个Relation上叠加分组/聚集运算,grouping_planner函数首先通过query_planner函数生成一个新的关系,然后在此关系上attached分组/聚集等操作。

/*-------------------- * grouping_planner *    Perform planning steps related to grouping, aggregation, etc. *    执行与与分组/聚集相关的"规划步骤". *    分组/聚集等操作是在一个Relation上叠加分组/聚集运算, *    PG首先通过query_planner函数生成一个新的关系,然后在此关系上attached分组/聚集等操作 * * This function adds all required top-level processing to the scan/join * Path(s) produced by query_planner. * * 该函数还处理了所有需要在顶层处理的扫描/连接路径(通过query_planner函数生成) * * 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.) * * 如果标志inheritance_update为true,这个函数的调用者是inheritance_planner,在结果路径中 * 不应包含ModifyTable步骤(inheritance_planner会创建一个单独的覆盖所有目标表的ModifyTable节点). * * 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是我们希望搜索的元组比例: * 0:正常情况下,期望扫描所有的元组 * 大于0小于1:按给定的比例扫描 * 大于等于1:扫描的元组数量(比如通过LIMIT语句指定) * * 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. * * 该函数没有返回值,有用的输出是root->upperrel->Paths,另外,root->processed_tlist中存储最终的投影列 * * Note that we have not done set_cheapest() on the final rel; it's convenient * to leave this to the caller. *-------------------- */static voidgrouping_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 */    //如果存在LIMIT/OFFSET子句,调整tuple_fraction    if (parse->limitCount || parse->limitOffset)//存在LIMIT/OFFSET语句    {        tuple_fraction = preprocess_limit(root, tuple_fraction,                                          &offset_est, &count_est);//获取元组数量        /*         * If we have a known LIMIT, and don't have an unknown OFFSET, we can         * estimate the effects of using a bounded sort.         * 如果我们有一个已知LIMIT，并且没有未知的OFFSET，我们可以估算使用有界排序的效果。         */        if (count_est > 0 && offset_est >= 0)            limit_tuples = (double) count_est + (double) offset_est;//    }    /* Make tuple_fraction accessible to lower-level routines */    //使tuple_fraction可被低级别的处理过程访问(在优化器信息中设置)    root->tuple_fraction = tuple_fraction;//设置值    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.         * 如果语句的最外层(顶级)存在ORDER BY子句，假设我们必须获取所有元组。         * 这可能过于简单，但无论如何，准确估计的几率是相当低的。         * XXX试图摆脱这种情况，让plan_set_operations同时生成快速启动和最便宜的路径。         */        if (parse->sortClause)            root->tuple_fraction = 0.0;//存在排序操作,需扫描所有的元组        /*         * 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.         * 为集合操作构造路径。         * 除了最外层的SORT/LIMIT操作外不需要作其他操作。注意，递归联合的任何特殊工作都是plan_set_operations负责。         */        current_rel = plan_set_operations(root);//调用集合操作的"规划"函数        /*         * 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.         * 我们不需要调用preprocess_targetlist函数，因为执行这些操作必须在SELECT查询NODE中。         * 相反，使用plan_set_operations函数返回的targetlist(因为这告诉它是否返回了所有的resjunk列)，         * 并从原始投影列链表tlist中传输所有的排序sort键信息。         */        Assert(parse->commandType == CMD_SELECT);        tlist = root->processed_tlist;  /* 从plan_set_operations函数的返回结果中获取;from plan_set_operations */        /* for safety, copy processed_tlist instead of modifying in-place */        //为了安全起见，复制processed_tlist，而不是就地修改        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 */        //从集合操作结果投影列中获取PathTarget格式的结果列        final_target = current_rel->cheapest_total_path->pathtarget;        /* And check whether it's parallel safe */        //检查是否并行安全        final_target_parallel_safe =            is_parallel_safe(root, (Node *) final_target->exprs);        /* The setop result tlist couldn't contain any SRFs */        //集合操作结果投影列不能包含任何的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).         * 无法在这里处理[KEY]更新/共享(解析器应该已经检查过了，但需要确认)。         */        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         * 计算表示结果排序需求的pathkeys         */        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 */        //没有集合操作,执行常规的规划过程        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 */        //递归查询应包含集合操作,检查!        Assert(!root->hasRecursion);//检查        /* Preprocess grouping sets and GROUP BY clause, if any */        //预处理grouping sets语句和GROUP BY 子句        if (parse->groupingSets)//        {            gset_data = preprocess_grouping_sets(root);//预处理grouping sets语句        }        else        {            /* Preprocess regular GROUP BY clause, if any */            //如处理常规的GROUP BY 子句            if (parse->groupClause)                parse->groupClause = preprocess_groupclause(root, NIL);//处理普通的Group By语句        }        /* Preprocess targetlist */        //预处理投影列        tlist = preprocess_targetlist(root);//处理投影列        /*         * 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.         * 现在已经完成了对查询语句targetlist的hacking工作。         * 剩下的大部分规划工作将使用tlists的PathTarget来完成，         * 但是需要保留完整的信息，这样我们就可以将它的修饰信息(如resname等)转移到完成计划的最顶层tlist中。         */        root->processed_tlist = tlist;//赋值        /*         * 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.         * 收集关于聚集操作的统计数据以估计成本，并在所有聚集操作上标上已解决的aggtranstypes。         * 必须在将tlist切割成各种PathKeys之前完成这项工作，         * 否则一些Aggref节点的副本中正确transtypes可能会被替换。         *          * 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.         * 注意:目前，我们没有检测到重复的聚合。         * 这可能会导致一些过高估算的成本，这对于我们的目的来说是好的，因为所有的Path都会耗费相同的成本。         * 但在某些时候，可能希望在计划器中进行检测，而不是在执行器executor启动期间。         */        MemSet(&agg_costs, 0, sizeof(AggClauseCosts));        if (parse->hasAggs)//存在聚合函数        {            get_agg_clause_costs(root, (Node *) tlist, AGGSPLIT_SIMPLE,                                 &agg_costs);//收集用于估算成本的统计信息            get_agg_clause_costs(root, parse->havingQual, AGGSPLIT_SIMPLE,                                 &agg_costs);//收集用于估算成本的统计信息        }        /*         * 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.         * 在tlist中找到所有的窗口函数。         * (我们不需要在其他地方查找，因为ORDER BY中使用的表达式也在那里。)         * 注意，它们可以通过不断折叠来消除，在这种情况下，我们不需要做更多的工作。         */        if (parse->hasWindowFuncs)//窗口函数        {            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.         * 重新处理MAX/MIN聚集操作，如果有的话。         * 注意:在这里和query_planner()调用之间添加逻辑时要小心。         * 在MIN/MAX优化情况下需要的所有东西都必须在plan .c中重复。         */        if (parse->hasAggs)//预处理最大最小聚合            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.         * 计算query_planner结果子计划需要返回的行数是否有硬性限制。         * 即使我们知道总的强制限制，如果查询在tlist中有任何分组/聚合操作或SRFs，它也不适用。         */        if (parse->groupClause ||            parse->groupingSets ||            parse->distinctClause ||            parse->hasAggs ||            parse->hasWindowFuncs ||            parse->hasTargetSRFs ||            root->hasHavingQual)//存在Group By/Grouping Set等语句,则limit_tuples设置为-1            root->limit_tuples = -1.0;        else            root->limit_tuples = limit_tuples;//否则,正常赋值        /* Set up data needed by standard_qp_callback */        //配置standard_qp_callback函数需要的相关数据        qp_extra.tlist = tlist;//赋值        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.         * 为这个查询的扫描/连接部分(即FROM/WHERE子句表示的处理)生成最好的未排序和预排序路径。         * (注意，可能没有任何预先设置的路径。)         * 我们还生成(在standard_qp_callback中)查询语句的sort子句和distinct子句对应的PathKey。         */         //为查询中的扫描/连接部分生成最优的未排序/预排序路径(如FROM/WHERE语句表示的处理过程)        current_rel = query_planner(root, tlist,                                    standard_qp_callback, &qp_extra);        /*         * Convert the query's result tlist into PathTarget format.         * 转换查询结果为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().         * 注意:在query_planner()之后才需要这样做，因为目标列的宽度估算可以使用在query_planner()中获得的每个VAR信息。         */        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.         * 如果存在ORDER BY子句，考虑是否使用post-sort投影，如使用则计算前面已调整过的步骤目标列。         */        if (parse->sortClause)//存在sort语句?        {            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,则直接赋值            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.         * 如果要处理窗口函数，任何分组步骤的输出都需要满足窗口函数的要求;         * 否则，它应该是sort_input_target。         */        if (activeWindows)//存在窗口函数?        {            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.         * 如果要进行分组或聚合，最外层的扫描/连接计划节点必须发出分组步骤需要的内容;         * 否则，它应该设置grouping_target。         */        have_grouping = (parse->groupClause || parse->groupingSets ||                         parse->hasAggs || root->hasHavingQual);        if (have_grouping)        {//存在group等分组语句            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.         * 如果targetlist中有任何SRFs，我们必须将这些PathKeys分别划分为SRF-computing和SRF-free 目标列。         * 用一个没有SRF的版本替换每个指定的目标，并记住后面需要添加的其他投影步骤链表。         */        if (parse->hasTargetSRFs)//存在SRFs        {            /* final_target doesn't recompute any SRFs in sort_input_target */            //在sort_input_target中不需要重复计算SRFs            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 */            //初始化链表            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. */         //应用扫描/连接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.         * 保存刚刚计算的各种upper- >upper_targets[]信息。         * 核心代码不使用这个功能，但是它为扩展提供了一个方便的地方来获取信息。         * 为了保持一致性，我们保存了所有的中间目标列，即使这个查询可能不需要一些相应的上层关系。         */         //赋值        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.         * 如果我们有分组和/或聚合，考虑如何实现它。需要构建一个表示此阶段输出的上层关系。         */        if (have_grouping)//存在分组操作        {            current_rel = create_grouping_paths(root,                                                current_rel,                                                grouping_target,                                                grouping_target_parallel_safe,                                                &agg_costs,                                                gset_data);//创建分组访问路径            /* 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.         * 如果有窗口函数，考虑如何实现这些函数。         * 我们建立一个新的上层关系表示这个阶段的输出。         */        if (activeWindows)//存在窗口函数        {            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.         * 如果有一个DISTINCT子句，考虑如何实现它。构建一个表示此阶段输出的上层关系。         */        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.     * 如果指定了ORDER BY，考虑实现它的方法，并生成一个仅包含ORDER和final_target的Path的上层关系。     * 我们可以在排序成本中应用初始的limit_tuples限制，但前提是没有延迟的SRFs。     */    if (parse->sortClause)//存在sort语句?    {        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.     * 可以构建最终的关系了!     */    final_rel = fetch_upper_rel(root, UPPERREL_FINAL, NULL);//获取最终的RelOptInfo(用于替换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.     * 如果关系被标记为consider_parallel，并且在LIMIT子句中没有任何非并行安全的地方，     * 那么final_rel也可以被标记为consider_parallel。     * 请注意，如果查询有rowMarks或不是SELECT语句，则认为对查询中的每个关系consider_parallel都为false。     */    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.     * 如current_rel属于某个单独的FDW,设置final_rel信息     */    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.     * 为final_rel生成访问路径.     * 插入所有筛选后的访问路径,包含需添加的LockRows/Limit/ModifyTable步骤     */    foreach(lc, current_rel->pathlist)//逐一遍历访问路径    {        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.)         * 如果存在FOR [KEY] UPDATE/SHARE子句，则添加LockRows节点。         * (注意:我们在这里有意测试的是parse->rowMarks，而不是root->rowMarks。         * 如果只有非锁定行标记，则应该由ModifyTable节点处理。         * 但是，root->rowMarks是进入LockRows节点的行标记。         */        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.         * 如果存在LIMIT/OFFSET子句,添加LIMIT节点         */        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.         * 如为INSERT/UPDATE/DELETE,而且不是从inheritance_planner函数中调用,则添加ModifyTable节点         */        if (parse->commandType != CMD_SELECT && !inheritance_update)//非查询语句        {            List       *withCheckOptionLists;            List       *returningLists;            List       *rowMarks;            /*             * Set up the WITH CHECK OPTION and RETURNING lists-of-lists, if             * needed.             * 如需要,添加WITH CHECK OPTION and RETURNING信息             */            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.             * 如果存在FOR [KEY] UPDATE/SHARE子句，那么LockRows节点将处理获取非带锁标记的行，             * 否则我们需要使用ModifyTable来完成。             */            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 */        //添加到final_rel中        add_path(final_rel, path);    }    /*     * Generate partial paths for final_rel, too,xxwssssssssssssssssss if outer query levels might     * be able to make use of them.     * 并行执行访问路径     */    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.     * 如查询中存在FDW,添加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 */    //通过扩展添加访问路径    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() */    //注意:目前的做法是让调用放来执行set_cheap()函数}

二、参考资料

allpaths.c