PostgreSQL 源码解读（80）- 查询语句#65（cre

本节介绍了创建计划create_plan函数中连接(join)计划的实现过程，主要的逻辑在函数create_join_plan中实现。

一、数据结构

Plan
所有计划节点通过将Plan结构作为第一个字段从Plan结构“派生”。这确保了在将节点转换为计划节点时能正常工作。(在执行器中以通用方式传递时，节点指针经常被转换为Plan *)

/* ----------------
 *      Plan node
 *
 * All plan nodes "derive" from the Plan structure by having the
 * Plan structure as the first field.  This ensures that everything works
 * when nodes are cast to Plan's.  (node pointers are frequently cast to Plan*
 * when passed around generically in the executor)
 * 所有计划节点通过将Plan结构作为第一个字段从Plan结构“派生”。
 * 这确保了在将节点转换为计划节点时，一切都能正常工作。
 * (在执行器中以通用方式传递时，节点指针经常被转换为Plan *)
 *
 * We never actually instantiate any Plan nodes; this is just the common
 * abstract superclass for all Plan-type nodes.
 * 从未实例化任何Plan节点;这只是所有Plan-type节点的通用抽象超类。
 * ----------------
 */
typedef struct Plan
{
    NodeTag     type;//节点类型

    /*
     * 成本估算信息;estimated execution costs for plan (see costsize.c for more info)
     */
    Cost        startup_cost;   /* 启动成本;cost expended before fetching any tuples */
    Cost        total_cost;     /* 总成本;total cost (assuming all tuples fetched) */

    /*
     * 优化器估算信息;planner's estimate of result size of this plan step
     */
    double      plan_rows;      /* 行数;number of rows plan is expected to emit */
    int         plan_width;     /* 平均行大小(Byte为单位);average row width in bytes */

    /*
     * 并行执行相关的信息;information needed for parallel query
     */
    bool        parallel_aware; /* 是否参与并行执行逻辑?engage parallel-aware logic? */
    bool        parallel_safe;  /* 是否并行安全;OK to use as part of parallel plan? */

    /*
     * Plan类型节点通用的信息.Common structural data for all Plan types.
     */
    int         plan_node_id;   /* unique across entire final plan tree */
    List       *targetlist;     /* target list to be computed at this node */
    List       *qual;           /* implicitly-ANDed qual conditions */
    struct Plan *lefttree;      /* input plan tree(s) */
    struct Plan *righttree;
    List       *initPlan;       /* Init Plan nodes (un-correlated expr
                                 * subselects) */

    /*
     * Information for management of parameter-change-driven rescanning
     * parameter-change-driven重扫描的管理信息.
     * 
     * extParam includes the paramIDs of all external PARAM_EXEC params
     * affecting this plan node or its children.  setParam params from the
     * node's initPlans are not included, but their extParams are.
     *
     * allParam includes all the extParam paramIDs, plus the IDs of local
     * params that affect the node (i.e., the setParams of its initplans).
     * These are _all_ the PARAM_EXEC params that affect this node.
     */
    Bitmapset  *extParam;
    Bitmapset  *allParam;
} Plan;

二、源码解读

create_join_plan函数创建Join Plan节点.Join可以分为Merge Join/Hash Join/NestLoop Join三种,相应的实现函数是create_nestloop_plan/create_mergejoin_plan/create_hashjoin_plan.

//------------------------------------------------------------------ create_join_plan
/*
 * create_join_plan
 *    Create a join plan for 'best_path' and (recursively) plans for its
 *    inner and outer paths.
 *    创建连接计划Plan节点.
 */
static Plan *
create_join_plan(PlannerInfo *root, JoinPath *best_path)
{
    Plan       *plan;
    List       *gating_clauses;

    switch (best_path->path.pathtype)
    {
        case T_MergeJoin://Merge Join
            plan = (Plan *) create_mergejoin_plan(root,
                                                  (MergePath *) best_path);
            break;
        case T_HashJoin://Hash Join
            plan = (Plan *) create_hashjoin_plan(root,
                                                 (HashPath *) best_path);
            break;
        case T_NestLoop://NestLoop Join
            plan = (Plan *) create_nestloop_plan(root,
                                                 (NestPath *) best_path);
            break;
        default://目前仅支持上述三种
            elog(ERROR, "unrecognized node type: %d",
                 (int) best_path->path.pathtype);
            plan = NULL;        /* keep compiler quiet */
            break;
    }

    /*
     * If there are any pseudoconstant clauses attached to this node, insert a
     * gating Result node that evaluates the pseudoconstants as one-time
     * quals.
     * 如果这个节点上附加了伪常量子句，插入一个gating Result节点，该节点将伪常量计算为一次性条件quals。
     */
    gating_clauses = get_gating_quals(root, best_path->joinrestrictinfo);
    if (gating_clauses)
        plan = create_gating_plan(root, (Path *) best_path, plan,
                                  gating_clauses);

#ifdef NOT_USED

    /*
     * * Expensive function pullups may have pulled local predicates * into
     * this path node.  Put them in the qpqual of the plan node. * JMH,
     * 6/15/92
     * pullups函数可能已经把本地谓词上拉到该访问路径节点中,把这些信息放在Plan节点的qpqual中
     */
    if (get_loc_restrictinfo(best_path) != NIL)
        set_qpqual((Plan) plan,
                   list_concat(get_qpqual((Plan) plan),
                               get_actual_clauses(get_loc_restrictinfo(best_path))));
#endif

    return plan;
}

//------------------------------------------ create_nestloop_plan

static NestLoop *
create_nestloop_plan(PlannerInfo *root,
                     NestPath *best_path)
{
    NestLoop   *join_plan;
    Plan       *outer_plan;
    Plan       *inner_plan;
    List       *tlist = build_path_tlist(root, &best_path->path);
    List       *joinrestrictclauses = best_path->joinrestrictinfo;
    List       *joinclauses;
    List       *otherclauses;
    Relids      outerrelids;
    List       *nestParams;
    Relids      saveOuterRels = root->curOuterRels;
    ListCell   *cell;
    ListCell   *prev;
    ListCell   *next;

    /* NestLoop can project, so no need to be picky about child tlists */
    //NestLoop可以执行投影操作,所以不需要关心子计划的tlists
    //递归调用生成外表计划
    outer_plan = create_plan_recurse(root, best_path->outerjoinpath, 0);

    /* For a nestloop, include outer relids in curOuterRels for inner side */
    //对于nestloop,对应内侧的curOuterRels中需要包含外表的relids
    root->curOuterRels = bms_union(root->curOuterRels,
                                   best_path->outerjoinpath->parent->relids);
    //递归调用生成内表计划
    inner_plan = create_plan_recurse(root, best_path->innerjoinpath, 0);

    /* Restore curOuterRels */
    //恢复curOuterRels
    bms_free(root->curOuterRels);
    root->curOuterRels = saveOuterRels;

    /* Sort join qual clauses into best execution order */
    //排序连接条件
    joinrestrictclauses = order_qual_clauses(root, joinrestrictclauses);

    /* Get the join qual clauses (in plain expression form) */
    /* Any pseudoconstant clauses are ignored here */
    //获取连接条件子句,在这里,会忽略伪常量
    if (IS_OUTER_JOIN(best_path->jointype))
    {
        extract_actual_join_clauses(joinrestrictclauses,
                                    best_path->path.parent->relids,
                                    &joinclauses, &otherclauses);//外连接
    }
    else
    {
        /* We can treat all clauses alike for an inner join */
        //内连接
        joinclauses = extract_actual_clauses(joinrestrictclauses, false);
        otherclauses = NIL;
    }

    /* Replace any outer-relation variables with nestloop params */
    //使用nestloop参数替代外表变量
    if (best_path->path.param_info)
    {
        joinclauses = (List *)
            replace_nestloop_params(root, (Node *) joinclauses);
        otherclauses = (List *)
            replace_nestloop_params(root, (Node *) otherclauses);
    }

    /*
     * Identify any nestloop parameters that should be supplied by this join
     * node, and move them from root->curOuterParams to the nestParams list.
     * 确定这个连接节点应该提供的所有nestloop连接参数，
     * 并将它们从root->curOuterParams移动到nestParams链表中。
     */
    outerrelids = best_path->outerjoinpath->parent->relids;
    nestParams = NIL;
    prev = NULL;
    for (cell = list_head(root->curOuterParams); cell; cell = next)//遍历curOuterParams
    {
        NestLoopParam *nlp = (NestLoopParam *) lfirst(cell);//获取参数

        next = lnext(cell);
        if (IsA(nlp->paramval, Var) &&
            bms_is_member(nlp->paramval->varno, outerrelids))//Var变量,而且是外层的relids
        {
            root->curOuterParams = list_delete_cell(root->curOuterParams,
                                                    cell, prev);
            nestParams = lappend(nestParams, nlp);
        }
        else if (IsA(nlp->paramval, PlaceHolderVar) &&//PHV
                 bms_overlap(((PlaceHolderVar *) nlp->paramval)->phrels,
                             outerrelids) &&
                 bms_is_subset(find_placeholder_info(root,
                                                     (PlaceHolderVar *) nlp->paramval,
                                                     false)->ph_eval_at,
                               outerrelids))
        {
            root->curOuterParams = list_delete_cell(root->curOuterParams,
                                                    cell, prev);
            nestParams = lappend(nestParams, nlp);
        }
        else
            prev = cell;//直接赋值
    }

    join_plan = make_nestloop(tlist,
                              joinclauses,
                              otherclauses,
                              nestParams,
                              outer_plan,
                              inner_plan,
                              best_path->jointype,
                              best_path->inner_unique);//构造nestloop访问节点

    copy_generic_path_info(&join_plan->join.plan, &best_path->path);

    return join_plan;
}

//------------------------------------------ create_mergejoin_plan

static MergeJoin *
create_mergejoin_plan(PlannerInfo *root,
                      MergePath *best_path)
{
    MergeJoin  *join_plan;
    Plan       *outer_plan;
    Plan       *inner_plan;
    List       *tlist = build_path_tlist(root, &best_path->jpath.path);
    List       *joinclauses;
    List       *otherclauses;
    List       *mergeclauses;
    List       *outerpathkeys;
    List       *innerpathkeys;
    int         nClauses;
    Oid        *mergefamilies;
    Oid        *mergecollations;
    int        *mergestrategies;
    bool       *mergenullsfirst;
    PathKey    *opathkey;
    EquivalenceClass *opeclass;
    int         i;
    ListCell   *lc;
    ListCell   *lop;
    ListCell   *lip;
    Path       *outer_path = best_path->jpath.outerjoinpath;
    Path       *inner_path = best_path->jpath.innerjoinpath;

    /*
     * MergeJoin can project, so we don't have to demand exact tlists from the
     * inputs.  However, if we're intending to sort an input's result, it's
     * best to request a small tlist so we aren't sorting more data than
     * necessary.
     * MergeJoin可以进行投影运算，因此不必从输入中要求精确的tlist。
    * 然而，如果打算对输入的结果进行排序，最好是请求一个小的tlist，这样就不会对多余的数据进行排序。
     */
    //对外表生成计划Plan
    outer_plan = create_plan_recurse(root, best_path->jpath.outerjoinpath,
                                     (best_path->outersortkeys != NIL) ? CP_SMALL_TLIST : 0);
    //对内部生成计划Plan
    inner_plan = create_plan_recurse(root, best_path->jpath.innerjoinpath,
                                     (best_path->innersortkeys != NIL) ? CP_SMALL_TLIST : 0);

    /* Sort join qual clauses into best execution order */
    /* NB: do NOT reorder the mergeclauses */
    //排序连接条件
    joinclauses = order_qual_clauses(root, best_path->jpath.joinrestrictinfo);

    /* Get the join qual clauses (in plain expression form) */
    /* Any pseudoconstant clauses are ignored here */
    //获取连接约束条件子句(以扁平化的形式)
    if (IS_OUTER_JOIN(best_path->jpath.jointype))
    {
        extract_actual_join_clauses(joinclauses,
                                    best_path->jpath.path.parent->relids,
                                    &joinclauses, &otherclauses);
    }
    else
    {
        /* We can treat all clauses alike for an inner join */
        //以内连接的方式处理所有条件子句
        joinclauses = extract_actual_clauses(joinclauses, false);
        otherclauses = NIL;
    }

    /*
     * Remove the mergeclauses from the list of join qual clauses, leaving the
     * list of quals that must be checked as qpquals.
     * 从join qual子句链表中删除mergeclauses，将必须检查为qpquals的quals链表保留下来。
     */
    mergeclauses = get_actual_clauses(best_path->path_mergeclauses);
    joinclauses = list_difference(joinclauses, mergeclauses);

    /*
     * Replace any outer-relation variables with nestloop params.  There
     * should not be any in the mergeclauses.
     * 使用nestloop参数替代外表变量.这些变量不应在mergeclauses中出现.
     */
    if (best_path->jpath.path.param_info)
    {
        joinclauses = (List *)
            replace_nestloop_params(root, (Node *) joinclauses);//连接条件
        otherclauses = (List *)
            replace_nestloop_params(root, (Node *) otherclauses);//其他条件
    }

    /*
     * Rearrange mergeclauses, if needed, so that the outer variable is always
     * on the left; mark the mergeclause restrictinfos with correct
     * outer_is_left status.
     * 如果需要，重新安排mergeclauses，使外部变量总是在左边;
     * 用正确的outer_is_left状态标记mergeclause restrictinfos。
     */
    mergeclauses = get_switched_clauses(best_path->path_mergeclauses,
                                        best_path->jpath.outerjoinpath->parent->relids);

    /*
     * Create explicit sort nodes for the outer and inner paths if necessary.
     * 如需要创建显式的Sort节点
     */
    if (best_path->outersortkeys)
    {
        Relids      outer_relids = outer_path->parent->relids;
        Sort       *sort = make_sort_from_pathkeys(outer_plan,
                                                   best_path->outersortkeys,
                                                   outer_relids);

        label_sort_with_costsize(root, sort, -1.0);
        outer_plan = (Plan *) sort;
        outerpathkeys = best_path->outersortkeys;
    }
    else
        outerpathkeys = best_path->jpath.outerjoinpath->pathkeys;

    if (best_path->innersortkeys)
    {
        Relids      inner_relids = inner_path->parent->relids;
        Sort       *sort = make_sort_from_pathkeys(inner_plan,
                                                   best_path->innersortkeys,
                                                   inner_relids);

        label_sort_with_costsize(root, sort, -1.0);
        inner_plan = (Plan *) sort;
        innerpathkeys = best_path->innersortkeys;
    }
    else
        innerpathkeys = best_path->jpath.innerjoinpath->pathkeys;

    /*
     * If specified, add a materialize node to shield the inner plan from the
     * need to handle mark/restore.
     * 如指定物化,则添加物化节点
     */
    if (best_path->materialize_inner)
    {
        Plan       *matplan = (Plan *) make_material(inner_plan);

        /*
         * We assume the materialize will not spill to disk, and therefore
         * charge just cpu_operator_cost per tuple.  (Keep this estimate in
         * sync with final_cost_mergejoin.)
         * 假设materialize不会溢出到磁盘，因此每个元组的成本为cpu_operator_cost。
         * (让这个估计与final_cost_mergejoin保持同步。)
         */
        copy_plan_costsize(matplan, inner_plan);
        matplan->total_cost += cpu_operator_cost * matplan->plan_rows;

        inner_plan = matplan;
    }

    /*
     * Compute the opfamily/collation/strategy/nullsfirst arrays needed by the
     * executor.  The information is in the pathkeys for the two inputs, but
     * we need to be careful about the possibility of mergeclauses sharing a
     * pathkey, as well as the possibility that the inner pathkeys are not in
     * an order matching the mergeclauses.
     * 计算执行器需要的opfamily/collation/strategy/nullsfirst数组。
     * 信息在这两个输入的pathkeys中，但是需要注意mergeclauses共享一个pathkey的可能性，
     * 以及内表路径键不符合mergeclauses顺序的可能性。
     */
    nClauses = list_length(mergeclauses);
    Assert(nClauses == list_length(best_path->path_mergeclauses));
    mergefamilies = (Oid *) palloc(nClauses * sizeof(Oid));//申请内存
    mergecollations = (Oid *) palloc(nClauses * sizeof(Oid));
    mergestrategies = (int *) palloc(nClauses * sizeof(int));
    mergenullsfirst = (bool *) palloc(nClauses * sizeof(bool));

    opathkey = NULL;
    opeclass = NULL;
    lop = list_head(outerpathkeys);
    lip = list_head(innerpathkeys);
    i = 0;
    foreach(lc, best_path->path_mergeclauses)//遍历条件
    {
        RestrictInfo *rinfo = lfirst_node(RestrictInfo, lc);
        EquivalenceClass *oeclass;
        EquivalenceClass *ieclass;
        PathKey    *ipathkey = NULL;
        EquivalenceClass *ipeclass = NULL;
        bool        first_inner_match = false;

        /* fetch outer/inner eclass from mergeclause */
        //从mergeclause中获取outer/inner等价类
        if (rinfo->outer_is_left)
        {
            oeclass = rinfo->left_ec;
            ieclass = rinfo->right_ec;
        }
        else
        {
            oeclass = rinfo->right_ec;
            ieclass = rinfo->left_ec;
        }
        Assert(oeclass != NULL);
        Assert(ieclass != NULL);

        /*
         * We must identify the pathkey elements associated with this clause
         * by matching the eclasses (which should give a unique match, since
         * the pathkey lists should be canonical).  In typical cases the merge
         * clauses are one-to-one with the pathkeys, but when dealing with
         * partially redundant query conditions, things are more complicated.
         * 必须通过匹配等价类eclasses来标识与此子句关联的pathkey元素
         * (它应该提供唯一的匹配，因为pathkey链表应该是规范的)。
         * 在典型的情况下，merge子句与pathkey是一对一的，但是在处理部分冗余查询条件时，事情就有些复杂了。
         * 
         * lop and lip reference the first as-yet-unmatched pathkey elements.
         * If they're NULL then all pathkey elements have been matched.
         * lop和lip引用第一个尚未匹配的pathkey元素。如果它们为空，那么所有的pathkey元素都已匹配。
         *
         * The ordering of the outer pathkeys should match the mergeclauses,
         * by construction (see find_mergeclauses_for_outer_pathkeys()). There
         * could be more than one mergeclause for the same outer pathkey, but
         * no pathkey may be entirely skipped over.
         * 通过处理，外表pathkey顺序应该与mergeclauses匹配(参见find_mergeclauses_for_outer_pathkeys()函数)。
         * 同一个外表pathkey可以有多个mergeclause，但是不能完全跳过所有pathkey。
         */
        if (oeclass != opeclass)    /* multiple matches are not interesting */
        {
            /* doesn't match the current opathkey, so must match the next */
            //与当前的opathkey不匹配,那么必须与接下来的匹配
            if (lop == NULL)
                elog(ERROR, "outer pathkeys do not match mergeclauses");
            opathkey = (PathKey *) lfirst(lop);
            opeclass = opathkey->pk_eclass;
            lop = lnext(lop);
            if (oeclass != opeclass)
                elog(ERROR, "outer pathkeys do not match mergeclauses");
        }

        /*
         * The inner pathkeys likewise should not have skipped-over keys, but
         * it's possible for a mergeclause to reference some earlier inner
         * pathkey if we had redundant pathkeys.  For example we might have
         * mergeclauses like "o.a = i.x AND o.b = i.y AND o.c = i.x".  The
         * implied inner ordering is then "ORDER BY x, y, x", but the pathkey
         * mechanism drops the second sort by x as redundant, and this code
         * must cope.
         * 同样，内表pathkey也不应该有skipped-over keys，但是如果我们有冗余的路径键，
         * mergeclause可以引用一些早期的内部路径键。
         * 例如，我们可能存在下面的mergeclauses，比如"o.a = i.x AND o.b = i.y AND o.c = i.x"。
         * 隐含的内部排序是“x, y, x的排序”，但是pathkey机制将按x排序视为多余并删除，在这里必须处理这种情况。
         *
         * It's also possible for the implied inner-rel ordering to be like
         * "ORDER BY x, y, x DESC".  We still drop the second instance of x as
         * redundant; but this means that the sort ordering of a redundant
         * inner pathkey should not be considered significant.  So we must
         * detect whether this is the first clause matching an inner pathkey.
         * 对于隐含的内表排序，也有可能是“ORDER BY x, y, x DESC”。
         * 仍然将x的第二个实例视为冗余并删除;
         * 但是这意味着冗余的内表pathkey的排序顺序不应该被认为是重要的。
         * 因此，我们必须检测这是否是与内表pathkey匹配的第一个子句。
         * 
         */
        if (lip)
        {
            ipathkey = (PathKey *) lfirst(lip);
            ipeclass = ipathkey->pk_eclass;
            if (ieclass == ipeclass)
            {
                /* successful first match to this inner pathkey */
                //成功匹配
                lip = lnext(lip);
                first_inner_match = true;
            }
        }
        if (!first_inner_match)
        {
            /* redundant clause ... must match something before lip */
            //多余的条件子句,必须在lip前匹配某些pathkey
            ListCell   *l2;

            foreach(l2, innerpathkeys)
            {
                if (l2 == lip)
                    break;
                ipathkey = (PathKey *) lfirst(l2);
                ipeclass = ipathkey->pk_eclass;
                if (ieclass == ipeclass)
                    break;
            }
            if (ieclass != ipeclass)
                elog(ERROR, "inner pathkeys do not match mergeclauses");
        }

        /*
         * The pathkeys should always match each other as to opfamily and
         * collation (which affect equality), but if we're considering a
         * redundant inner pathkey, its sort ordering might not match.  In
         * such cases we may ignore the inner pathkey's sort ordering and use
         * the outer's.  (In effect, we're lying to the executor about the
         * sort direction of this inner column, but it does not matter since
         * the run-time row comparisons would only reach this column when
         * there's equality for the earlier column containing the same eclass.
         * There could be only one value in this column for the range of inner
         * rows having a given value in the earlier column, so it does not
         * matter which way we imagine this column to be ordered.)  But a
         * non-redundant inner pathkey had better match outer's ordering too.
         * 对于opfamily和collation(这会影响等式)，pathkey应该总是匹配的，
         * 但是如果我们考虑一个冗余的内表pathkey，它的排序顺序可能不匹配。
         * 在这种情况下，我们可以忽略内表pathkey的排序顺序，而使用外表访问路径。
         * (实际上，是在内表列的排序方向上欺骗执行器，但这无关紧要，
         *  因为运行时行比较只在包含相同eclass的前一列相等时才会到达这一列。
         *  对于在前一列中具有给定值的内部行范围，在此列中只能有一个值，
         *  因此我们认为该列的顺序如何并不重要。)
         * 而一个非冗余的内部路径也最好与外部的排序匹配。
         */
        if (opathkey->pk_opfamily != ipathkey->pk_opfamily ||
            opathkey->pk_eclass->ec_collation != ipathkey->pk_eclass->ec_collation)
            elog(ERROR, "left and right pathkeys do not match in mergejoin");
        if (first_inner_match &&
            (opathkey->pk_strategy != ipathkey->pk_strategy ||
             opathkey->pk_nulls_first != ipathkey->pk_nulls_first))
            elog(ERROR, "left and right pathkeys do not match in mergejoin");

        /* OK, save info for executor */
        mergefamilies[i] = opathkey->pk_opfamily;
        mergecollations[i] = opathkey->pk_eclass->ec_collation;
        mergestrategies[i] = opathkey->pk_strategy;
        mergenullsfirst[i] = opathkey->pk_nulls_first;
        i++;
    }

    /*
     * Note: it is not an error if we have additional pathkey elements (i.e.,
     * lop or lip isn't NULL here).  The input paths might be better-sorted
     * than we need for the current mergejoin.
     * 注意:如果有额外的pathkey元素(例如， lop或lip在这里不是空的)。
     * 输入路径可能比当前合并连接所需的排序更好。
     */

    /*
     * Now we can build the mergejoin node.
     * 创建mergejoin节点
     */
    join_plan = make_mergejoin(tlist,
                               joinclauses,
                               otherclauses,
                               mergeclauses,
                               mergefamilies,
                               mergecollations,
                               mergestrategies,
                               mergenullsfirst,
                               outer_plan,
                               inner_plan,
                               best_path->jpath.jointype,
                               best_path->jpath.inner_unique,
                               best_path->skip_mark_restore);

    /* Costs of sort and material steps are included in path cost already */
    //排序和物化步骤一包含在访问路径的成本中
    copy_generic_path_info(&join_plan->join.plan, &best_path->jpath.path);

    return join_plan;
}

//------------------------------------------ create_hashjoin_plan

static HashJoin *
create_hashjoin_plan(PlannerInfo *root,
                     HashPath *best_path)
{
    HashJoin   *join_plan;
    Hash       *hash_plan;
    Plan       *outer_plan;
    Plan       *inner_plan;
    List       *tlist = build_path_tlist(root, &best_path->jpath.path);
    List       *joinclauses;
    List       *otherclauses;
    List       *hashclauses;
    Oid         skewTable = InvalidOid;
    AttrNumber  skewColumn = InvalidAttrNumber;
    bool        skewInherit = false;

    /*
     * HashJoin can project, so we don't have to demand exact tlists from the
     * inputs.  However, it's best to request a small tlist from the inner
     * side, so that we aren't storing more data than necessary.  Likewise, if
     * we anticipate batching, request a small tlist from the outer side so
     * that we don't put extra data in the outer batch files.
     * HashJoin可以进行投影运算，因此我们不必从输入中要求精确的tlist。
     * 但是，最好从内部请求一个小tlist，这样就不需要存储过多的数据。
     * 同样，如果我们进行预批处理，从外部请求一个小tlist，这样就不会在外部批处理文件中添加额外的数据。
     */
    outer_plan = create_plan_recurse(root, best_path->jpath.outerjoinpath,
                                     (best_path->num_batches > 1) ? CP_SMALL_TLIST : 0);

    inner_plan = create_plan_recurse(root, best_path->jpath.innerjoinpath,
                                     CP_SMALL_TLIST);

    /* Sort join qual clauses into best execution order */
    joinclauses = order_qual_clauses(root, best_path->jpath.joinrestrictinfo);
    /* There's no point in sorting the hash clauses ... */

    /* Get the join qual clauses (in plain expression form) */
    /* Any pseudoconstant clauses are ignored here */
    if (IS_OUTER_JOIN(best_path->jpath.jointype))
    {
        extract_actual_join_clauses(joinclauses,
                                    best_path->jpath.path.parent->relids,
                                    &joinclauses, &otherclauses);
    }
    else
    {
        /* We can treat all clauses alike for an inner join */
        joinclauses = extract_actual_clauses(joinclauses, false);
        otherclauses = NIL;
    }

    /*
     * Remove the hashclauses from the list of join qual clauses, leaving the
     * list of quals that must be checked as qpquals.
     * 从join qual子句列表中删除hashclause，将必须检查为qpquals的quals列表保留下来。
     */
    hashclauses = get_actual_clauses(best_path->path_hashclauses);
    joinclauses = list_difference(joinclauses, hashclauses);

    /*
     * Replace any outer-relation variables with nestloop params.  There
     * should not be any in the hashclauses.
     * 用nestloop参数替换任何外部关系变量。而且不应在hashclauses中出现。
     */
    if (best_path->jpath.path.param_info)
    {
        joinclauses = (List *)
            replace_nestloop_params(root, (Node *) joinclauses);
        otherclauses = (List *)
            replace_nestloop_params(root, (Node *) otherclauses);
    }

    /*
     * Rearrange hashclauses, if needed, so that the outer variable is always
     * on the left.
     * 重新安排hashclausees,以便外表的Var出现在左侧
     */
    hashclauses = get_switched_clauses(best_path->path_hashclauses,
                                       best_path->jpath.outerjoinpath->parent->relids);

    /*
     * If there is a single join clause and we can identify the outer variable
     * as a simple column reference, supply its identity for possible use in
     * skew optimization.  (Note: in principle we could do skew optimization
     * with multiple join clauses, but we'd have to be able to determine the
     * most common combinations of outer values, which we don't currently have
     * enough stats for.)
     * 如果有一个连接条件子句，并且可以将外表变量标识为一个简单的列引用，
     * 那么可以通过提供它的标识以表在列数据倾斜优化中使用。
     * (注意:原则上可以使用多个连接子句进行倾斜优化，
     * 但我们必须能够确定最常见的外部值组合，目前我们还没有足够的统计数据。)
     */
    if (list_length(hashclauses) == 1)
    {
        OpExpr     *clause = (OpExpr *) linitial(hashclauses);
        Node       *node;

        Assert(is_opclause(clause));
        node = (Node *) linitial(clause->args);
        if (IsA(node, RelabelType))
            node = (Node *) ((RelabelType *) node)->arg;
        if (IsA(node, Var))
        {
            Var        *var = (Var *) node;
            RangeTblEntry *rte;

            rte = root->simple_rte_array[var->varno];
            if (rte->rtekind == RTE_RELATION)
            {
                skewTable = rte->relid;
                skewColumn = var->varattno;
                skewInherit = rte->inh;
            }
        }
    }

    /*
     * Build the hash node and hash join node.
     * 创建hash节点和hash join节点
     */
    hash_plan = make_hash(inner_plan,
                          skewTable,
                          skewColumn,
                          skewInherit);//为内表创建hash表

    /*
     * Set Hash node's startup & total costs equal to total cost of input
     * plan; this only affects EXPLAIN display not decisions.
     * 设置哈希节点的启动和总成本等于输入的计划总成本;
     * 这只影响解释显示而不是决策。
     */
    copy_plan_costsize(&hash_plan->plan, inner_plan);
    hash_plan->plan.startup_cost = hash_plan->plan.total_cost;

    /*
     * If parallel-aware, the executor will also need an estimate of the total
     * number of rows expected from all participants so that it can size the
     * shared hash table.
     * 如果需要并行，执行器还需要估计所有参与者预期的行数，以便对共享哈希表进行大小计算。
     */
    if (best_path->jpath.path.parallel_aware)
    {
        hash_plan->plan.parallel_aware = true;
        hash_plan->rows_total = best_path->inner_rows_total;
    }

    join_plan = make_hashjoin(tlist,
                              joinclauses,
                              otherclauses,
                              hashclauses,
                              outer_plan,
                              (Plan *) hash_plan,
                              best_path->jpath.jointype,
                              best_path->jpath.inner_unique);//创建hash join节点

    copy_generic_path_info(&join_plan->join.plan, &best_path->jpath.path);

    return join_plan;
}

三、跟踪分析

测试脚本如下

testdb=# explain select dw.*,grjf.grbh,grjf.xm,grjf.ny,grjf.je 
testdb-# from t_dwxx dw,lateral (select gr.grbh,gr.xm,jf.ny,jf.je 
testdb(#                         from t_grxx gr inner join t_jfxx jf 
testdb(#                                        on gr.dwbh = dw.dwbh 
testdb(#                                           and gr.grbh = jf.grbh) grjf
testdb-# where dw.dwbh in ('1001','1002')
testdb-# order by dw.dwbh;
                                     QUERY PLAN                                               
--------------------------------------------------------------------------------------------------
 Sort  (cost=2010.12..2010.17 rows=20 width=47)
   Sort Key: dw.dwbh
   ->  Nested Loop  (cost=14.24..2009.69 rows=20 width=47)
         ->  Hash Join  (cost=13.95..2002.56 rows=20 width=32)
               Hash Cond: ((gr.dwbh)::text = (dw.dwbh)::text)
               ->  Seq Scan on t_grxx gr  (cost=0.00..1726.00 rows=100000 width=16)
               ->  Hash  (cost=13.92..13.92 rows=2 width=20)
                     ->  Index Scan using t_dwxx_pkey on t_dwxx dw  (cost=0.29..13.92 rows=2 width=20)
                           Index Cond: ((dwbh)::text = ANY ('{1001,1002}'::text[]))
         ->  Index Scan using idx_t_jfxx_grbh on t_jfxx jf  (cost=0.29..0.35 rows=1 width=20)
               Index Cond: ((grbh)::text = (gr.grbh)::text)

启动gdb,设置断点,进入create_join_plan函数

(gdb) b create_join_plan
Breakpoint 1 at 0x7b8426: file createplan.c, line 973.
(gdb) c
Continuing.

Breakpoint 1, create_join_plan (root=0x2ef8a00, best_path=0x2f5ad40) at createplan.c:973
973     switch (best_path->path.pathtype)

查看输入参数,pathtype为T_NestLoop

(gdb) p *best_path
$3 = {path = {type = T_NestPath, pathtype = T_NestLoop, parent = 0x2f5a570, pathtarget = 0x2f5a788, param_info = 0x0, 
    parallel_aware = false, parallel_safe = true, parallel_workers = 0, rows = 20, startup_cost = 14.241722117799656, 
    total_cost = 2009.6908721177995, pathkeys = 0x0}, jointype = JOIN_INNER, inner_unique = false, 
  outerjoinpath = 0x2f58bb0, innerjoinpath = 0x2f56080, joinrestrictinfo = 0x0}

进入create_nestloop_plan

973     switch (best_path->path.pathtype)
(gdb) n
984             plan = (Plan *) create_nestloop_plan(root,
(gdb) step
create_nestloop_plan (root=0x2f49180, best_path=0x2f5ad40) at createplan.c:3678
3678        List       *tlist = build_path_tlist(root, &best_path->path);    

nestloop join->创建tlist,获取连接条件等

3678        List       *tlist = build_path_tlist(root, &best_path->path);
(gdb) n
3679        List       *joinrestrictclauses = best_path->joinrestrictinfo;
(gdb) 
3684        Relids      saveOuterRels = root->curOuterRels;
(gdb) p root->curOuterRels
$1 = (Relids) 0x0

nestloop join->调用create_plan_recurse创建outer_plan

(gdb) n
3690        outer_plan = create_plan_recurse(root, best_path->outerjoinpath, 0);
(gdb) 

Breakpoint 1, create_join_plan (root=0x2f49180, best_path=0x2f58bb0) at createplan.c:973
973     switch (best_path->path.pathtype)

nestloop join->外表对应的outer_plan为T_HashJoin

(gdb) p *best_path
$2 = {path = {type = T_HashPath, pathtype = T_HashJoin, parent = 0x2f572d0, pathtarget = 0x2f57508, param_info = 0x0, 
    parallel_aware = false, parallel_safe = true, parallel_workers = 0, rows = 20, startup_cost = 13.949222117799655, 
    total_cost = 2002.5604721177997, pathkeys = 0x0}, jointype = JOIN_INNER, inner_unique = true, 
  outerjoinpath = 0x2f512f0, innerjoinpath = 0x2f51e98, joinrestrictinfo = 0x2f577a8}
(gdb) 

nestloop join->进入create_hashjoin_plan

(gdb) n
980             plan = (Plan *) create_hashjoin_plan(root,
(gdb) step
create_hashjoin_plan (root=0x2f49180, best_path=0x2f58bb0) at createplan.c:4093
4093        List       *tlist = build_path_tlist(root, &best_path->jpath.path);

hash join->创建outer plan

(gdb) 
4108        outer_plan = create_plan_recurse(root, best_path->jpath.outerjoinpath,
(gdb) p *best_path->jpath.outerjoinpath
$4 = {type = T_Path, pathtype = T_SeqScan, parent = 0x2f06090, pathtarget = 0x2f062c8, param_info = 0x0, 
  parallel_aware = false, parallel_safe = true, parallel_workers = 0, rows = 100000, startup_cost = 0, total_cost = 1726, 
  pathkeys = 0x0}

hash join->创建inner plan

(gdb) p *best_path->jpath.innerjoinpath
$5 = {type = T_IndexPath, pathtype = T_IndexScan, parent = 0x2f04b60, pathtarget = 0x2f04d98, param_info = 0x0, 
  parallel_aware = false, parallel_safe = true, parallel_workers = 0, rows = 2, startup_cost = 0.28500000000000003, 
  total_cost = 13.924222117799655, pathkeys = 0x2f51e20}

hash join->获取连接条件

(gdb) n
4115        joinclauses = order_qual_clauses(root, best_path->jpath.joinrestrictinfo);
(gdb) 
4120        if (IS_OUTER_JOIN(best_path->jpath.jointype))
(gdb) p *joinclauses
$6 = {type = T_List, length = 1, head = 0x2f57780, tail = 0x2f57780}

hash join->处理连接条件&hash条件

(gdb) n
4137        hashclauses = get_actual_clauses(best_path->path_hashclauses);
(gdb) 
4138        joinclauses = list_difference(joinclauses, hashclauses);
(gdb) 
4144        if (best_path->jpath.path.param_info)
(gdb) p *hashclauses
$8 = {type = T_List, length = 1, head = 0x2f5d690, tail = 0x2f5d690}
(gdb) p *joinclauses
Cannot access memory at address 0x0

hash join->变换位置,把外表的Var放在左侧

(gdb) n
4156        hashclauses = get_switched_clauses(best_path->path_hashclauses,
(gdb) 

hash join->Hash连接条件只有一个,进行数据倾斜优化

(gdb) 
4167        if (list_length(hashclauses) == 1)
(gdb) n
4169            OpExpr     *clause = (OpExpr *) linitial(hashclauses);
(gdb) n
4172            Assert(is_opclause(clause));
(gdb) 
4173            node = (Node *) linitial(clause->args);
(gdb) 
4174            if (IsA(node, RelabelType))
(gdb) 
4175                node = (Node *) ((RelabelType *) node)->arg;
(gdb) 
4176            if (IsA(node, Var))
(gdb) 
4178                Var        *var = (Var *) node;
(gdb) 
4181                rte = root->simple_rte_array[var->varno];
(gdb) p *node
$9 = {type = T_Var}
(gdb) p *(Var *)node
$10 = {xpr = {type = T_Var}, varno = 3, varattno = 1, vartype = 1043, vartypmod = 14, varcollid = 100, varlevelsup = 0, 
  varnoold = 3, varoattno = 1, location = 208}
(gdb) n
4182                if (rte->rtekind == RTE_RELATION)
(gdb) 
4184                    skewTable = rte->relid;
(gdb) 
4185                    skewColumn = var->varattno;
(gdb) 
4186                    skewInherit = rte->inh;
(gdb) 

hash join->开始创建创建hash节点和hash join节点
创建hash节点(构建Hash表)

4194        hash_plan = make_hash(inner_plan,
(gdb) n
4203        copy_plan_costsize(&hash_plan->plan, inner_plan);
(gdb) 
4204        hash_plan->plan.startup_cost = hash_plan->plan.total_cost;
(gdb) p *hash_plan
$11 = {plan = {type = T_Hash, startup_cost = 0.28500000000000003, total_cost = 13.924222117799655, plan_rows = 2, 
    plan_width = 20, parallel_aware = false, parallel_safe = true, plan_node_id = 0, targetlist = 0x2f5d250, qual = 0x0, 
    lefttree = 0x2f58428, righttree = 0x0, initPlan = 0x0, extParam = 0x0, allParam = 0x0}, skewTable = 16742, 
  skewColumn = 1, skewInherit = false, rows_total = 0}

hash join->创建hash join节点

(gdb) n
4217        join_plan = make_hashjoin(tlist,
(gdb) 
4226        copy_generic_path_info(&join_plan->join.plan, &best_path->jpath.path);
(gdb) 
4228        return join_plan;
(gdb) p *join_plan
$13 = {join = {plan = {type = T_HashJoin, startup_cost = 13.949222117799655, total_cost = 2002.5604721177997, 
      plan_rows = 20, plan_width = 32, parallel_aware = false, parallel_safe = true, plan_node_id = 0, 
      targetlist = 0x2f5cb28, qual = 0x0, lefttree = 0x2f5ae98, righttree = 0x2f5d830, initPlan = 0x0, extParam = 0x0, 
      allParam = 0x0}, jointype = JOIN_INNER, inner_unique = true, joinqual = 0x0}, hashclauses = 0x2f5d7f8}

hash join->回到create_nestloop_plan

(gdb) n
create_nestloop_plan (root=0x2f49180, best_path=0x2f5ad40) at createplan.c:3694
3694                                       best_path->outerjoinpath->parent->relids);
(gdb) n
3693        root->curOuterRels = bms_union(root->curOuterRels,        

nestloop join->创建内表Plan

(gdb) n
3696        inner_plan = create_plan_recurse(root, best_path->innerjoinpath, 0);
(gdb) p *best_path->innerjoinpath
$16 = {type = T_IndexPath, pathtype = T_IndexScan, parent = 0x2f06858, pathtarget = 0x2f06a70, param_info = 0x2f56910, 
  parallel_aware = false, parallel_safe = true, parallel_workers = 0, rows = 1, startup_cost = 0.29249999999999998, 
  total_cost = 0.34651999999999999, pathkeys = 0x2f56608}

nestloop join->获取连接条件子句

(gdb) n
3699        bms_free(root->curOuterRels);
(gdb) 
3700        root->curOuterRels = saveOuterRels;
(gdb) 
3703        joinrestrictclauses = order_qual_clauses(root, joinrestrictclauses);
(gdb) 
3707        if (IS_OUTER_JOIN(best_path->jointype))
(gdb) p *joinrestrictclauses
Cannot access memory at address 0x0

nestloop join->获取连接条件&参数化处理(相关值为NULL)

(gdb) n
3716            joinclauses = extract_actual_clauses(joinrestrictclauses, false);
(gdb) 
3717            otherclauses = NIL;
(gdb) 
3721        if (best_path->path.param_info)
(gdb) p *joinclauses
Cannot access memory at address 0x0
(gdb) p *best_path->path.param_info
Cannot access memory at address 0x0

nestloop join->获取外表的relids(外表为1和3号RTE的连接)

(gdb) n
3733        outerrelids = best_path->outerjoinpath->parent->relids;
(gdb) 
3734        nestParams = NIL;
(gdb) p *outerrelids
$17 = {nwords = 1, words = 0x2f574ec}
(gdb) p *outerrelids->words
$18 = 10

nestloop join->遍历当前的外表参数链表

(gdb) n
3735        prev = NULL;
(gdb) 
3736        for (cell = list_head(root->curOuterParams); cell; cell = next)
(gdb) p *root->curOuterParams
$19 = {type = T_List, length = 1, head = 0x2f5df98, tail = 0x2f5df98}

nestloop join->查看该参数信息,3号RTE编号为2的字段(即grbh)

(gdb) n
3738            NestLoopParam *nlp = (NestLoopParam *) lfirst(cell);
(gdb) 
3740            next = lnext(cell);
(gdb) p *(NestLoopParam *)nlp
$21 = {type = T_NestLoopParam, paramno = 0, paramval = 0x2f54e50}
(gdb) p *nlp->paramval
$22 = {xpr = {type = T_Var}, varno = 3, varattno = 2, vartype = 1043, vartypmod = 14, varcollid = 100, varlevelsup = 0, 
  varnoold = 3, varoattno = 2, location = 273}

nestloop join->把条件从root->curOuterParams移动到nestParams链表中

(gdb) n
3741            if (IsA(nlp->paramval, Var) &&
(gdb) n
3742                bms_is_member(nlp->paramval->varno, outerrelids))
(gdb) 
3741            if (IsA(nlp->paramval, Var) &&
(gdb) 
3744                root->curOuterParams = list_delete_cell(root->curOuterParams,
(gdb) 
3746                nestParams = lappend(nestParams, nlp);
(gdb) 
3736        for (cell = list_head(root->curOuterParams); cell; cell = next)
(gdb) p *nestParams
$23 = {type = T_List, length = 1, head = 0x2f5df98, tail = 0x2f5df98}
(gdb) p *(Node *)nestParams->head->data.ptr_value
$24 = {type = T_NestLoopParam}
(gdb) p *(NestLoopParam *)nestParams->head->data.ptr_value
$25 = {type = T_NestLoopParam, paramno = 0, paramval = 0x2f54e50}
(gdb) set $nlp=(NestLoopParam *)nestParams->head->data.ptr_value
(gdb) p $nlp->paramval
$26 = (Var *) 0x2f54e50
(gdb) p *$nlp->paramval
$27 = {xpr = {type = T_Var}, varno = 3, varattno = 2, vartype = 1043, vartypmod = 14, varcollid = 100, varlevelsup = 0, 
  varnoold = 3, varoattno = 2, location = 273}
(gdb) 

nestloop join->创建nestloop join节点

(gdb) n
3771                                  best_path->inner_unique);
(gdb) 
3764        join_plan = make_nestloop(tlist,
(gdb) 
3773        copy_generic_path_info(&join_plan->join.plan, &best_path->path);
(gdb) 
3775        return join_plan;
(gdb) p *join_plan
$28 = {join = {plan = {type = T_NestLoop, startup_cost = 14.241722117799656, total_cost = 2009.6908721177995, 
      plan_rows = 20, plan_width = 47, parallel_aware = false, parallel_safe = true, plan_node_id = 0, 
      targetlist = 0x2f5c770, qual = 0x0, lefttree = 0x2f5d8c8, righttree = 0x2f59ed0, initPlan = 0x0, extParam = 0x0, 
      allParam = 0x0}, jointype = JOIN_INNER, inner_unique = false, joinqual = 0x0}, nestParams = 0x2f5dfc0}
(gdb) 

DONE!

四、参考资料

createplan.c
PG Document:Query Planning