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PostgreSQL 源码解读(74)- 查询语句#59(Review - subquery_...

原创 PostgreSQL 作者:husthxd 时间:2018-11-01 16:16:31 0 删除 编辑

本节回过头来Review subquery_planner函数的实现逻辑,该函数对(子)查询进行执行规划。对于查询树中的每个(子)查询(sub-SELECT),都会递归执行此处理过程。

一、源码解读

subquery_planner函数由函数standard_planner调用,生成最终的结果Relation(成本最低),其输出作为生成实际执行计划的输入,在此函数中会调用grouping_planner执行主要的计划过程

/*--------------------
 * subquery_planner
 *    Invokes the planner on a subquery.  We recurse to here for each
 *    sub-SELECT found in the query tree.
 *    对子查询进行执行规划。对于查询树中的每个子查询(sub-SELECT),都会递归此处理过程。    
 *
 * glob is the global state for the current planner run.
 * parse is the querytree produced by the parser & rewriter.
 * parent_root is the immediate parent Query's info (NULL at the top level).
 * hasRecursion is true if this is a recursive WITH query.
 * tuple_fraction is the fraction of tuples we expect will be retrieved.
 * tuple_fraction is interpreted as explained for grouping_planner, below.
 * glob-当前计划器运行的全局状态。
 * parse-由解析器和重写器生成的查询树querytree。
 * parent_root是父查询的信息(如为顶层则为空)。
 * hasRecursion-如果这是一个带查询的递归,值为T。
 * tuple_fraction-扫描元组的比例。tuple_fraction在grouping_planner中详细解释。
 *
 * Basically, this routine does the stuff that should only be done once
 * per Query object.  It then calls grouping_planner.  At one time,
 * grouping_planner could be invoked recursively on the same Query object;
 * that's not currently true, but we keep the separation between the two
 * routines anyway, in case we need it again someday.
 * 基本上,这个函数包含完成了每个Query只需要执行一次的任务。
 * 该函数调用grouping_planner一次。在同一个Query上,每次递归grouping_planner都调用一次;
 * 当然,这不是通常的情况,但我们仍然保持这两个例程(subquery_planner和grouping_planner)之间的分离,
 * 以防有一天我们再次需要它。
 * 
 * subquery_planner will be called recursively to handle sub-Query nodes
 * found within the query's expressions and rangetable.
 * 函数subquery_planner将被递归调用,以处理表达式和RTE中的子查询节点。 
 *
 * Returns the PlannerInfo struct ("root") that contains all data generated
 * while planning the subquery.  In particular, the Path(s) attached to
 * the (UPPERREL_FINAL, NULL) upperrel represent our conclusions about the
 * cheapest way(s) to implement the query.  The top level will select the
 * best Path and pass it through createplan.c to produce a finished Plan.
 * 返回PlannerInfo struct(“root”),它包含在计划子查询时生成的所有数据。
 * 特别地,访问路径附加到(UPPERREL_FINAL, NULL) 上层关系中,以代表优化器已找到查询成本最低的方法.
 * 顶层将选择最佳路径并将其通过createplan.c传递以制定一个已完成的计划。
 *--------------------
 */
/*
输入:
    glob-PlannerGlobal
    parse-Query结构体指针
    parent_root-父PlannerInfo Root节点
    hasRecursion-是否递归?
    tuple_fraction-扫描Tuple比例
输出:
    PlannerInfo指针
*/
PlannerInfo *
subquery_planner(PlannerGlobal *glob, Query *parse,
                 PlannerInfo *parent_root,
                 bool hasRecursion, double tuple_fraction)
{
    PlannerInfo *root;//返回值
    List       *newWithCheckOptions;//
    List       *newHaving;//Having子句
    bool        hasOuterJoins;//是否存在Outer Join?
    RelOptInfo *final_rel;//
    ListCell   *l;//临时变量

    /* Create a PlannerInfo data structure for this subquery */
    //创建一个规划器数据结构:PlannerInfo
    root = makeNode(PlannerInfo);//构造返回值
    root->parse = parse;
    root->glob = glob;
    root->query_level = parent_root ? parent_root->query_level + 1 : 1;
    root->parent_root = parent_root;
    root->plan_params = NIL;
    root->outer_params = NULL;
    root->planner_cxt = CurrentMemoryContext;
    root->init_plans = NIL;
    root->cte_plan_ids = NIL;
    root->multiexpr_params = NIL;
    root->eq_classes = NIL;
    root->append_rel_list = NIL;
    root->rowMarks = NIL;
    memset(root->upper_rels, 0, sizeof(root->upper_rels));
    memset(root->upper_targets, 0, sizeof(root->upper_targets));
    root->processed_tlist = NIL;
    root->grouping_map = NULL;
    root->minmax_aggs = NIL;
    root->qual_security_level = 0;
    root->inhTargetKind = INHKIND_NONE;
    root->hasRecursion = hasRecursion;
    if (hasRecursion)
        root->wt_param_id = SS_assign_special_param(root);
    else
        root->wt_param_id = -1;
    root->non_recursive_path = NULL;
    root->partColsUpdated = false;

    /*
     * If there is a WITH list, process each WITH query and build an initplan
     * SubPlan structure for it.
     * 如果有一个WITH链表,使用查询处理每个链表,并为其构建一个initplan子计划结构。
     */
    if (parse->cteList)
        SS_process_ctes(root);//处理With 语句

    /*
     * Look for ANY and EXISTS SubLinks in WHERE and JOIN/ON clauses, and try
     * to transform them into joins.  Note that this step does not descend
     * into subqueries; if we pull up any subqueries below, their SubLinks are
     * processed just before pulling them up.
     * 查找WHERE和JOIN/ON子句中的ANY/EXISTS子句,并尝试将它们转换为JOIN。
     * 注意,此步骤不会下降为子查询;如果我们上拉子查询,它们的SubLinks将在调出它们上拉前被处理。
     */
    if (parse->hasSubLinks)
        pull_up_sublinks(root); //上拉子链接

    /*
     * Scan the rangetable for set-returning functions, and inline them if
     * possible (producing subqueries that might get pulled up next).
     * Recursion issues here are handled in the same way as for SubLinks.
     * 扫描RTE中的set-returning函数,
     * 如果可能,内联它们(生成下一个可能被上拉的子查询)。
     * 这里递归问题的处理方式与SubLinks相同。
     */
    inline_set_returning_functions(root);//

    /*
     * Check to see if any subqueries in the jointree can be merged into this
     * query.
     * 检查连接树中的子查询是否可以合并到该查询中(上拉子查询)
     */
    pull_up_subqueries(root);//上拉子查询

    /*
     * If this is a simple UNION ALL query, flatten it into an appendrel. We
     * do this now because it requires applying pull_up_subqueries to the leaf
     * queries of the UNION ALL, which weren't touched above because they
     * weren't referenced by the jointree (they will be after we do this).
     * 如果这是一个简单的UNION ALL查询,则将其ftatten为appendrel结构。
     * 我们现在这样做是因为它需要对UNION ALL的叶子查询应用pull_up_subqueries,
     * 上面没有涉及到这些查询,因为它们没有被jointree引用(在我们这样做之后它们将被引用)。
     */
    if (parse->setOperations)
        flatten_simple_union_all(root);//扁平化处理UNION ALL

    /*
     * Detect whether any rangetable entries are RTE_JOIN kind; if not, we can
     * avoid the expense of doing flatten_join_alias_vars().  Also check for
     * outer joins --- if none, we can skip reduce_outer_joins().  And check
     * for LATERAL RTEs, too.  This must be done after we have done
     * pull_up_subqueries(), of course.
     * 检测是否有任何RTE中的元素是RTE_JOIN类型;如果没有,可以避免执行refin_join_alias_vars()的开销。
     * 检查外部连接——如果没有,可以跳过reduce_outer_join()函数。同样的,我们会检查LATERAL RTEs。
     * 当然,这必须在我们完成pull_up_subqueries()调用之后完成。
     */
     //判断RTE中是否存在RTE_JOIN?
    root->hasJoinRTEs = false;
    root->hasLateralRTEs = false;
    hasOuterJoins = false;
    foreach(l, parse->rtable)
    {
        RangeTblEntry *rte = lfirst_node(RangeTblEntry, l);

        if (rte->rtekind == RTE_JOIN)
        {
            root->hasJoinRTEs = true;
            if (IS_OUTER_JOIN(rte->jointype))
                hasOuterJoins = true;
        }
        if (rte->lateral)
            root->hasLateralRTEs = true;
    }

    /*
     * Preprocess RowMark information.  We need to do this after subquery
     * pullup (so that all non-inherited RTEs are present) and before
     * inheritance expansion (so that the info is available for
     * expand_inherited_tables to examine and modify).
     * 预处理RowMark信息。
     * 我们需要在子查询上拉(以便所有非继承的RTEs都存在)和继承展开之后完成
     * (以便expand_inherited_tables可以使用这个信息来检查和修改)。
     */
     //预处理RowMark信息
    preprocess_rowmarks(root);

    /*
     * Expand any rangetable entries that are inheritance sets into "append
     * relations".  This can add entries to the rangetable, but they must be
     * plain base relations not joins, so it's OK (and marginally more
     * efficient) to do it after checking for join RTEs.  We must do it after
     * pulling up subqueries, else we'd fail to handle inherited tables in
     * subqueries.
     * 将继承集的任何可范围条目展开为“append relations”。
     * 将相关的relation添加到RTE中,但它们必须是纯基础关系而不是连接,
     * 因此在检查连接RTEs之后执行它是可以的(而且更有效)。
     * 我们必须在启动子查询后执行,否则我们将无法在子查询中处理继承表。
     */
     //展开继承表
    expand_inherited_tables(root);

    /*
     * Set hasHavingQual to remember if HAVING clause is present.  Needed
     * because preprocess_expression will reduce a constant-true condition to
     * an empty qual list ... but "HAVING TRUE" is not a semantic no-op.
     * 如果存在HAVING子句,则务必设置hasHavingQual属性。
     * 因为preprocess_expression将把constant-true条件减少为空的条件qual列表…
     * 但是,“HAVING TRUE”并没有语义错误。
     */
     //是否存在Having表达式
    root->hasHavingQual = (parse->havingQual != NULL);

    /* Clear this flag; might get set in distribute_qual_to_rels */
    //清除hasPseudoConstantQuals标记,该标记可能在distribute_qual_to_rels函数中设置
    root->hasPseudoConstantQuals = false;

    /*
     * Do expression preprocessing on targetlist and quals, as well as other
     * random expressions in the querytree.  Note that we do not need to
     * handle sort/group expressions explicitly, because they are actually
     * part of the targetlist.
     * 对targetlist和quals以及querytree中的其他随机表达式进行表达式预处理。
     * 注意,我们不需要显式地处理sort/group表达式,因为它们实际上是targetlist的一部分。
     */
     //预处理表达式:targetList(投影列)
    parse->targetList = (List *)
        preprocess_expression(root, (Node *) parse->targetList,
                              EXPRKIND_TARGET);

    /* Constant-folding might have removed all set-returning functions */
    //Constant-folding 可能已经把set-returning函数去掉
    if (parse->hasTargetSRFs)
        parse->hasTargetSRFs = expression_returns_set((Node *) parse->targetList);

    newWithCheckOptions = NIL;
    foreach(l, parse->withCheckOptions)//witch Check Options
    {
        WithCheckOption *wco = lfirst_node(WithCheckOption, l);

        wco->qual = preprocess_expression(root, wco->qual,
                                          EXPRKIND_QUAL);
        if (wco->qual != NULL)
            newWithCheckOptions = lappend(newWithCheckOptions, wco);
    }
    parse->withCheckOptions = newWithCheckOptions;
     //返回列信息returningList
    parse->returningList = (List *)
        preprocess_expression(root, (Node *) parse->returningList,
                              EXPRKIND_TARGET);
     //预处理条件表达式
    preprocess_qual_conditions(root, (Node *) parse->jointree);
     //预处理Having表达式
    parse->havingQual = preprocess_expression(root, parse->havingQual,
                                              EXPRKIND_QUAL);
     //窗口函数
    foreach(l, parse->windowClause)
    {
        WindowClause *wc = lfirst_node(WindowClause, l);

        /* partitionClause/orderClause are sort/group expressions */
        wc->startOffset = preprocess_expression(root, wc->startOffset,
                                                EXPRKIND_LIMIT);
        wc->endOffset = preprocess_expression(root, wc->endOffset,
                                              EXPRKIND_LIMIT);
    }
     //Limit子句
    parse->limitOffset = preprocess_expression(root, parse->limitOffset,
                                               EXPRKIND_LIMIT);
    parse->limitCount = preprocess_expression(root, parse->limitCount,
                                              EXPRKIND_LIMIT);
     //On Conflict子句
    if (parse->onConflict)
    {
        parse->onConflict->arbiterElems = (List *)
            preprocess_expression(root,
                                  (Node *) parse->onConflict->arbiterElems,
                                  EXPRKIND_ARBITER_ELEM);
        parse->onConflict->arbiterWhere =
            preprocess_expression(root,
                                  parse->onConflict->arbiterWhere,
                                  EXPRKIND_QUAL);
        parse->onConflict->onConflictSet = (List *)
            preprocess_expression(root,
                                  (Node *) parse->onConflict->onConflictSet,
                                  EXPRKIND_TARGET);
        parse->onConflict->onConflictWhere =
            preprocess_expression(root,
                                  parse->onConflict->onConflictWhere,
                                  EXPRKIND_QUAL);
        /* exclRelTlist contains only Vars, so no preprocessing needed */
    }
     //集合操作(AppendRelInfo)
    root->append_rel_list = (List *)
        preprocess_expression(root, (Node *) root->append_rel_list,
                              EXPRKIND_APPINFO);
     //RTE
    /* Also need to preprocess expressions within RTEs */
    foreach(l, parse->rtable)
    {
        RangeTblEntry *rte = lfirst_node(RangeTblEntry, l);
        int         kind;
        ListCell   *lcsq;

        if (rte->rtekind == RTE_RELATION)
        {
            if (rte->tablesample)
                rte->tablesample = (TableSampleClause *)
                    preprocess_expression(root,
                                          (Node *) rte->tablesample,
                                          EXPRKIND_TABLESAMPLE);//数据表采样语句
        }
        else if (rte->rtekind == RTE_SUBQUERY)//子查询
        {
            /*
             * We don't want to do all preprocessing yet on the subquery's
             * expressions, since that will happen when we plan it.  But if it
             * contains any join aliases of our level, those have to get
             * expanded now, because planning of the subquery won't do it.
             * That's only possible if the subquery is LATERAL.
             * 我们还不想对子查询的表达式进行预处理,因为这将在计划时发生。
             * 但是,如果它包含当前级别的任何连接别名,那么现在就必须扩展这些别名,
             * 因为子查询的计划无法做到这一点。只有在子查询是LATERAL的情况下才有可能。
             */
            if (rte->lateral && root->hasJoinRTEs)
                rte->subquery = (Query *)
                    flatten_join_alias_vars(root, (Node *) rte->subquery);
        }
        else if (rte->rtekind == RTE_FUNCTION)//函数
        {
            /* Preprocess the function expression(s) fully */
            //预处理函数表达式
            kind = rte->lateral ? EXPRKIND_RTFUNC_LATERAL : EXPRKIND_RTFUNC;
            rte->functions = (List *)
                preprocess_expression(root, (Node *) rte->functions, kind);
        }
        else if (rte->rtekind == RTE_TABLEFUNC)//TABLE FUNC
        {
            /* Preprocess the function expression(s) fully */
            kind = rte->lateral ? EXPRKIND_TABLEFUNC_LATERAL : EXPRKIND_TABLEFUNC;
            rte->tablefunc = (TableFunc *)
                preprocess_expression(root, (Node *) rte->tablefunc, kind);
        }
        else if (rte->rtekind == RTE_VALUES)//VALUES子句
        {
            /* Preprocess the values lists fully */
            kind = rte->lateral ? EXPRKIND_VALUES_LATERAL : EXPRKIND_VALUES;
            rte->values_lists = (List *)
                preprocess_expression(root, (Node *) rte->values_lists, kind);
        }

        /*
         * Process each element of the securityQuals list as if it were a
         * separate qual expression (as indeed it is).  We need to do it this
         * way to get proper canonicalization of AND/OR structure.  Note that
         * this converts each element into an implicit-AND sublist.
         * 处理securityQuals列表的每个元素,就好像它是一个单独的qual表达式(事实也是如此)。
         * 之所以这样做,是因为需要获得适当的规范化AND/OR结构。
         * 注意,这将把每个元素转换为隐含的子列表。
         */
        foreach(lcsq, rte->securityQuals)
        {
            lfirst(lcsq) = preprocess_expression(root,
                                                 (Node *) lfirst(lcsq),
                                                 EXPRKIND_QUAL);
        }
    }

    /*
     * Now that we are done preprocessing expressions, and in particular done
     * flattening join alias variables, get rid of the joinaliasvars lists.
     * They no longer match what expressions in the rest of the tree look
     * like, because we have not preprocessed expressions in those lists (and
     * do not want to; for example, expanding a SubLink there would result in
     * a useless unreferenced subplan).  Leaving them in place simply creates
     * a hazard for later scans of the tree.  We could try to prevent that by
     * using QTW_IGNORE_JOINALIASES in every tree scan done after this point,
     * but that doesn't sound very reliable.
     * 现在,已经完成了预处理表达式,特别是扁平化连接别名变量,现在可以去掉joinaliasvars链表了。
     * 它们不再匹配树中其他部分中的表达式,因为我们没有在那些链表中预处理表达式
     * (而且是不希望这样做,例如,在那里展开一个SubLink将导致无用的未引用的子计划)。
     * 把它们放在链表中只会给以后扫描树造成问题。
     * 我们可以在这之后的每一次树扫描中使用QTW_IGNORE_JOINALIASES来防止这种情况,虽然这听起来不太可靠。
     */
    if (root->hasJoinRTEs)
    {
        foreach(l, parse->rtable)
        {
            RangeTblEntry *rte = lfirst_node(RangeTblEntry, l);

            rte->joinaliasvars = NIL;
        }
    }

    /*
     * In some cases we may want to transfer a HAVING clause into WHERE. We
     * cannot do so if the HAVING clause contains aggregates (obviously) or
     * volatile functions (since a HAVING clause is supposed to be executed
     * only once per group).  We also can't do this if there are any nonempty
     * grouping sets; moving such a clause into WHERE would potentially change
     * the results, if any referenced column isn't present in all the grouping
     * sets.  (If there are only empty grouping sets, then the HAVING clause
     * must be degenerate as discussed below.)
     * 在某些情况下,我们可能想把“HAVING”条件转移到WHERE子句中。
     * 如果HAVING子句包含聚合(显式的)或易变volatile函数(因为每个GROUP只执行一次HAVING子句),就不能这样做。
     * 如果有任何非空GROUPING SET,也不能这样做;
     * 如果在所有GROUPING SET中没有出现任何引用列,将这样的子句移动到WHERE可能会改变结果。
     * (如果只有空的GROUP SET分组集,则可以按照下面讨论的那样简化HAVING子句->WHERE中。)
     *
     * Also, it may be that the clause is so expensive to execute that we're
     * better off doing it only once per group, despite the loss of
     * selectivity.  This is hard to estimate short of doing the entire
     * planning process twice, so we use a heuristic: clauses containing
     * subplans are left in HAVING.  Otherwise, we move or copy the HAVING
     * clause into WHERE, in hopes of eliminating tuples before aggregation
     * instead of after.
     * 而且,执行子句的成本非常高,所以最好每组只执行一次,尽管这样会导致选择性selectivity。
     * 如果不把整个规划过程重复一遍,这是很难估计的,因此我们使用启发式的方法:
     * 包含子计划的条款在HAVING的后面。
     * 否则,我们将把HAVING子句移动到WHERE中,希望在聚合之前而不是聚合之后消除元组。
     * 
     * If the query has explicit grouping then we can simply move such a
     * clause into WHERE; any group that fails the clause will not be in the
     * output because none of its tuples will reach the grouping or
     * aggregation stage.  Otherwise we must have a degenerate (variable-free)
     * HAVING clause, which we put in WHERE so that query_planner() can use it
     * in a gating Result node, but also keep in HAVING to ensure that we
     * don't emit a bogus aggregated row. (This could be done better, but it
     * seems not worth optimizing.)
     * 如果查询有显式分组,那么可以简单地将这样的子句移动到WHERE中;
     * 任何失败的GROUP子句都不会出现在输出中,因为它的元组不会到达分组或聚合阶段。
     * 否则,我们必须有一个退化的(无变量的)HAVING子句,把它放在WHERE中,
     * 以便query_planner()可以在一个控制结果节点中使用它,但同时还要确保不会发出一个伪造的聚合行。
     * (这本来可以做得更好,但似乎不值得继续深入优化。)
     *
     * Note that both havingQual and parse->jointree->quals are in
     * implicitly-ANDed-list form at this point, even though they are declared
     * as Node *.
     * 请注意,现在不管是qual还是parse->jointree->quals,即使它们被声明为节点 *,
     * 但它们在这个点上都是都是隐式的链表形式。
     */
    newHaving = NIL;
    foreach(l, (List *) parse->havingQual)
    {
        Node       *havingclause = (Node *) lfirst(l);

        if ((parse->groupClause && parse->groupingSets) ||
            contain_agg_clause(havingclause) ||
            contain_volatile_functions(havingclause) ||
            contain_subplans(havingclause))
        {
            /* keep it in HAVING */
            newHaving = lappend(newHaving, havingclause);
        }
        else if (parse->groupClause && !parse->groupingSets)
        {
            /* move it to WHERE */
            parse->jointree->quals = (Node *)
                lappend((List *) parse->jointree->quals, havingclause);
        }
        else
        {
            /* put a copy in WHERE, keep it in HAVING */
            parse->jointree->quals = (Node *)
                lappend((List *) parse->jointree->quals,
                        copyObject(havingclause));
            newHaving = lappend(newHaving, havingclause);
        }
    }
    parse->havingQual = (Node *) newHaving;

    /* Remove any redundant GROUP BY columns */
    //移除多余的GROUP BY 列
    remove_useless_groupby_columns(root);

    /*
     * If we have any outer joins, try to reduce them to plain inner joins.
     * This step is most easily done after we've done expression
     * preprocessing.
     * 如果存在外连接,则尝试将它们转换为普通的内部连接。
     * 在我们完成表达式预处理之后,这个步骤相对容易完成。
     */
    if (hasOuterJoins)
        reduce_outer_joins(root);

    /*
     * Do the main planning.  If we have an inherited target relation, that
     * needs special processing, else go straight to grouping_planner.
     * 执行主要的计划过程。
     * 如果存在继承的目标关系,则需要特殊处理,否则直接执行grouping_planner。
     */
    if (parse->resultRelation &&
        rt_fetch(parse->resultRelation, parse->rtable)->inh)
        inheritance_planner(root);
    else
        grouping_planner(root, false, tuple_fraction);

    /*
     * Capture the set of outer-level param IDs we have access to, for use in
     * extParam/allParam calculations later.
     * 获取我们可以访问的outer-level的参数IDs,以便稍后在extParam/allParam计算中使用。
     */
    SS_identify_outer_params(root);

    /*
     * If any initPlans were created in this query level, adjust the surviving
     * Paths' costs and parallel-safety flags to account for them.  The
     * initPlans won't actually get attached to the plan tree till
     * create_plan() runs, but we must include their effects now.
     * 如果在此查询级别中创建了initplan,则调整现存的访问路径成本和并行安全标志,以反映这些成本。
     * 在create_plan()运行之前,initPlans实际上不会被附加到计划树中,但是我们现在必须包含它们的效果。
     */
    final_rel = fetch_upper_rel(root, UPPERREL_FINAL, NULL);
    SS_charge_for_initplans(root, final_rel);

    /*
     * Make sure we've identified the cheapest Path for the final rel.  (By
     * doing this here not in grouping_planner, we include initPlan costs in
     * the decision, though it's unlikely that will change anything.)
     * 确保我们已经为最终的关系确定了成本最低的路径
     * (我们没有在grouping_planner中这样做,而是在最终决定中加入了initPlan的成本,尽管这不太可能改变任何事情)。
     */
    set_cheapest(final_rel);

    return root;
}
 

二、参考资料

allpaths.c
PG Document:Query Planning

来自 “ ITPUB博客 ” ,链接:http://blog.itpub.net/6906/viewspace-2374817/,如需转载,请注明出处,否则将追究法律责任。

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