先前的章節已介紹了函數query_planner中子函數reconsider_outer_join_clauses和generate_base_implied_equalities的主要實現邏輯,本節繼續介紹query_planner中qp_callback（回調函數）、fix_placeholder_input_needed_levels函數的實現邏輯。

query_planner代碼片段:

     //...
     /*
      * We have completed merging equivalence sets, so it's now possible to
      * generate pathkeys in canonical form; so compute query_pathkeys and
      * other pathkeys fields in PlannerInfo.
      */
     (*qp_callback) (root, qp_extra);//調用回調函數,處理PathKeys
 
     /*
      * Examine any "placeholder" expressions generated during subquery pullup.
      * Make sure that the Vars they need are marked as needed at the relevant
      * join level.  This must be done before join removal because it might
      * cause Vars or placeholders to be needed above a join when they weren't
      * so marked before.
      */
     fix_placeholder_input_needed_levels(root);//檢查在子查詢上拉時生成的PH表達式,確保Vars是OK的
 

     //...

一、數據結構

PlannerInfo與RelOptInfo結構體貫徹邏輯優化和物理優化過程的始終.
PlannerInfo

 /*----------
  * PlannerInfo
  *      Per-query information for planning/optimization
  *
  * This struct is conventionally called "root" in all the planner routines.
  * It holds links to all of the planner's working state, in addition to the
  * original Query.  Note that at present the planner extensively modifies
  * the passed-in Query data structure; someday that should stop.
  *----------
  */
 struct AppendRelInfo;
 
 typedef struct PlannerInfo
 {
     NodeTag     type;//Node標識
 
     Query      *parse;          /* 查詢樹,the Query being planned */
 
     PlannerGlobal *glob;        /* 當前的planner全局信息,global info for current planner run */
 
     Index       query_level;    /* 查詢層次,1標識最高層,1 at the outermost Query */
 
     struct PlannerInfo *parent_root;    /* 如為子計劃,則這里存儲父計劃器指針,NULL標識最高層,NULL at outermost Query */
 
     /*
      * plan_params contains the expressions that this query level needs to
      * make available to a lower query level that is currently being planned.
      * outer_params contains the paramIds of PARAM_EXEC Params that outer
      * query levels will make available to this query level.
      */
     List       *plan_params;    /* list of PlannerParamItems, see below */
     Bitmapset  *outer_params;
 
     /*
      * simple_rel_array holds pointers to "base rels" and "other rels" (see
      * comments for RelOptInfo for more info).  It is indexed by rangetable
      * index (so entry 0 is always wasted).  Entries can be NULL when an RTE
      * does not correspond to a base relation, such as a join RTE or an
      * unreferenced view RTE; or if the RelOptInfo hasn't been made yet.
      */
     /* RelOptInfo數組,存儲"base rels",比如基表/子查詢等.該數組與RTE的順序一一對應,而且是從1開始,因此[0]無用 */
     struct RelOptInfo **simple_rel_array;   /* All 1-rel RelOptInfos */
     int         simple_rel_array_size;  /* 數組大小,allocated size of array */
 
     /*
      * simple_rte_array is the same length as simple_rel_array and holds
      * pointers to the associated rangetable entries.  This lets us avoid
      * rt_fetch(), which can be a bit slow once large inheritance sets have
      * been expanded.
      */
     RangeTblEntry **simple_rte_array;   /* RTE數組,rangetable as an array */
 
     /*
      * append_rel_array is the same length as the above arrays, and holds
      * pointers to the corresponding AppendRelInfo entry indexed by
      * child_relid, or NULL if none.  The array itself is not allocated if
      * append_rel_list is empty.
      */
     struct AppendRelInfo **append_rel_array;//先前已介紹,在處理集合操作如UNION ALL時使用
 
     /*
      * all_baserels is a Relids set of all base relids (but not "other"
      * relids) in the query; that is, the Relids identifier of the final join
      * we need to form.  This is computed in make_one_rel, just before we
      * start making Paths.
      */
     Relids      all_baserels;//"base rels"
 
     /*
      * nullable_baserels is a Relids set of base relids that are nullable by
      * some outer join in the jointree; these are rels that are potentially
      * nullable below the WHERE clause, SELECT targetlist, etc.  This is
      * computed in deconstruct_jointree.
      */
     Relids      nullable_baserels;//Nullable-side端的"base rels"
 
     /*
      * join_rel_list is a list of all join-relation RelOptInfos we have
      * considered in this planning run.  For small problems we just scan the
      * list to do lookups, but when there are many join relations we build a
      * hash table for faster lookups.  The hash table is present and valid
      * when join_rel_hash is not NULL.  Note that we still maintain the list
      * even when using the hash table for lookups; this simplifies life for
      * GEQO.
      */
     List       *join_rel_list;  /* 參與連接的Relation的RelOptInfo鏈表,list of join-relation RelOptInfos */
     struct HTAB *join_rel_hash; /* 可加快鏈表訪問的hash表,optional hashtable for join relations */
 
     /*
      * When doing a dynamic-programming-style join search, join_rel_level[k]
      * is a list of all join-relation RelOptInfos of level k, and
      * join_cur_level is the current level.  New join-relation RelOptInfos are
      * automatically added to the join_rel_level[join_cur_level] list.
      * join_rel_level is NULL if not in use.
      */
     List      **join_rel_level; /* RelOptInfo指針鏈表數組,k層的join存儲在[k]中,lists of join-relation RelOptInfos */
     int         join_cur_level; /* 當前的join層次,index of list being extended */
 
     List       *init_plans;     /* 查詢的初始化計劃鏈表,init SubPlans for query */
 
     List       *cte_plan_ids;   /* CTE子計劃ID鏈表,per-CTE-item list of subplan IDs */
 
     List       *multiexpr_params;   /* List of Lists of Params for MULTIEXPR
                                      * subquery outputs */
 
     List       *eq_classes;     /* 活動的等價類鏈表,list of active EquivalenceClasses */
 
     List       *canon_pathkeys; /* 規范化PathKey鏈表,list of "canonical" PathKeys */
 
     List       *left_join_clauses;  /* 外連接約束條件鏈表(左),list of RestrictInfos for mergejoinable
                                      * outer join clauses w/nonnullable var on
                                      * left */
 
     List       *right_join_clauses; /* 外連接約束條件鏈表(右),list of RestrictInfos for mergejoinable
                                      * outer join clauses w/nonnullable var on
                                      * right */
 
     List       *full_join_clauses;  /* 全連接約束條件鏈表,list of RestrictInfos for mergejoinable
                                      * full join clauses */
 
     List       *join_info_list; /* 特殊連接信息鏈表,list of SpecialJoinInfos */
 
     List       *append_rel_list;    /* AppendRelInfo鏈表,list of AppendRelInfos */
 
     List       *rowMarks;       /* list of PlanRowMarks */
 
     List       *placeholder_list;   /* PHI鏈表,list of PlaceHolderInfos */
 
     List       *fkey_list;      /* 外鍵信息鏈表,list of ForeignKeyOptInfos */
 
     List       *query_pathkeys; /* uery_planner()要求的PathKeys,desired pathkeys for query_planner() */
 
     List       *group_pathkeys; /* groupClause pathkeys, if any */
     List       *window_pathkeys;    /* pathkeys of bottom window, if any */
     List       *distinct_pathkeys;  /* distinctClause pathkeys, if any */
     List       *sort_pathkeys;  /* sortClause pathkeys, if any */
 
     List       *part_schemes;   /* 已規范化的分區Schema,Canonicalised partition schemes used in the
                                  * query. */
 
     List       *initial_rels;   /* 嘗試連接的RelOptInfo鏈表,RelOptInfos we are now trying to join */
 
     /* Use fetch_upper_rel() to get any particular upper rel */
     List       *upper_rels[UPPERREL_FINAL + 1]; /* 上層的RelOptInfo鏈表, upper-rel RelOptInfos */
 
     /* Result tlists chosen by grouping_planner for upper-stage processing */
     struct PathTarget *upper_targets[UPPERREL_FINAL + 1];//
 
     /*
      * grouping_planner passes back its final processed targetlist here, for
      * use in relabeling the topmost tlist of the finished Plan.
      */
     List       *processed_tlist;//最后需處理的投影列
 
     /* Fields filled during create_plan() for use in setrefs.c */
     AttrNumber *grouping_map;   /* for GroupingFunc fixup */
     List       *minmax_aggs;    /* List of MinMaxAggInfos */
 
     MemoryContext planner_cxt;  /* 內存上下文,context holding PlannerInfo */
 
     double      total_table_pages;  /* 所有的pages,# of pages in all tables of query */
 
     double      tuple_fraction; /* query_planner輸入參數:元組處理比例,tuple_fraction passed to query_planner */
     double      limit_tuples;   /* query_planner輸入參數:limit_tuples passed to query_planner */
 
     Index       qual_security_level;    /* 表達式的最新安全等級,minimum security_level for quals */
     /* Note: qual_security_level is zero if there are no securityQuals */
 
     InheritanceKind inhTargetKind;  /* indicates if the target relation is an
                                      * inheritance child or partition or a
                                      * partitioned table */
     bool        hasJoinRTEs;    /* 存在RTE_JOIN的RTE,true if any RTEs are RTE_JOIN kind */
     bool        hasLateralRTEs; /* 存在標記為LATERAL的RTE,true if any RTEs are marked LATERAL */
     bool        hasDeletedRTEs; /* 存在已在jointree刪除的RTE,true if any RTE was deleted from jointree */
     bool        hasHavingQual;  /* 存在Having子句,true if havingQual was non-null */
     bool        hasPseudoConstantQuals; /* true if any RestrictInfo has
                                          * pseudoconstant = true */
     bool        hasRecursion;   /* 遞歸語句,true if planning a recursive WITH item */
 
     /* These fields are used only when hasRecursion is true: */
     int         wt_param_id;    /* PARAM_EXEC ID for the work table */
     struct Path *non_recursive_path;    /* a path for non-recursive term */
 
     /* These fields are workspace for createplan.c */
     Relids      curOuterRels;   /* outer rels above current node */
     List       *curOuterParams; /* not-yet-assigned NestLoopParams */
 
     /* optional private data for join_search_hook, e.g., GEQO */
     void       *join_search_private;
 
     /* Does this query modify any partition key columns? */
     bool        partColsUpdated;
 } PlannerInfo;
 

RelOptInfo

 typedef struct RelOptInfo
 {
     NodeTag     type;//節點標識
 
     RelOptKind  reloptkind;//RelOpt類型
 
     /* all relations included in this RelOptInfo */
     Relids      relids;         /*Relids(rtindex)集合 set of base relids (rangetable indexes) */
 
     /* size estimates generated by planner */
     double      rows;           /*結果元組的估算數量 estimated number of result tuples */
 
     /* per-relation planner control flags */
     bool        consider_startup;   /*是否考慮啟動成本?是,需要保留啟動成本低的路徑 keep cheap-startup-cost paths? */
     bool        consider_param_startup; /*是否考慮參數化?的路徑 ditto, for parameterized paths? */
     bool        consider_parallel;  /*是否考慮并行處理路徑 consider parallel paths? */
 
     /* default result targetlist for Paths scanning this relation */
     struct PathTarget *reltarget;   /*掃描該Relation時默認的結果 list of Vars/Exprs, cost, width */
 
     /* materialization information */
     List       *pathlist;       /*訪問路徑鏈表 Path structures */
     List       *ppilist;        /*路徑鏈表中使用參數化路徑進行 ParamPathInfos used in pathlist */
     List       *partial_pathlist;   /* partial Paths */
     struct Path *cheapest_startup_path;//代價最低的啟動路徑
     struct Path *cheapest_total_path;//代價最低的整體路徑
     struct Path *cheapest_unique_path;//代價最低的獲取唯一值的路徑
     List       *cheapest_parameterized_paths;//代價最低的參數化?路徑鏈表
 
     /* parameterization information needed for both base rels and join rels */
     /* (see also lateral_vars and lateral_referencers) */
     Relids      direct_lateral_relids;  /*使用lateral語法,需依賴的Relids rels directly laterally referenced */
     Relids      lateral_relids; /* minimum parameterization of rel */
 
     /* information about a base rel (not set for join rels!) */
     //reloptkind=RELOPT_BASEREL時使用的數據結構
     Index       relid;          /* Relation ID */
     Oid         reltablespace;  /* 表空間 containing tablespace */
     RTEKind     rtekind;        /* 基表?子查詢?還是函數等等?RELATION, SUBQUERY, FUNCTION, etc */
     AttrNumber  min_attr;       /* 最小的屬性編號 smallest attrno of rel (often <0) */
     AttrNumber  max_attr;       /* 最大的屬性編號 largest attrno of rel */
     Relids     *attr_needed;    /* 數組 array indexed [min_attr .. max_attr] */
     int32      *attr_widths;    /* 屬性寬度 array indexed [min_attr .. max_attr] */
     List       *lateral_vars;   /* 關系依賴的Vars/PHVs LATERAL Vars and PHVs referenced by rel */
     Relids      lateral_referencers;    /*依賴該關系的Relids rels that reference me laterally */
     List       *indexlist;      /* 該關系的IndexOptInfo鏈表 list of IndexOptInfo */
     List       *statlist;       /* 統計信息鏈表 list of StatisticExtInfo */
     BlockNumber pages;          /* 塊數 size estimates derived from pg_class */
     double      tuples;         /* 元組數 */
     double      allvisfrac;     /* ? */
     PlannerInfo *subroot;       /* 如為子查詢,存儲子查詢的root if subquery */
     List       *subplan_params; /* 如為子查詢,存儲子查詢的參數 if subquery */
     int         rel_parallel_workers;   /* 并行執行,需要多少個workers? wanted number of parallel workers */
 
     /* Information about foreign tables and foreign joins */
     //FWD相關信息
     Oid         serverid;       /* identifies server for the table or join */
     Oid         userid;         /* identifies user to check access as */
     bool        useridiscurrent;    /* join is only valid for current user */
     /* use "struct FdwRoutine" to avoid including fdwapi.h here */
     struct FdwRoutine *fdwroutine;
     void       *fdw_private;
 
     /* cache space for remembering if we have proven this relation unique */
     //已知的,可保證唯一的Relids鏈表
     List       *unique_for_rels;    /* known unique for these other relid
                                      * set(s) */
     List       *non_unique_for_rels;    /* 已知的,不唯一的Relids鏈表 known not unique for these set(s) */
 
     /* used by various scans and joins: */
     List       *baserestrictinfo;   /* 如為基本關系,存儲約束條件 RestrictInfo structures (if base rel) */
     QualCost    baserestrictcost;   /* 解析約束表達式的成本? cost of evaluating the above */
     Index       baserestrict_min_security;  /* 最低安全等級 min security_level found in
                                              * baserestrictinfo */
     List       *joininfo;       /* 連接語句的約束條件信息 RestrictInfo structures for join clauses
                                  * involving this rel */
     bool        has_eclass_joins;   /* 是否存在等價類連接? T means joininfo is incomplete */
 
     /* used by partitionwise joins: */
     bool        consider_partitionwise_join;    /* 分區? consider partitionwise
                                                  * join paths? (if
                                                  * partitioned rel) */
     Relids      top_parent_relids;  /* Relids of topmost parents (if "other"
                                      * rel) */
 
     /* used for partitioned relations */
     //分區表使用
     PartitionScheme part_scheme;    /* 分區的schema Partitioning scheme. */
     int         nparts;         /* 分區數 number of partitions */
     struct PartitionBoundInfoData *boundinfo;   /* 分區邊界信息 Partition bounds */
     List       *partition_qual; /* 分區約束 partition constraint */
     struct RelOptInfo **part_rels;  /* 分區的RelOptInfo數組 Array of RelOptInfos of partitions,
                                      * stored in the same order of bounds */
     List      **partexprs;      /* 非空分區鍵表達式 Non-nullable partition key expressions. */
     List      **nullable_partexprs; /* 可為空的分區鍵表達式 Nullable partition key expressions. */
     List       *partitioned_child_rels; /* RT Indexes鏈表 List of RT indexes. */
 } RelOptInfo;

PlaceHolderInfo

/*
  * For each distinct placeholder expression generated during planning, we
  * store a PlaceHolderInfo node in the PlannerInfo node's placeholder_list.
  * This stores info that is needed centrally rather than in each copy of the
  * PlaceHolderVar.  The phid fields identify which PlaceHolderInfo goes with
  * each PlaceHolderVar.  Note that phid is unique throughout a planner run,
  * not just within a query level --- this is so that we need not reassign ID's
  * when pulling a subquery into its parent.
  *
  * The idea is to evaluate the expression at (only) the ph_eval_at join level,
  * then allow it to bubble up like a Var until the ph_needed join level.
  * ph_needed has the same definition as attr_needed for a regular Var.
  *
  * The PlaceHolderVar's expression might contain LATERAL references to vars
  * coming from outside its syntactic scope.  If so, those rels are *not*
  * included in ph_eval_at, but they are recorded in ph_lateral.
  *
  * Notice that when ph_eval_at is a join rather than a single baserel, the
  * PlaceHolderInfo may create constraints on join order: the ph_eval_at join
  * has to be formed below any outer joins that should null the PlaceHolderVar.
  *
  * We create a PlaceHolderInfo only after determining that the PlaceHolderVar
  * is actually referenced in the plan tree, so that unreferenced placeholders
  * don't result in unnecessary constraints on join order.
  */
 
 typedef struct PlaceHolderInfo
 {
     NodeTag     type;
 
     Index       phid;           /* PH的ID,ID for PH (unique within planner run) */
     PlaceHolderVar *ph_var;     /* copy of PlaceHolderVar tree */
     Relids      ph_eval_at;     /* lowest level we can evaluate value at */
     Relids      ph_lateral;     /* relids of contained lateral refs, if any */
     Relids      ph_needed;      /* highest level the value is needed at */
     int32       ph_width;       /* estimated attribute width */
 } PlaceHolderInfo;

二、源碼解讀

standard_qp_callback
標準的query_planner回調函數,在生成計劃的期間處理query_pathkeys和其他pathkeys

 /*
  * Compute query_pathkeys and other pathkeys during plan generation
  */
 static void
 standard_qp_callback(PlannerInfo *root, void *extra)
 {
     Query      *parse = root->parse;//查詢樹
     standard_qp_extra *qp_extra = (standard_qp_extra *) extra;//參數
     List       *tlist = qp_extra->tlist;
     List       *activeWindows = qp_extra->activeWindows;
 
     /*
      * Calculate pathkeys that represent grouping/ordering requirements.  The
      * sortClause is certainly sort-able, but GROUP BY and DISTINCT might not
      * be, in which case we just leave their pathkeys empty.
      */
     if (qp_extra->groupClause &&
         grouping_is_sortable(qp_extra->groupClause))//group語句&要求排序
         root->group_pathkeys =
             make_pathkeys_for_sortclauses(root,
                                           qp_extra->groupClause,
                                           tlist);//構建pathkeys
     else
         root->group_pathkeys = NIL;
 
     /* We consider only the first (bottom) window in pathkeys logic */
     if (activeWindows != NIL)//窗口函數
     {
         WindowClause *wc = linitial_node(WindowClause, activeWindows);
 
         root->window_pathkeys = make_pathkeys_for_window(root,
                                                          wc,
                                                          tlist);
     }
     else
         root->window_pathkeys = NIL;
 
     if (parse->distinctClause &&
         grouping_is_sortable(parse->distinctClause))//存在distinct語句&按相關字段排序
         root->distinct_pathkeys =
             make_pathkeys_for_sortclauses(root,
                                           parse->distinctClause,
                                           tlist);//構建pathkeys
     else
         root->distinct_pathkeys = NIL;
 
     root->sort_pathkeys =
         make_pathkeys_for_sortclauses(root,
                                       parse->sortClause,
                                       tlist);//構建常規的排序pathkeys
 
     /*
      * Figure out whether we want a sorted result from query_planner.
      *
      * If we have a sortable GROUP BY clause, then we want a result sorted
      * properly for grouping.  Otherwise, if we have window functions to
      * evaluate, we try to sort for the first window.  Otherwise, if there's a
      * sortable DISTINCT clause that's more rigorous than the ORDER BY clause,
      * we try to produce output that's sufficiently well sorted for the
      * DISTINCT.  Otherwise, if there is an ORDER BY clause, we want to sort
      * by the ORDER BY clause.
      *
      * Note: if we have both ORDER BY and GROUP BY, and ORDER BY is a superset
      * of GROUP BY, it would be tempting to request sort by ORDER BY --- but
      * that might just leave us failing to exploit an available sort order at
      * all.  Needs more thought.  The choice for DISTINCT versus ORDER BY is
      * much easier, since we know that the parser ensured that one is a
      * superset of the other.
      */
     if (root->groupremove_useless_joins_pathkeys)
         root->query_pathkeys = root->group_pathkeys;
     else if (root->window_pathkeys)
         root->query_pathkeys = root->window_pathkeys;
     else if (list_length(root->distinct_pathkeys) >
              list_length(root->sort_pathkeys))
         root->query_pathkeys = root->distinct_pathkeys;
     else if (root->sort_pathkeys)
         root->query_pathkeys = root->sort_pathkeys;
     else
         root->query_pathkeys = NIL;
 }

/*
  * make_pathkeys_for_sortclauses
  *      Generate a pathkeys list that represents the sort order specified
  *      by a list of SortGroupClauses
  *
  * The resulting PathKeys are always in canonical form.  (Actually, there
  * is no longer any code anywhere that creates non-canonical PathKeys.)
  *
  * We assume that root->nullable_baserels is the set of base relids that could
  * have gone to NULL below the SortGroupClause expressions.  This is okay if
  * the expressions came from the query's top level (ORDER BY, DISTINCT, etc)
  * and if this function is only invoked after deconstruct_jointree.  In the
  * future we might have to make callers pass in the appropriate
  * nullable-relids set, but for now it seems unnecessary.
  *
  * 'sortclauses' is a list of SortGroupClause nodes
  * 'tlist' is the targetlist to find the referenced tlist entries in
  */
 List *
 make_pathkeys_for_sortclauses(PlannerInfo *root,
                               List *sortclauses,
                               List *tlist)
 {
     List       *pathkeys = NIL;
     ListCell   *l;
 
     foreach(l, sortclauses)
     {
         SortGroupClause *sortcl = (SortGroupClause *) lfirst(l);
         Expr       *sortkey;
         PathKey    *pathkey;
 
         sortkey = (Expr *) get_sortgroupclause_expr(sortcl, tlist);
         Assert(OidIsValid(sortcl->sortop));
         pathkey = make_pathkey_from_sortop(root,
                                            sortkey,
                                            root->nullable_baserels,
                                            sortcl->sortop,
                                            sortcl->nulls_first,
                                            sortcl->tleSortGroupRef,
                                            true);
 
         /* Canonical form eliminates redundant ordering keys */
         if (!pathkey_is_redundant(pathkey, pathkeys))//不是多余的Key的情況下,才保留
             pathkeys = lappend(pathkeys, pathkey);
     }
     return pathkeys;
 }

測試腳本:

testdb=# select t1.dwbh,t2.grbh
from t_dwxx t1 left join t_grxx t2 on t1.dwbh = t2.dwbh
where t1.dwbh = '1001'
order by t1.dwbh;

跟蹤分析,進入make_pathkeys_for_sortclauses函數:

...
(gdb) step
make_pathkeys_for_sortclauses (root=0x1702958, sortclauses=0x170d068, tlist=0x1746758) at pathkeys.c:878
878   List     *pathkeys = NIL;
(gdb) p *(SortGroupClause *)sortclauses->head->data.ptr_value
$5 = {type = T_SortGroupClause, tleSortGroupRef = 1, eqop = 98, sortop = 664, nulls_first = false, hashable = true}

...
(gdb) n
889     pathkey = make_pathkey_from_sortop(root,
(gdb) 
898     if (!pathkey_is_redundant(pathkey, pathkeys))
(gdb) p *pathkey
$11 = {type = T_PathKey, pk_eclass = 0x17486c0, pk_opfamily = 1994, pk_strategy = 1, pk_nulls_first = false}

函數pathkey_is_redundant通過等價類判斷排序是否多余(redundant),在本例中,已存在限制條件dwbh='1001',因此該排序是多余的,返回NULL.

901   return pathkeys;
(gdb) p *pathkeys
Cannot access memory at address 0x0

fix_placeholder_input_needed_levels

 /*
  * fix_placeholder_input_needed_levels
  *      Adjust the "needed at" levels for placeholder inputs
  *
  * This is called after we've finished determining the eval_at levels for
  * all placeholders.  We need to make sure that all vars and placeholders
  * needed to evaluate each placeholder will be available at the scan or join
  * level where the evaluation will be done.  (It might seem that scan-level
  * evaluations aren't interesting, but that's not so: a LATERAL reference
  * within a placeholder's expression needs to cause the referenced var or
  * placeholder to be marked as needed in the scan where it's evaluated.)
  * Note that this loop can have side-effects on the ph_needed sets of other
  * PlaceHolderInfos; that's okay because we don't examine ph_needed here, so
  * there are no ordering issues to worry about.
  */
 void
 fix_placeholder_input_needed_levels(PlannerInfo *root)
 {
     ListCell   *lc;
 
     foreach(lc, root->placeholder_list)//遍歷鏈表
     {
         PlaceHolderInfo *phinfo = (PlaceHolderInfo *) lfirst(lc);
         List       *vars = pull_var_clause((Node *) phinfo->ph_var->phexpr,
                                            PVC_RECURSE_AGGREGATES |
                                            PVC_RECURSE_WINDOWFUNCS |
                                            PVC_INCLUDE_PLACEHOLDERS);//獲取Vars
 
         add_vars_to_targetlist(root, vars, phinfo->ph_eval_at, false);//添加到投影列中
         list_free(vars);
     }
 }

考察下面的SQL語句:

testdb=# explain verbose select t1.dwbh,t2.grbh,t2.constant_field
from t_dwxx t1 left join (select a.dwbh,a.grbh,'TEST' as constant_field from t_grxx a) t2 on t1.dwbh = t2.dwbh  
where t1.dwbh = '1001'
order by t1.dwbh;
                               QUERY PLAN                               
------------------------------------------------------------------------
 Nested Loop Left Join  (cost=0.00..16.06 rows=2 width=108)
   Output: t1.dwbh, a.grbh, ('TEST'::text) -- PlaceHolderVar
   Join Filter: ((t1.dwbh)::text = (a.dwbh)::text)
   ->  Seq Scan on public.t_dwxx t1  (cost=0.00..1.04 rows=1 width=38)
         Output: t1.dwmc, t1.dwbh, t1.dwdz
         Filter: ((t1.dwbh)::text = '1001'::text)
   ->  Seq Scan on public.t_grxx a  (cost=0.00..15.00 rows=2 width=108)
         Output: a.grbh, a.dwbh, 'TEST'::text
         Filter: ((a.dwbh)::text = '1001'::text)
(9 rows)

子查詢上拉與t_dwxx進行連接,上拉過程中,不能簡單的把子查詢中的"'TEST' as constant_field"作為上層查詢的Var來看待(如果不作特殊處理,跟外連接就不等價了,因為該值有可能是NULL),PG因此引入了PlaceHolderVar這么一個Var來對這種變量進行特殊處理.

跟蹤分析:

(gdb) b planmain.c:161
Breakpoint 1 at 0x769602: file planmain.c, line 161.
(gdb) c
Continuing.

Breakpoint 1, query_planner (root=0x1702b08, tlist=0x1749c20, qp_callback=0x76e97d <standard_qp_callback>, 
    qp_extra=0x7ffd35e059c0) at planmain.c:163
163   reconsider_outer_join_clauses(root);

注意root中的placeholder_list

(gdb) p *root
$1 = {type = T_PlannerInfo, ..., placeholder_list = 0x174bf00, ...}

查看其內存結構:

#1個PHV
(gdb) p *root->placeholder_list
$2 = {type = T_List, length = 1, head = 0x174bee0, tail = 0x174bee0}
(gdb) p *(Node *)root->placeholder_list->head->data.ptr_value
$3 = {type = T_PlaceHolderInfo}
(gdb) p *(PlaceHolderInfo *)root->placeholder_list->head->data.ptr_value
$4 = {type = T_PlaceHolderInfo, phid = 1, ph_var = 0x174be18, ph_eval_at = 0x174bec8, ph_lateral = 0x0, 
  ph_needed = 0x174bf30, ph_width = 32}
(gdb) set $phi=(PlaceHolderInfo *)root->placeholder_list->head->data.ptr_value
(gdb) p *$phi->ph_var
$5 = {xpr = {type = T_PlaceHolderVar}, phexpr = 0x174be48, phrels = 0x174beb0, phid = 1, phlevelsup = 0}
#該PHV位于編號為4的RTE中
(gdb) p *$phi->ph_eval_at
$6 = {nwords = 1, words = 0x174becc}
(gdb) p *$phi->ph_eval_at->words
$7 = 16
(gdb) p *$phi->ph_needed
$8 = {nwords = 1, words = 0x174bf34}
#該PHV在編號為1的RTE中需要用到
(gdb) p *$phi->ph_needed->words
$9 = 1

三、參考資料

planmain.c
what exactly is a PlaceHolderVAr