PostgreSQL 原始碼解讀（46）- 查詢語句#31（query_planner函式#7）
摘要：
先前的章節已介紹了函式query_planner中子函式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（回撥函式）、fix_place...
先前的章節已介紹了函式query_planner中子函式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（回撥函式）、fix_placeholder_input_needed_levels函式的實現邏輯。
query_planner程式碼片段:
//... /* * 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);//呼叫回撥函式,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表示式,確保Vars是OK的 //...
一、資料結構
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
{
NodeTagtype;//Node標識
Query*parse;/* 查詢樹,the Query being planned */
PlannerGlobal *glob;/* 當前的planner全域性資訊,global info for current planner run */
Indexquery_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,則這裡儲存父計劃器指標,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陣列,儲存"base rels",比如基表/子查詢等.該陣列與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 */
intsimple_rel_array_size;/* 陣列大小,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陣列,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UIO/">NION 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.
*/
Relidsall_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.
*/
Relidsnullable_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;/* 參與連線的Relation的RelOptInfo連結串列,list of join-relation RelOptInfos */
struct HTAB *join_rel_hash; /* 可加快連結串列訪問的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指標連結串列陣列,k層的join儲存在[k]中,lists of join-relation RelOptInfos */
intjoin_cur_level; /* 當前的join層次,index of list being extended */
List*init_plans;/* 查詢的初始化計劃連結串列,init SubPlans for query */
List*cte_plan_ids;/* CTE子計劃ID連結串列,per-CTE-item list of subplan IDs */
List*multiexpr_params;/* List of Lists of Params for MULTIEXPR
* subquery outputs */
List*eq_classes;/* 活動的等價類連結串列,list of active EquivalenceClasses */
List*canon_pathkeys; /* 規範化PathKey連結串列,list of "canonical" PathKeys */
List*left_join_clauses;/* 外連線約束條件連結串列(左),list of RestrictInfos for mergejoinable
* outer join clauses w/nonnullable var on
* left */
List*right_join_clauses; /* 外連線約束條件連結串列(右),list of RestrictInfos for mergejoinable
* outer join clauses w/nonnullable var on
* right */
List*full_join_clauses;/* 全連線約束條件連結串列,list of RestrictInfos for mergejoinable
* full join clauses */
List*join_info_list; /* 特殊連線資訊連結串列,list of SpecialJoinInfos */
List*append_rel_list;/* AppendRelInfo連結串列,list of AppendRelInfos */
List*rowMarks;/* list of PlanRowMarks */
List*placeholder_list;/* PHI連結串列,list of PlaceHolderInfos */
List*fkey_list;/* 外來鍵資訊連結串列,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;/* 已規範化的分割槽Schema,Canonicalised partition schemes used in the
* query. */
List*initial_rels;/* 嘗試連線的RelOptInfo連結串列,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連結串列, 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;//最後需處理的投影列
/* 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;/* 記憶體上下文,context holding PlannerInfo */
doubletotal_table_pages;/* 所有的pages,# of pages in all tables of query */
doubletuple_fraction; /* query_planner輸入引數:元組處理比例,tuple_fraction passed to query_planner */
doublelimit_tuples;/* query_planner輸入引數:limit_tuples passed to query_planner */
Indexqual_security_level;/* 表示式的最新安全等級,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 */
boolhasJoinRTEs;/* 存在RTE_JOIN的RTE,true if any RTEs are RTE_JOIN kind */
boolhasLateralRTEs; /* 存在標記為LATERAL的RTE,true if any RTEs are marked LATERAL */
boolhasDeletedRTEs; /* 存在已在jointree刪除的RTE,true if any RTE was deleted from jointree */
boolhasHavingQual;/* 存在Having子句,true if havingQual was non-null */
boolhasPseudoConstantQuals; /* true if any RestrictInfo has
* pseudoconstant = true */
boolhasRecursion;/* 遞迴語句,true if planning a recursive WITH item */
/* These fields are used only when hasRecursion is true: */
intwt_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 */
RelidscurOuterRels;/* 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? */
boolpartColsUpdated;
} PlannerInfo;
RelOptInfo
typedef struct RelOptInfo
{
NodeTagtype;//節點標識
RelOptKindreloptkind;//RelOpt型別
/* all relations included in this RelOptInfo */
Relidsrelids;/*Relids(rtindex)集合 set of base relids (rangetable indexes) */
/* size estimates generated by planner */
doublerows;/*結果元組的估算數量 estimated number of result tuples */
/* per-relation planner control flags */
boolconsider_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? */
boolconsider_param_startup; /*是否考慮引數化?的路徑 ditto, for parameterized paths? */
boolconsider_parallel;/*是否考慮並行處理路徑 consider parallel paths? */
/* default result targetlist for Paths scanning this relation */
struct PathTarget *reltarget;/*掃描該Relation時預設的結果 list of Vars/Exprs, cost, width */
/* materialization information */
List*pathlist;/*訪問路徑連結串列 Path structures */
List*ppilist;/*路徑連結串列中使用引數化路徑進行 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;//代價最低的引數化?路徑連結串列
/* parameterization information needed for both base rels and join rels */
/* (see also lateral_vars and lateral_referencers) */
Relidsdirect_lateral_relids;/*使用lateral語法,需依賴的Relids rels directly laterally referenced */
Relidslateral_relids; /* minimum parameterization of rel */
/* information about a base rel (not set for join rels!) */
//reloptkind=RELOPT_BASEREL時使用的資料結構
Indexrelid;/* Relation ID */
Oidreltablespace;/* 表空間 containing tablespace */
RTEKindrtekind;/* 基表?子查詢?還是函式等等?RELATION, SUBQUERY, FUNCTION, etc */
AttrNumbermin_attr;/* 最小的屬性編號 smallest attrno of rel (often <0) */
AttrNumbermax_attr;/* 最大的屬性編號 largest attrno of rel */
Relids*attr_needed;/* 陣列 array indexed [min_attr .. max_attr] */
int32*attr_widths;/* 屬性寬度 array indexed [min_attr .. max_attr] */
List*lateral_vars;/* 關係依賴的Vars/PHVs LATERAL Vars and PHVs referenced by rel */
Relidslateral_referencers;/*依賴該關係的Relids rels that reference me laterally */
List*indexlist;/* 該關係的IndexOptInfo連結串列 list of IndexOptInfo */
List*statlist;/* 統計資訊連結串列 list of StatisticExtInfo */
BlockNumber pages;/* 塊數 size estimates derived from pg_class */
doubletuples;/* 元組數 */
doubleallvisfrac;/* ? */
PlannerInfo *subroot;/* 如為子查詢,儲存子查詢的root if subquery */
List*subplan_params; /* 如為子查詢,儲存子查詢的引數 if subquery */
intrel_parallel_workers;/* 並行執行,需要多少個workers? wanted number of parallel workers */
/* Information about foreign tables and foreign joins */
//FWD相關資訊
Oidserverid;/* identifies server for the table or join */
Oiduserid;/* identifies user to check access as */
booluseridiscurrent;/* 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連結串列
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連結串列 known not unique for these set(s) */
/* used by various scans and joins: */
List*baserestrictinfo;/* 如為基本關係,儲存約束條件 RestrictInfo structures (if base rel) */
QualCostbaserestrictcost;/* 解析約束表示式的成本? cost of evaluating the above */
Indexbaserestrict_min_security;/* 最低安全等級 min security_level found in
* baserestrictinfo */
List*joininfo;/* 連線語句的約束條件資訊 RestrictInfo structures for join clauses
* involving this rel */
boolhas_eclass_joins;/* 是否存在等價類連線? T means joininfo is incomplete */
/* used by partitionwise joins: */
boolconsider_partitionwise_join;/* 分割槽? consider partitionwise
* join paths? (if
* partitioned rel) */
Relidstop_parent_relids;/* Relids of topmost parents (if "other"
* rel) */
/* used for partitioned relations */
//分割槽表使用
PartitionScheme part_scheme;/* 分割槽的schema Partitioning scheme. */
intnparts;/* 分割槽數 number of partitions */
struct PartitionBoundInfoData *boundinfo;/* 分割槽邊界資訊 Partition bounds */
List*partition_qual; /* 分割槽約束 partition constraint */
struct RelOptInfo **part_rels;/* 分割槽的RelOptInfo陣列 Array of RelOptInfos of partitions,
* stored in the same order of bounds */
List**partexprs;/* 非空分割槽鍵表示式 Non-nullable partition key expressions. */
List**nullable_partexprs; /* 可為空的分割槽鍵表示式 Nullable partition key expressions. */
List*partitioned_child_rels; /* RT Indexes連結串列 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 { NodeTagtype; Indexphid;/* PH的ID,ID for PH (unique within planner run) */ PlaceHolderVar *ph_var;/* copy of PlaceHolderVar tree */ Relidsph_eval_at;/* lowest level we can evaluate value at */ Relidsph_lateral;/* relids of contained lateral refs, if any */ Relidsph_needed;/* highest level the value is needed at */ int32ph_width;/* estimated attribute width */ } PlaceHolderInfo;
二、原始碼解讀
standard_qp_callback
標準的query_planner回撥函式,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;//引數
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)//視窗函式
{
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&按相關欄位排序
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))//不是多餘的Key的情況下,才保留
pathkeys = lappend(pathkeys, pathkey);
}
return pathkeys;
}
測試指令碼:
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函式:
... (gdb) step make_pathkeys_for_sortclauses (root=0x1702958, sortclauses=0x170d068, tlist=0x1746758) at pathkeys.c:878 878List*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 889pathkey = make_pathkey_from_sortop(root, (gdb) 898if (!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}
函式pathkey_is_redundant通過等價類判斷排序是否多餘(redundant),在本例中,已存在限制條件dwbh='1001',因此該排序是多餘的,返回NULL.
901return 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)//遍歷連結串列
{
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);//新增到投影列中
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進行連線,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,跟外連線就不等價了,因為該值有可能是NULL),PG因此引入了PlaceHolderVar這麼一個Var來對這種變數進行特殊處理.
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 163reconsider_outer_join_clauses(root);
注意root中的placeholder_list
(gdb) p *root
$1 = {type = T_PlannerInfo, ..., placeholder_list = 0x174bf00, ...}
檢視其記憶體結構:
#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
$5 = {xpr = {type = T_PlaceHolderVar}, phexpr = 0x174be48, phrels = 0x174beb0, phid = 1, phlevelsup = 0}
#該PHV位於編號為4的RTE中
(gdb) p *$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